Sky Observers' Glossary-0

8/4/2019 Sky Observers' Glossary-0

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Sky Observers' Glossary

For London and the UK

The Sun

The Sun rises roughly in the East and sets roughly in the West. During the year, the Sun'sbehaviour changes following the seasons.

From London (UK), the Spring Equinox occurs around 21 March. At this time, the length ofdaylight is 12 hours and the Sun reaches a noonday altitude of38.5. (This figure is 90 MINUSthe Latitude of London, 51.5).

Subsequently, the length of day increases, as does the noonday altitude. On the Summer Solstice(around 21 June) the length of daylight is 16 hours 38 minutes and the Sun's noonday altitude is62. (38.5 PLUS 23.5 - the tilt of the Earth's axis).

The days then get shorter. On the Autumn Equinox (around 23 September) the length ofdaylight is back to 12 hours and the Sun reaches the same noonday altitude as in March (38.5).

The length of day continues to decrease. On the Winter Solstice (around 21 December) thelength of daylight is only 7 hours 56 Minutes and the Sun's noonday altitude is a mere 15.(38.5 MINUS 23.5 - the tilt of the Earth's axis).

The seasonal cycle continues as the length of the day and noonday altitude increase back to thevalues for the Spring Equinox.

The figures given here are also correct for places on the same latitude as London (UK). Theexact dates of the equinoxes and solstices can vary by a day or so. For places closer to theEquator, noon-day altitude of the Sun is higher and the variation in day length throughout theyear is less. For places closer to the North Pole, the noon-day altitude of the Sun is lower and thevariation in day length throughout the year is more. In the Southern Hemisphere, the seasons arereversed.

The Sun appears to move around the Earth in a year. It blocks out different stars each month.Each month different stars are visible in the sky.

Twilight

If the Earth had no atmosphere, it would become dark as soon as the Sun disappeared below thehorizon. The Earth's atmosphere disperses light from the Sun below the horizon causing it to getdark slowly. This is called twilight.

There are three types of twilight. During Civil Twilight, it is possible to read without artificiallight. Civil Twilight ends when the Sun is 6 below the horizon. The bright stars and planetsbecome visible at this time. During Nautical Twilight it is possible to see the horizon at sea for


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measurements to be made. This ends when the Sun is 12 below the horizon. At the end ofAstronomical Twilight, even the faintest stars are visible. This ends with the Sun 18 below thehorizon.

In June from London, Astronomical Twilight never ends since the Sun never reaches 18 belowthe horizon. Further North, there is a significant twilight during June (The White Nights).

Conjunction

Two objects close together in the same part of the sky. When the Moon is in conjunction with aplanet, that is a good time to find the planet as it will be close to the Moon in the sky. Twoplanets can be in conjunction with each other. If the planets are bright a planetary conjunctioncan be a spectacular sight in the sky.

When a planet is in conjunction with the Sun, it will rise at sunrise and set at sunset; normally aplanet is not visible when it is in conjunction with the Sun.

The Moon

The Moon travels around the Earth taking a month to do so. Its motion against the starrybackground can easily be seen from night to night. The Moon moves roughly its own diameterevery hour.

The Moon is a dark body illuminated by the Sun. As the angle between Sun, Moon and Earthchanges, the amount of the sunlit Moon visible from the Earth varies. This produces the Phasesof the Moon: New, Crescent, Half, Gibbous, Full.

At New Moon, the Moon is in conjunction with the Sun is not usually visible (except during aneclipse). The New Moon rises and sets with the Sun.

Each day, the Moon moves eastwards from the Sun by about 13. After a few days, a thinCrescent Moon will be visible in the evening setting soon after the Sun. As each day passes, thephase of the Moon increases (the crescent gets fatter - the Moon is said to waxing) and its timeof setting gets later (about 50 minutes per day). Tides are mainly caused by the Moon - times of

high tides also get later by 50 minutes each day.

The Half Moon occurs about 7 days after New Moon and is usually due south at sunset (fromLondon, UK), setting around midnight. As the Moon continues waxing (and setting later) itdisplays a shape called the Gibbous Moon.

Two weeks after the New Moon, the Moon reaches opposition to the Sun (the Full Moon). TheFull Moon rises at sunset and sets at sunrise, being visible all night long. In the summer (whenthe Sun is high) the Full Moon hangs low in the sky. In the winter (when the Sun is low) the FullMoon rides high in the sky.

After Full Moon, the Moon begins waning. The second half of the lunar cycle occurs later andlater, mainly in the morning. The Half Moon that occurs three weeks after the New Moon rises at

midnight and is due south at sunrise. Near the end of the cycle the Moon is a thin crescent risingshortly before the Sun.

The Moon's orbit around the Earth is not a circle but an ellipse. When it is at its closest to theEarth, it is said to be at perigee; when at its furthest from Earth, it is at apogee.

Eclipses

Eclipses of the Sun (also called Solar Eclipses) occur when the Moon covers the Sun. There arethree types:


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Total - where the Moon covers the Sun completely. These are only visible in a smallband along the Earth. For one location they occur roughly every 400 years.

Annular - where the Moon is too far to cover the Sun completely leaving a ring(annulus) visible. These are slightly more common than total eclipses.

Partial - when the Sun looks like it has had a bite taken out of it. All total and annular

eclipses have a large area where a partial eclipse is visible. For one location, they can beseen every two or three years.

In a calendar year, there must be two solar eclipses and there can be as many as five.

Eclipses of the Moon (Lunar Eclipses) occur when the Moon (at full phase) passes into theshadow of the Earth. These eclipses can be total: the Full Moon slowly disappears to be replacedby ghostly reddish Moon for up to 1 hour 43 minutes. Partial eclipses are also possible.

Lunar eclipses are less common than solar eclipses. In a year none may occur; the maximumnumber is three. When lunar eclipses occur, however, they are visible from over half the Earth sothey are easier to see. From a single location, several lunar eclipses will be seen per decade.

Planets

One of five star-like objects in the sky that are not fixed in position like the stars. They areMercury, Venus, Mars, Jupiter and Saturn.

The word planet means wanderer in Greek. The planets move against the starry backgroundbecause they are worlds close to the Earth that are moving around the Sun.

The five naked eye planets have been known since ancient times. From 1781, two more majorplanets have been discovered by telescope (Uranus and Neptune) and a number of minorplanets (Ceres, Pluto, Sedna).

Inferior Planets

The Inferior Planets are closer to the Sun than the Earth. Mercury and Venus are the Inferior

Planets. An Inferior Planet can only been seen in the vicinity of the Sun; either in the eveningafter sunset or in the morning before sunrise. It can never be visible all night long.

An Inferior Planet can pass behind the Sun. This is called Superior Conjunction. It will not bevisible at this point as it will be too close to the Sun in the sky. The planet then moves to the Eastof the Sun where it becomes visible in the evening sky after sunset. It is then misleadingly calledan evening star. As an evening star, Mercury is normally visible for a few weeks beforedisappearing while Venus can be visible for several months at a time.

For observers in the Northern Hemisphere facing South, a planet that is East of the Sun is to theleft of it, following it in the sky as it moves from left to right during the day. This is why theplanet will be visible after sunset.

When the planet is as far from the Sun as it can be in the evening sky (as seen from the Earth),this is called Greatest Elongation East. Mercury can never be more than 28 away from the Sun(the amount varies because Mercury has a very eliptical orbit and can be as low as 18). ForVenus the greatest elongation is 47.

After an evening appearance, the planet then moves in front of the Sun as seen from the Earth.This is called Inferior Conjunction. If the alignment is exact, the planet can actually pass infront of the Sun as a small black spot. This is called a Transit and is a rare phenomenon.


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After Inferior Conjunction the planet moves West of the Sun into the morning sky (where it iscalled a morning star). The Greatest Elongation West is the farthest the planet can be from theSun in the morning sky (as seen from Earth).

For observers in the Northern Hemisphere facing South, a planet that is West of the Sun is to theright of it, preceeding it in the sky as it moves from left to right during the day. This is why the

planet will be visible before sunrise.After a period of morning visibility, the planet moves back to Superior Conjunction and thewhole cycle begins again. A complete cycle (say, from Superior Conjunction back to SuperiorConjunction) is called the Synodic Period orSynodic Cycle.

Mercury's Synodic Cycle takes just under four months. The planet is at its brightest on either sideof Superior Conjunction. For Venus, the cycle takes 16 months and the planet is at its brightestabout a month on either side of Inferior Conjunction. The length of the Synodic Cycle dependson a planet's orbital period as well as the length of the year (the Earth's orbital period).

The Inclination of the Ecliptic

The visibility of a planet is determined by its Elongation from the Sun and something called the

Inclination of the Ecliptic. The Ecliptic is the path in the sky that the Sun, Moon and planetsappear to travel close to. It runs through the familiar constellations of the Zodiac.

In high latitudes (like in the UK), the Inclination (orsteepness) of the Ecliptic varies throughoutthe year. This affects the visibility of planets close to the Sun, especially the inferior planets.

Somethimes, the ecliptic makes a very shallow angle with the horizon. When this happens,planets will appear very low down after sunset or before sunrise. The period of visibility will beshort: the planet will either rise very close to the time of sunrise, or set very close to the time ofsunset.

At other times, the ecliptic makes a steep angle with the horizon. Planets will then appear muchhigher in the sky. The period of visibility will be long: the planet will either rise a long time

before sunrise or set a long time after sunset.In the case of Venus, this can make the difference between the planet making a spectacularappearance, high up or being barely visible close to a bright horizon. In the case of Mercury, asteep ecliptic means the planet will be visible, while a shallow ecliptic means that the planetcannot be seen.

In the Northern Hemisphere, the steepest ecliptic is found in the evening sky between Februaryand April and in the morning sky between August and October. These are the best times to seethe inferior planets: as evening objects in the late winter and early spring or as morning objectsin the late summer or early autumn. If the planets have a good elongation from the Sun at thesetimes, Venus will dominate the sky and be visible for several hours after sunset or before dawn;Mercury will be easily visible to the naked eye.

Conversely, the shallowest ecliptic is found in the morning sky between February and April andin the evening sky between August and October. These are the worst times to see the inferiorplanets: as morning objects in the late winter and early spring or as evening objects in the latesummer or early autumn. Even if the planets have a good elongation from the Sun at these times,Venus will be visible but only briefly after sunset or before dawn and will be low down close tothe horizon; Mercury will be unobservable.


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The steepness of the ecliptic also effects the Moon. The best time to see the thinnest crescentMoon that occurs after New Moon is in the evening sky during the late winter or early spring.The effects of the ecliptic are shown in the diagram below which is after sunset.

In the diagram above, the Sun (yellow circle) is shown below the horizon (black line) at the endof Civil Twilight in the Northern Hemisphere. The Earth's Equator is the blue line. In thetemperate regions, the Equator will be at an angle. The positions P1 and P2 are planets (greycircles) at the same elongation from the Sun. In the case of P1, the planet is south of the Sun andthe ecliptic (red line) is at a shallow angle; the planet appears very low above the horizon when itbegins to get dark. For P2, the planet is north of the Sun and the ecliptic (green line) is at a steep

angle; the planet is much higher in the sky. The black dashed lines are the paths of the planets toreach the horizon as the Earth rotates. These paths are parallel to the Equator. For P1, the dashedline is the shorter meaning that the planet will reach the horizon very quickly; it will set in ashort time. For P2, the dashed line is the longer meaning that the planet will take a long time toreach the horizon; it will remain visible for a long time before setting.

Transits

Only the Inferior Planets can transit the Sun. A transit occurs when an Inferior Planet passesdirectly between the Earth and the Sun appearing as a black spot against the Sun. Transits arerare.

Transits of Mercury occur about 13 times per century; 9 in November and 4 in May. The transit

is not visible to the naked eye and must be viewed by projection. The last one was in November2006.

Transits of Venus only occur in June and December. A pair of transits is separated by 8 years buteach pair occurs after 105 or 121 years. A transit of Venus can be seen with the naked eye if afilter is used. The dates of Venus transits below indicate how rare the event is. The last transit ofVenus occurred in June 2004 but, before that, none occurred in the 20th Century.

6 December 1631


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4 December 1639

5 June 1761

3 June 1769

8 December 1874

6 December 1882 8 June 2004

6 June 2012

10 December 2117

8 December 2125

Superior Planets

The Superior Planets are Mars, Jupiter and Saturn. These planets are further from the Sun thanthe Earth.

A Superior Planet cannot pass between the Earth and the Sun so there is no Inferior Conjunction.

Since it can only go behind the Sun (Superior Conjunction) this is normally referred to, simply,as Conjunction. The planet will not be seen when it is in conjunction with the Sun.

The planet then moves to the West of the Sun. It appears in the morning sky. As the months pass,the planet will rise earlier and earlier as it moves away from the Sun in the sky, as seen from theEarth.

Eventually, the planet will be in Opposition. At Opposition, the planet rises at sunset and sets atsunrise and is visible all night long. A Superior Planet is at its closest to the Earth at Oppositionand is then at its brightest. The elongation of a planet at opposition is 180.

After Opposition, the planet will set earlier and earlier and will be seen in the evening sky.Eventually, the planet will set very shortly after the Sun and will then disappear in the evening

twilight as it reaches Conjunction again.A complete cycle (say, from Opposition back to Opposition) is called the Synodic Period orSynodic Cycle.

For Mars, the Synodic Cycle takes 26 months. So Mars is prominent in the sky every alternateyear. For Jupiter the cycle takes just over 13 months and so occurs a month later each year. ForSaturn, the cycle is just under 54 weeks.

There are two other superior planets. Uranus can just about be seen with the naked eye on a darkMoonless night. A small telescope or a good pair of binocculars are required to see Neptune.Both these planets travel very slowly around the sky and have Synodic Periods just over a yearlong.

The Altitude of the EclipticThe Ecliptic is essentially the path of the Sun in the sky over a year. In June the Sun is very highin the sky and the days are long. In December the Sun is very low in the sky and the days areshort.

The planets also follow the ecliptic. This means that their altitudes in the sky and their periodsabove the horizon vary depending on which part of the ecliptic they are in.


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The best time to observe the superior planets is when they are at opposition. They are thenopposite the Sun. If the Sun is high in the sky, a planet in opposition will be low. If the Sun islow in the sky, a planet in opposition will be high.

It follows that the best time of the year to see a planet in opposition is December. At that timethe Sun will be low in the sky and the days will be short. A planet in opposition will be high in

the sky and the nights are long. In June, a planet in opposition will be low in the sky and thenights will be short.

The Moon is also affected by the altitude of the ecliptic. The Full Moon is essentially a Moon inopposition. This explains why the winter Full Moon is high in the sky while the summer FullMoon is low in the sky.

Occultations

An occultation occurs when one object covers or occults another. The Moon can occult fourbright stars: Aldebaran (Taurus), Regulus (Leo), Spica (Virgo) and Antares (Scorpius). Theseoccultations occur at irregular intervals. The Moon can also occult planets. These occultationsare even rarer. Rarest of all is an occultation of a star by a planet and one planet occulting

another.Stars

Stars are distant suns. They vary in brightness and colour. Human beings have connectedunrelated stars into groups called Constellations. Modern astronomers recognise 88Constellations.

Different cultures (Mayan, Chinese) have used different groupings. Of the modernConstellations, 48 are from Greek and Roman times. These 48 constellations (mostly in theNorthern sky) are known as the Classical Constellations. They include the 12 constellations(also known as signs) of the Zodiac (e.g. Scorpius, Cancer, Leo) in which the planets, Sun andMoon are always found, and other famous names like Orion, Ursa Major (the Great Bear), andHercules.

When Europeans began exploring the Southern Hemishere, new stars were discovered and 40new Constellations were created. These include Crux Australis (The Southern Cross), Centaurusand Carina.

The Milky Way is a collection of stars too numerous to be seen individually. To see it, a darkmoonless night away from city street lights is required. The best time of year is Winter orSummer.

Many stars when viewed through a telescope turn out to be doubles - two stars forming a singlesystem and in orbit around each other. These are called Binaries. Albireo in Cygnus is an easyexample to see and the two stars are different colours. In some cases the two stars are not relatedand only happen to be in the same direction in the sky. These are Optical Doubles. Mizor (and

Alcor) in the Great Bear is the best known example.Some stars are bunched together over a small area of the sky. These are called Star Clusters.There are two types.

The more scattered are called Open Clusters. The most famous in the Northern hemisphere skyis The Pleiades (also known as The Seven Sisters) in Taurus. This is easilly visible with thenaked eye and is superb in binocculars. Others include Praesepe (the Beehive) in Cancer and theDouble Cluster in Perseus.


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Spherical and more bunched together are the Globular Clusters. The brightest in the NorthernHemisphere is the Great Cluster in Hercules (known by astronomers as M13). The best ones areonly visible in the Southern Hemisphere.

Some stars vary in brightness and are called Variable Stars. Some are actually changing theamount of light they emit (Intrinsic Variables). The easiest to see is Mira (the Wonderful) in

Cetus. This star varies over about 400 days from being as bright as the stars in the Great Bear tobeing below naked eye visibility. Most variable stars are not this obvious. Another group of starsvary because they are really a double and one member eclipses the other. These are EclisingVariables. The best known is Algol (the Demon star) in Perseus which appears to wink for anhour or so every few days. Explosive Variables are just that - exploding stars. They areunpredictable and rare. They have been known to outshine Venus in the past.

Apart from the stars there are more nebulous objects visible in the sky. A Nebula is a interstellargas cloud. The easiest to see is the Orion Nebula (visible in Winter in Orion). A Galaxy is afuzzy patch that is actually a very distant system of stars, clusters and nebulae. The AndromedaGalaxy (Autumn in Andromeda) is the furthest object visible to the naked eye.

Meteors

Meteors (also called shooting stars) are small particles (usually smaller than a grain of sand)that burn up when they enter the Earth's atmosphere leaving a brief streak of light. On any darknight 10 to 15 can be seen every hour. The numbers increase after midnight.

At some times of the year, the Earth passes through a stream of particles and we can see ameteor shower. The numbers of shooting stars is then higher, occasionally very much more.These meteor showers appear to come from a particular region of the sky and are named after theconstellation in that region. The most regular and famous meteor showers are the Perseids(apparently from Persus) every August and the Leonids (Leo) in November.

Shooting stars are nothing to do with stars.

Some particles are so large that they reach the Earth's surface. They are then called Meteorites.

Comets

Comets are small bodies compared to planets. They are covered with ice and travel around theSun in (usually) non-circular orbits.

When they are a long way from the Sun they are not visible to the naked eye. As they approachthe Sun, the radiation and gases from the Sun evaporate the ices and a tail appears. The tailalways points away from the Sun. cometary tails may be millions of kilometres long but they aremade up of very thin gases.

Comets do not streak across the sky. They hang amongst the stars, changing position night afternight. The head of a Comet is fuzzy. Comets appear at irregular intervals. Naked eye Comets arerare and can be spectacular.

Asteroids or Minor Planets

These are bodies in the solar system that are orbiting the sun and are smaller than planets.

Many go around the Sun between the orbits of Mars and Jupiter. Ceries is the largest of theseand is visible with binocculars. The brightest asterod is Vesta which can be just about visible tothe naked eye at opposition. Most of the others require telescopes to be visible.


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Other asteroids orbit among the inner planets, like Eros, which passes close to the Earth andIcaros which moves closer to the Sun than Mercury.

A small group follow in Jupiter's orbit. These are called Trojans and the first descovered wasAchilles.

Still others orbit amongst the outer planets. These are called Centaurs and the best known is

Charon.

Beyond the planet Neptune are a number of smaller icy bodies, the best known of which isPluto, but others include Sedna and Eris.

Polar Lights

Polar Lights are coloured glows in the upper atmosphere caused by the interaction of the gasmolecules with charged particles from the Sun. These particles are diverted into the polar regionsby the Earth's magnetic field. In the Northern Hemisphere they are called the Northern Lights(orAurora Borealis). They are not normally visible as far south as the UK but displays havebeen known at these latitudes.

A Brief History of Astronomy

From Creation Myths to BigBang Cosmology

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4000 BC

Early peoples think thatthe world is flat with a

crystalline sky overhead.

The Sun is thought to be

a god that rode across

the sky in a chariot,

travelling beneath the

Earth at night.

The world is thought to havebeen created in a small

amount of time by a deity ordeities.

Ancient views of a flat Earth

Astronomical records

from Mesopotamia

2000 BCMesopotamian priests begin keeping systematic astronomical

records. Observations of the stars and planets made in India

and China.


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1500 BC

The Sumarians, Babylonians,Indians, Chinese and Egyptians

develop astronomy. The stars

appear to form patterns in the

sky that are visible every year.

These patterns are considered

fixed and are called

constellations. The Chinese

divide the sky into 28

constellations; the Indians into

27.

The length of the day, the month andthe year is known. The five naked eyeplanets are known.

Chinese star map

600 BC

In Greece, Anaximander notices

that the stars appear to rotate

around a pole. He suggests that

the sky is a complete sphere

around the Earth. He thinks that

the Earth's surface must be

curved after hearing that

travellers saw new stars

appearing when moving north or

south. He pictures the Earth as a

cylinder.

The planets are known to move againstthe background of the stars whichappear fixed to a crystal sphere. Theword planet means "wanderer".

Stars rotating around the pole


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500 BCPythagoras and his followers teach that the

Earth is a sphere. The idea came about from

observations of Lunar Eclipses - the Earth's

shadow on the Moon is always circular.

The Pythagoreans think that the motions of the planetsare mathematically related to musical sounds andnumber. These ideas are called "The Music of theSpheres".

Pythagoras

Rotating Earth

350 BC

Heracleides suggests that the daily motion of the Sun,

Moon, planets and stars around the Earth could be

explained if the Earth rotated on its axis once every day.


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330 BC

Aristotle writes a series of books which

contain ideas that will influence humanity for

1800 years.

He talks about the four elements (earth, fire air andwater) which he says are only found on Earth. Theseelements each have their own tendencies: earth isheavy and falls, fire is light and rises. Motion is instraight lines. The heavier the object, the faster it falls.

A fifth element, the Aether, is only present in theobjects of the sky. Its natural motion is circular socelestial objects travel around the Earth in perfectcircles. Aristotle assumes that light travels infinitelyfast.

The Earth and the heavens are, therefore, subject todifferent natural laws. Things on Earth are corruptedand subject to change while the heavens areincorruptible and unchanging.

Aristotle

Eratosthenes

250 BC

Eratosthenes measures of the size of the Earth

from observations of the Sun in different parts of

the Earth. On the longest day of the year, the Sun

is overhead in southern Egypt but 7 from the

vertical in northern Egypt. Eratosthenes takes the

distance between these two points and multiplies it

by the ratio between a full circle (360) and the 7.

His measurement is within 1% of the correct value.


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Measuring the size of the Earth from the altitude of the Sun at two locations

Aristarchus accurately measures of the distanceto the Moon using trigonometry applied to Lunar

eclipses. He correctly shows that the moon is 25%

as large as the Earth.


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Aristarchus

Distance to the Moon

He makes the first attempt to find the distance to the Sun. His theory is good but

the measurements are difficult and his figure (19 times further than the Moon - 5%

of the correct value) is too low. Even so, the Sun is shown to be larger than the

Earth.


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Distance to the Sun

Aristarchus even suggests that the Earth goes around the larger Sun. This idea does

not take root because of lack of evidence and will not become accepted for 1800

years.


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140 BC

Hipparchus refines the distance between the

Earth and Moon usingTrigonometric Functions

which he had invented.

He thinks that he had observed positional changesamongst the so called "fixed stars" but he is unsure.He creates a very accurate map of the 1000 or sobrightest stars. This map will play an important role inastronomical history 1800 years later.

During his research he discovers that there are twotypes of year. The Tropical Year and the SiderealYear differ by 20 minutes. This causes the position ofthe Celestial Pole to move in a circle taking 26,000

years to complete one cycle. This phenomenon iscalled the Precession of the Equinoxes.

Hipparchus

130 BC

Seleucus thinks that the Moon is somehow responsible for the tides. This idea

would not be proved for nearly 1800 years until the time of Newton.

115 BC

Poseidonius recalculates the Earth's circumference as 70% of the

correct value. This figure would become accepted until modern

times. 1500 years later, Christopher Columbus would use this figure

when searching for finance for his expedition across the Atlantic.

Poseidonius also measures the distance between the Earth and the Sun to anaccuracy of 43%.

He also popularises astrology.

The astrological

symbol for

Scorpio


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Ptolemy

100 AD

Ptolemy

writes a book

(known by its

Arabic name,The

Almagest -

The

Greatest)

which

summarises

the

astronomical

knowledge of

the ancients,

especially

that of

Aristotle.

The cosmologyis based onEarth being thecentre of theUniverse withthe Sun, Moon,planets, and

stars (all set oncrystalspheres)revolvingaround theEarth in aseries of circlescalledEpicycles. Theplanets,Mercury andVenus always

lie close to theline joining theEarth and theSun.

The system iscumbersomebut could beused to predict

An Epicycle


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the motions ofthe planets tonaked eyeaccuracy.Tables are

created thatpredict thepositions of theplanets in thefuture. Herepublishes thestar map ofHipparchusand names the(48) classicalconstellationswith the namesthey are stillknown by inthe West.

Ptolemy writesthat the sphereof the stars is200 timesfurther awaythan the Moon.

The book also

contains asummary ofgeographicalknowledgewith estimatesof latitudes andlongitudes forplaces inEurope. Thesewould not beimproved for800 years.

The book isone of the fewto survive thechaos of theEuropean DarkAges. After thefall of theRoman


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Empire, thebook would betranslated intoArabic in theIslamic world,

and, later, intoLatin and willplay a part inEurope'sRenaissance

The (Western) Constellations

500


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In India, Aryabhatta, writes a book in which he states that the Sun is the centre of

the Solar System. This idea would not be accepted for another 1000 years.

Aryabhatta

600

Varahamihira writes that "Bodies fall towards

the earth as it is in the nature of the earth to

attract bodies", 1100 years before the idea

would become accepted.

850

The Arab mathematician, al-Khwarizmi, adds

and refines Ptolemy's geographical knowledge,

using astronomical observations to give the

latitudes and longitudes of over 2400 localities

in Europe and Asia.

He also champined the use of the Indian number

system working out the rules of arithmetic that wouldsimplify calculation. His numbers arrived in Europewhere they became known as Arabic Numerals.

al-Khwarizmi


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al-Battani

900The length of the year is calculated as 365

days 5 hours 48 minutes 24 seconds by the

Arab astronomer, al-Battani. At that time

(and for the next 600 years) Europe's

calendar is based on a year of 356 days 6

hours.

Al-Battani also updates the figures for the Precessionof the Equinoxes (54.5'' per year) and the tilt of theEarth's axis (23 35').

His observations show that the Earth's distance to theSun varies, putting a doubt on the idea of perfectcircular orbits.


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964

al-Sufi

publishe

s a book

aboutstars

listing a

number

of

objects

that are

hazy and

fuzzy.

These

turn out

to be the

star

clusters

and

galaxies

that will

provide

the

informati

on to

enlargethe size

of the

Universe

, 900

years

later.

This is thefirstwritten

mentionof theobject thatwouldlater beknown astheAndromeda

al-Sufi's Book


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Galaxy.

al-Biruni

1040

The Central Asian scientist, al-Biruni,

develops the experimental method of science

including the modern mathematical treatment

for handling errors.

His surveying techniques using triangulation allowhim to measure the radius of the Earth as 6339.6 km,a value that would not be improved for 500 years.

He suggests that the velocity of light is immense

compared to that of sound. He theorises that theappearance of the Milky Way is due to it being madeup of countless stars, an assertion that would not beverified until the invention of the telescope 500 yearslater.

1050

al-Zarqali (known in the West as Arzachel) discovers that thepoint in the year when the Earth is closest to the Sun moves

forward at a rate of 12.04'' per year. This is within 2% of the

modern value. He also suggests elliptical orbits for the planets.

1121

al-Khazini suggests that the centre of the Earth is the source

of all gravity.al-Zarqali


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The Ptolemaic Geo-centric (Earth Centred) System

1200

Many

Greek and

Arab

books aretranslated

into Latin

including

Ptolemy's

Almagest.

The influx

of classical

knowledge

helps the

Renaissan

ce begin in

Europe.

The(Christian)CatholicChurchadoptsAristotle'scosmology.In thecomingcenturies,disagreement with thiscosmologywouldbecome aheresy.

The modelenlargesAristotle'sideas of thecorruptEarth andthe perfectheavens.The mostcorrupt partof theUniverse is


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Hell whichis situated inthe centre ofthe Earth.Both Earth

and Hell areimperfectand both aresubject tochange,corruptionand decay.Man's Sincauses thecorruptionof the Earth.Above theEarth is theatmosphere.This is lesssubject tochange butchangesenough toproduce theweather.Aurora,meteors and

comets arealsoconsideredto beatmosphericphenomena.

The Moonbeingfurther fromthe Earth,changes

less. Itchanges itsphases andhas ablotchyappearancebut has theperfect


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circularmotion of acelestialobject. TheSun and

planetscome next.They don'tchange andalso movein circlesaround theEarth. Mostdistant is thecrystalspherecontainingthe stars.The starsareunchangingand eternal.God (themost perfectpart of theUniverse) ison theoutside of

this finalcrystalsphere. Allheavenlymotion is incircles (aperfectshape) or'circleswithincircles'.

Dante,would laterwrite aboutdescendingthe ninecircles toHell andascending


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Helocentric (Sun-Centred) System

1572

The Danish astronomer, Tycho Brahe,

studies a brilliant new star that

appears in the sky. The star is later

known to be a type of exploding star.

Brahe names the star a nova.

Brahe finds no parallax indicating that it is areal stellar object and not something close tothe Earth. The star fades after a couple ofyears. This is an indication that the starryheavens do change.

He also studies a comet and shows that it ismoving in an elongated orbit amongst the

planets. This indicates that comets are notatmospheric phenomena and that there are nocrystal spheres holding the planets sinceobjects can move freely between the planets.It also shows that not all heavenly motion iscircular.

This is the first observational evidence thatAristotle and Ptolemy's ideas may be


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Tycho Brahe

flawed. Brahe disagrees with Copernicus,however, and writes that the planets doindeed go around the Sun but that the Sun(carrying all the planets) orbits the Earth.This half-way idea is not taken seriously. He

measures the year to an accuracy of onesecond. This helps promote the introductionof the Gregorian Calendar(now theinternational standard) in 1582.

Tycho Brahe is the last of the Europeannaked-eye astronomers. His detailed andaccurate observations of the motion of Marswould lead to a better understanding ofplanetary orbits after his death.

1596

David Fabricius discovers that the star, Omicron Ceti,

varies its brightness over several months. This is

another blow to the idea of the unchanging heavens.

Omicron Ceti or Mira(Wonderful)

Giordano Bruno

1600

In Italy, Giordano Bruno believes and teaches that the

Universe is infinite, the Earth is moving around the Sun,

the stars are other suns with planets around them, and

life is not confined to the Earth.

He is eventually burnt at the stake for heresy!

1610


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The Italian mathematician and astronomer, Galileo Galilei, points the newly

invented telescope to the sky and revolutionises astronomy.

On the first night, Galileo sees stars that are invisible to the naked eye. If these stars were beingseen for the first time, the 'ancients' could not have known everything! The Milky Way is seen tobe a vast collection of stars too numerous to be seen individually.

Observing the Sun, he sees sunspots, imperfections in the 'perfect' Sun. He watches them moveacross the Sun as it rotates; the first time a celestial body has been observed to rotate on its axis.This leads to the thought that if the large Sun could rotate, why not the smaller Earth.

He sees Venus go through a complete cycle of phases. The planet appears to change its shapelike a miniature Moon from full to half to crescent. This could only happen if it was movingaround the Sun. If Venus was always between the Sun and the Earth (as Ptolemy thought) itwould only exhibit a crescent phase at all times.

Through the telescope, the Moon appears to have mountains and plains. This showed it to be aworld, no different to the Earth.

Looking at Jupiter Galileo discovers its four large moons, resembling little stars. This proves thatnot everything is moving directly around the Earth. It is also an indication that it is possible for abody to carry its moons with it as it moves around the Sun. If Jupiter could carry four moonswhy could the Earth not carry its single moon.

Galileo's observations support the Sun-centred Universe ofCopernicus and he advocates thissystem in all his writings. Unlike most academics of the time who only write in Latin, Galileowrites his books in the local language so that they could be read by everyone. The CatholicChurch is angered and forces him to deny that the Earth is moving around the Sun.

Although not the first to perform experiments, Galileo makes it fashionable and disproves someofAristotle's assertions. By dropping cannon balls from a tower (actually, the Leaning Tower ofPisa) he proves that heavy objects fall to the Earth at the same rate as light objects. He shows that

falling bodies accelerate as they fall to Earth. More interestingly, moving bodies could be subjectto two separate forces acting independently. This explains how objects could be carried on theEarth even if it was moving.

He also shows that the period of a pendulum swing is constant for a given length. This willeventually lead to accurate timepieces.

He attempts to measure the speed of light but fails due to lack of accurate equipment. Galileo'sphysics experiments set the stage for Newton's work.

1610

Using a telescope, Simon Marius describes a fuzzy patch in the constellation of

Andromeda. 300 years later, this object (the Andromeda Galaxy) will dramatically

expand humanity's notions about the Universe.

1620

The German mathematician,John Kepler, uses the copious observations of Mars

by Tycho Brahe to show that the planets move in elliptical orbits. The circular

motion of the ancients is finally removed. This is the first major use of the newly

discovered logarithms in a scientific calculation.


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In addition, Kepler shows that the closer a planet is to the Sun, the faster it moves. This is thesame effect that causes ballet dancers to rotate faster when they bring their arms in. The planetsare seen to be following mechanical laws similar to those on the Earth. This is a further blow tothe ancient idea of one law for the Earth, another for celestial objects.

Kepler discovers a simple mathematical relationship between the period of a planet to orbit the

Sun and its distance from the Sun. The square of the period is proportional to the cube of thedistance. This provides a scale for the Solar System. If any single distance in the Solar Systemcould be measured it would be possible to calculate all the others. Saturn, the furthest planet, isshown to be 10 times further from the Sun than the Earth.

Kepler suggests that the Sun somehow pulls the planets around it. He correctly predicts thepassage of the planets Mercury and Venus in front of the Sun. These are called transits and theywould later help in accurately determining the distance from the Earth to the Sun.

1639

In England,Jeremiah Horrocks observes the first transit of Venus. He suggests

that observations of this phenomenon from different parts of the Earth could be

used to measure the scale of the Solar System, hence the distance from the Earthto the Sun. Observations of an event or object from two vantage points is called

parallax.

He proves that the Moon's orbit around the Earth is an ellipse and suggests that the irregularitiesin the orbit were due somehow to the Sun. He also suggests that Jupiter and Saturn affect eachother's orbits.

1640

Godefroy Wendelin measures the distance between the Earth and the Sun using

the method first used by Aristarchus. His result is 60% of the actual figure.

1650

Giovanni Riccioli discovers a double star with the telescope. This is another

example of properties of stars not visible to the naked eye.

1656

Christian Huygens (from the Netherlands) discovers a Moon around Saturn. Since

this makes six planets (including the Earth) and six Moons, he declares the Solar

System complete! More bizarrely, he finds that Saturn itself is surrounded by a ring.

He discovers a new type of object, the Orion Nebula. This is a fuzzy cloud-like nebulous object

amongst the stars. Huygens guesses the distance to the brightest star, Sirius by assuming it is thesame luminosity as the Sun. He calculates the distance as being over 25,000 times the distancebetween the Earth and Sun. This is a very large distance but is actually only one twentieth of thecorrect distance.

1666


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Isaac Newton (England) begins work on his masterpiece, Principia. In this book, he

explains the motions of the Earth, Moon, and planets in terms of the same force of

gravity that pulls objects (like apples) to the Earth.

He shows mathematically that two bodies that attract each other gravitationally will orbit eachother in an elliptical path (explaining Kepler's results). The more massive body will appear tomove less while the less massive body will appear to move more. By studying these motions it ispossible to show that the Sun is far more massive than all the planets since they all appear tomove around it. The theory allows the motions of the Moon and planets to be calculated fromfirst principles. Most planetary orbits are shown to be almost circular apart from that of Mercury,which is strongly elliptical. Newton also confirmed that the planets affect each other's paths asthey orbit the Sun.

His equations of gravity show that all objects should fall to the Earth with the same acceleration,as Galileo found. Newton extends the experimental results ofGalileo into his three laws ofmotion. These explain why we do not feel the rotation of the Earth on its axis or its motionaround the Sun. They also explain why the planets did not need to be pushed around the Sun andremove the need for planetary 'crystal spheres'. According to Newton, the period of a pendulumcan be used to measure the force of gravity on the surface of the Earth.

Newton also explains the tides. They are caused mainly by the Moon (and to a lesser extent, theSun). His equations explain why there were two tides every day. The Moon is also shown to beresponsible for the Precession of the Equinoxes, discovered by Hipparchus.

For the first time, the laws in the heavens are shown to be the same as the laws on the Earth.

The motions of the Solar System (the Sun and planets) are now understood in detail. The stars,however, are still considered to be lights set on a distant crystal sphere beyond the planets.

Apart from his astronomical discoveries, Newton does important work on optics andmathematics.

With the publication of Newton's work, the Age of Reason is considered to have begun.1671

Jean Richer notices that a pendulum has a slower rate of swing at the equator than

at higher latitudes. He deduces that the Earth is not a perfect sphere but an oblate

spheroid (a sphere flattened at the poles).

1672

Giovanni Cassini measures the parallax of Mars. The observations are made from

Paris and French Guiana. This gives a value of the Earth - Sun distance that is 93%

of the actual value.

His discovery of four moons around Saturn destroys Huygens' view of Solar System perfection.

1676

Olaus Roemer observes Jupiter and its moons. He notes that the eclipses of the

moons with the planet were sometimes occurring later than predicted. This is

because Jupiter's distance from the Earth varies as both planets orbit the Sun. He


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correctly deduces that the delay is caused by the fact that light needs a few

minutes to travel from the eclipses at Jupiter to the Earth.

Using the best distance measurements available, Roemer calculates the speed of light. His figureis 75% of the correct value, an excellent value for the times. Aristotle's idea of an infinite speedfor light is shown to be wrong. The fact that light has a finite (though very large) speed meansthat the further we look into space, the further back in time we can see.

1718

By accurately comparing the positions of stars with those on Hipparchus' star

map, Edmund Halley shows that a small number of the stars had changed position

in the 2000 year period since that map was made. The movement of one star is

even noticeable when compared to maps made by Tycho Brahe, 150 years earlier.

This is now known as a star's Proper Motion. The amount of this motion is very small

but this is the beginning of the end of the idea that the stars are fixed on a crystal

sphere.

Assuming that stars were moving at the same rate as planets, it is possible to make an estimate ofstellar distances. At the estimated distances, the stars had to be sun-like in their real brilliance(luminosity). This is the first hint that the Sun is an ordinary star rather than the light at the centreof the Universe.

Halley also works out the orbit of the comet that bears his name. It is a highly elliptical orbit. Upto then, comets were thought to come and go at random. Halley shows that even comets followNewton's laws of gravity.

1728

James Bradley, attempts to determine stellar distances by observing stellar

parallax during the course of the year. The idea is to use a baseline that is twice the

distance between the Earth and the Sun. Observations of stars are made to see ifthe stars' positions change.

They do, but not in the way expected. Bradley discovers a phenomenon called the Aberration ofLight. This is the first direct proof that the Earth is in motion but does not yield stellar distances.It is caused by the fact that light has a finite speed. Bradley's observations give a value for thespeed of light which is close to the correct value.

Bradley also measures the diameter of Jupiter and finds that it is much larger than the Earth. Notonly is the Earth not the centre of the Solar System but it isn't even the largest of the planets.

1755

The German philosopher, Immanuel Kant speculates on the origin of the planets.He suggests a nebula condensing around the Sun.

He thinks that the Milky Way is an "Island Universe" of stars arranged as a flat disk, and thatsome of the nebulous objects in the sky may be other similar systems outside the Milky Way.This idea would not be accepted for 170 years.

1768


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James Cookleads an expedition to the South Pacific to observe a transit of Venus.

The observations are not successful but the geographical discoveries made

encourage others to explore the world scientifically.

1780

William Herschel discovers Uranus, the first new planet since ancient times. Thisinstantly doubles the size of the Solar System.

He attempts to measure stellar parallax by looking at stars that are close together in the sky. Heassumes that one star may be closer than the other so that the parallax movement will be easier toobserve and measure. In many cases, he finds movement but this is independent of the Earth'smotion around the Sun. The stars are actually in orbit around each other. These are called BinaryStars. This demonstrates that the stars are not fixed to a crystal sphere and that Newton's law ofgravity also operates amongst the stars.

Herschel also discovers many stars that change their brightness. These are called Variable Stars.Stars can no longer thought of as unchanging and uninteresting.

By counting stars, measuring their motions and applying statistics, Herschel makes the firstestimate of the size of the region occupied by the stars. This region is now called the Galaxy. Theobservations indicate that the Solar System is a tiny speck within the Galaxy. It is apparentlysituated close to the galactic centre because the Milky Way appears symmetrical in the sky.Herschel's estimate of the diameter of the Galaxy is enormous (9000 Light Years) but is actuallyless than 10% of the true value.

The Sun is shown to have a motion of its own relative to other stars. This motion is towards theconstellation of Hercules.

Herschel and others continue to speculate about the existence of other galaxies.

1798

Henry Cavendish applies Newton's equations to very accurate laboratory

experiments to measure the mass of the Earth.

1814

Joseph Fraunhofer passes light from the Sun through a high quality prism. This

breaks down the white light and displays a spectrum. He finds that the continuous

rainbow of colours is crossed by thousands of dark lines. These lines would unlock

many mysteries of the Universe.

1838

The first stellar distances are finally measured.

Friedrich Bessel measures the parallax of a faint star called 61 Cygni. It had been chosenbecause it has a large Proper Motion and is therefore assumed to be nearby.

Even the nearest star is over 270,000 times further away than the Sun!

The stars are so far away that they must be Sun-like in luminosity to be visible from the Earth.The Sun is thus shown to be an ordinary star seen from close up. Copernicus had been correctwhen he stated that the stars were too distant for a parallax to be easily visible.


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1846

The French mathematician, Urbain Leverrier (France) studies anomalies in the

motion of Uranus and predicts the existence of a new planet using the laws of

gravity. This planet (Neptune) is quickly detected and is considered another triumph

for Newton.

The orbit of the planet Mercury is found to have an anomaly which Newton's laws of gravitycannot explain. This has to wait forEinstein sixty years later.

1848

Armand Fizeau shows that lines in a spectrum change position when the light

source is moving to or from the observer. When the source is moving away the lines

are shifted towards the red end of the spectrum (a Red Shift); when the source is

moving closer the lines are shifted towards the blue end of the spectrum (a Blue

Shift). This is called the Doppler Effect.

1851Jean Foucault uses a large pendulum in a church to prove that the Earth is

rotating on its axis. This is now called a Foucault Pendulum and is the first direct

proof of the Earth's rotation postulated by Heracleides 2000 years previously.

Foucault also measures the speed of light in the laboratory to a high level of accuracy.

1854

Gustav Kirchhoffstudies the spectrum of glowing substances. He discovers that

each type of atom gives a different set of lines in the spectrum. This gives a method

of identifying the atoms present in glowing objects without needing a sample in the

laboratory.

1863

William Huggins applies the technique of spectroscopy to astronomy. He studies

the composition of the Sun, stars and planets. The same elements that exist on

Earth are found in space.

He finds that the Sun and stars are mainly made of Hydrogen. He measures the Doppler Effect ofthe star, Sirius and finds that it is moving away from us. Comets are shown to contain glowingcarbon compounds. Many Nebulae produce spectra that show that they are glowing gases ratherthan stars. He uses photography to obtain spectra of very faint objects.

Aristotle's 2100 year old idea that the heavens are made of a different element (the Aether) isfinally proved to be wrong.

1864

Pietro Secchi photographs the spectra of over 4000 stars. He finds differences

which would eventually lead to ideas of stellar evolution.

1868


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Joseph Lockyer discovers a new element in the Sun's spectrum. It is named after

the Greek word for Sun, Helium. 40 years later, Helium would be found on the Earth.

1882

In the USA, Albert Michelson and Edward Morley measure the speed of light to a

very high level of accuracy. They attempt to measure the absolute motion of theEarth around the Sun by finding a difference in the speed of light in different

directions. No difference is found.

Michelson and Morley consider that the experiment has failed because it could not be reconciledwith the physics of the day. It turns out to be the most glorious failed experiment in the history ofscience, eventually laying the groundwork forEinstein'sTheory of Relativity.

1893

Wilhelm Wien studies radiation of energy and light from hot objects. He shows

that the colour of a glowing body is related to its temperature in a definite

mathematical way. This is called Wien's Law and can be applied to the surfaces ofstars.

Red stars are coolest. Orange stars are hotter; then come yellow stars; hotter still are white stars.Blue stars are the hottest. Very cool stars give out their energy in the infra-red. The very hotteststars shine mainly in the ultra-violet.

The Sun's surface temperature is shown to be around 6,000C. Some stars are hotter than theSun.

Theoretically, Wien's energy pattern could not be explained by the physics of the day. Theexplanation would have to await the development ofQuantum Theory.

1895

Maximillian Wolfand Edward Barnard discover that dark areas in the Milky Way

are dark nebulae made up of gas and dust.

1900

The German physicist, Max Planck, develops the idea that energy exists in lumps

(called quanta) rather than continuous emissions. This idea explains Wien's work

on radiation.

1906

Jacobus Kapteyn repeats William Herschel's statistical analysis of the stars. He

discovers order in the motions of the stars. The stars are not moving at random in

the Galaxy. This is the phenomenon of star streaming. His measurements of the

size of the Galaxy increase its size but are still less than 60% of the correct figure.

The presence of dark nebulae hinder accurate measurements.

1912
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Henrietta Leavitt studies thousands of variable stars. She finds that a particular

type (called Cepheids) have regular periods and are easily distinguished by the way

their brightness changes (the Light Curve) and their spectra. She observes

examples of these stars in star clusters. Stars in these clusters are all at the same

distance from the earth. This allows her to discover a link between the period and

the luminosity. This is called the Period-Luminosity Law. The longer the star'speriod, the more luminous the star.

These stars provide a yardstick for measuring distant objects in the Universe. The period givesthe luminosity; the luminosity can be compared with the apparent brightness of the star as seenfrom the Earth; this gives the distance to the star. If the star is part of a group, cluster or nebula,the distance to that object is known.

1913

Walter Adams works out how to deduce a star's luminosity from its spectrum.

Once the luminosity is known, the distance can be calculated from the apparent

brightness.

This new tool allows Ejnar Hertzsprung and Henry Russell to measure the distance to nearbyCepheid variables thus providing the scale to Leavitt's cosmic yardstick.

Hertzsprung and Russell go on to find a relationship between the colour and luminosity of stars.Blue (hot) stars tend to be luminous, yellow (medium) stars tend to be less luminous, red (cool)stars tend to be faint. More than 90% of stars fit this classification and are called Main Sequencestars. Some red stars are too luminous for their colour. These are called Red Giants because theyare very large. Some white stars are too dim for their colour. These are small and very densestars called White Dwarfs.

A graph of these results is known as the Hertzsprung-Russell (or H-R) diagram. Special stars

(like Cepheid variables) occupy distinct zones in the H-R diagram. The diagram is veryimportant in the study of stellar structure and provides a foundation for ideas about stellarevolution.

Russell studies the spectrum of the Sun to determine its chemical composition. The Sun is 90%Hydrogen, 9% Helium and 1% everything else. Most stars have a similar composition.

Niels Bohr applies Planck's quantum ideas to atoms helping to explain why and how atomicspectra form.

1915

Physics and astronomy are revolutionised by Albert Einstein.

He brings Planck's ideas of quanta into prominence by using them to explain a previouslymysterious effect when light shines on metals (the Photoelectric Effect).

He explains a strange movement of small particles in a liquid (called Brownian Motion) byproving mathematically that it must be due to the random motions of atoms and molecules. Thisis the first direct proof of atomic theory and allows the size of these small particles to bedetermined.

His Special Theory ofRelativity explains why the Michelson and Morley experiment hadapparently failed. Absolute motion cannot be measured: all motion is relative. This leads to the
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idea that the velocity of light is the maximum speed that any material body can have. Noinformation can travel faster than light. When we look into distant space we are looking at thepast! A further development leads to the famous equation

E = mc2

which shows that matter is a concentrated form of energy. This would allow future scientists toexplain the source of the energy of the stars. Time and space turn out to be changeable anddependent on the position and motion of the observer. This defies common sense but would befound to be in accord with observation.

Einstein's General Theory of Relativity changes the way humans look at gravity. Newton hadenvisaged gravity as a force between all matter. Einstein sees matter as distorting the very fabricof space, causing it to curve. This curvature of space causes matter to move in non-linear paths.Under most conditions, the differences between the two theories of gravity are minimal.However, Einstein's theory explains the anomalies in the orbit of Mercury found by Leverriersixty years earlier.

General Relativity also predicts that light would be bent by a gravitational field. This would be

proved during a total eclipse of the sun a few years later. Another prediction is that a stronggravitational field would give a spectral red shift separate from that produced by the Dopplershift. This is proved when the spectrum of a very dense White Dwarf star is examined. The staris a companion of Sirius so the Doppler effect could be accounted for since the two stars movetogether.

The General Theory of Relativity gives an overall view of the entire Universe indicating that it isnot static. Einstein thinks that the Universe is static and disregards this part of his equations. Hewould soon be proved wrong.

1917

An idea to explain the formation of the Solar System is postulated byJames Jeans.

He suggests that a passing star had drawn material from the Sun. This material hadcondensed to form the planets (including the Earth).

If this idea is correct, the Sun's planetary system could be unique since stellar encounters arevery rare. Stars are too far apart to interact with others very frequently.

1918

Harlow Shapley applies Leavitt's Cepheid yardstick to Globular Clusters. These

are large spherical groups of stars. Most types of object are distributed randomly in

the sky. Globular Clusters, however, are bunched up together. 70% of them occupy

a 2% region of the sky. Shapley finds that these clusters are arranged in a sphere

centred on a point a long way from the Sun.

He assumes that the centre of these clusters is the centre of the Galaxy. If so, then that centre is50,000 Light Years away from the Solar System. Not only is the Earth not the centre of the SolarSystem; the Solar System is nowhere near the centre of the Galaxy. Shapley points out that theMilky Way looks symmetrical from the Earth because of the existence of dark nebulae(interstellar clouds) blocking out distant stars.


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Shapley's measurements to the centre of the Galaxy turn out to be an over-estimate, however.This is the first time that the size of the Universe is over-estimated. The currently accepted figureis 30,000 Light Years. In 1930 Robert Trumpler would show that interstellar dust dims theGlobular Clusters making them look further than they actually were.

1924

Arthur Eddington uses gas theory to study the interiors of stars. He shows thatstars are stable because there is a balance between two opposing tendencies. The

energy and gas pressure coming from the hot centre push the star outwards,

tending to expand it. Gravity pulls the star inwards, tending to contract it.

He estimates the interior temperature of the Sun to be in the millions of degrees. This is so hotthat Jeans' idea of planetary formation would not work.

Eddington discoveres the Mass-Luminosity Law for stars. More massive stars are moreluminous. His studies allow him to explain how Cepheid stars vary in brightness by pulsating.

1926

Edwin Schrdinger discovers a wave equation that puts Quantum Mechanics on afirm mathematical footing. This would lead to advances in the understanding of

atomic and molecular spectra that would increase knowledge in astronomical

objects.

1927

Jan Oort studies the star streaming discovered by Kapteyn. He shows that these

stellar movements are due to the stars in the Galaxy revolving about the centre.

The stars closer to the Galactic centre travel faster than the stars further away. Oort

uses the stellar motions to find the location of the Galactic centre: its position

agrees with Shapley's centre of Globular Clusters.

The centre of the Galaxy is confirmed to be 30,000 Light Years from the Sun's position. The Sunrequires 200 million years to orbit the Galactic centre. The Galaxy has enough matter to make100 thousand million stars like the Sun.

1929

Edwin Hubble studies the spiral nebulous object in the constellation of Andromeda

(first noted by Al-Sufi and Marius). Using the world's largest telescope, he

manages to see stars in the object. Some of the stars are Cepheids. This allows him

to determine their distance and hence the distance of the spiral. The distance of

800,000 Light Years he finds is far outside the domain of our Galaxy even though it

is an under-estimate.

The Andromeda spiral is in fact a galaxy outside our own and is now called the AndromedaGalaxy.

Our Galaxy, with its thousands of millions of stars, is not unique.
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More galaxies are quickly found; there are billions now known. The Universe is far, far largerthan previously thought. Hubble finds that there are three types of galaxies: spiral, elliptical andirregular. From its overall properties our Galaxy appeared to be a spiral.

Vesto Slipher had previously measured the velocities of many nebulae by taking photographs oftheir spectra.

Hubble analyses the velocities of the ones now recognised as galaxies. He finds that theoverwhelming majority of galaxies are moving away from us. Their spectra show a Red Shift.He shows that there is a simple mathematical relationship between the distance of the galaxy andits velocity away from us. This relationship is now called Hubble's Law.

Hubble's Law provides another yardstick with which to measure distance. The Red Shift of agalaxy can be measured from its spectrum. This gives its velocity of recession from us. Hubble'sLaw provides the distance.

The simplest way to explain these observations is to assume that the Universe is expanding.Einstein's General Theory of Relativity had already predicted that the Universe would not stableif it was static. Hubble's work shows that the Universe is, indeed, not static.

Modern Cosmology (the study of the overall structure of the Universe) can be said to have begunwith Hubble's work.

1930

Abb Lematre and George Gamow explain the observation of the expanding

Universe by postulating that it began in a huge explosion. This is colourfully known

as the Big Bang Theory.

Lematre suggests that all the matter in the Universe was once contained in a very dense "cosmicegg". This object exploded and the matter was spread out through space. We see the effects ofthis explosion when we observe the galaxies moving away from each other.

Gamow predicts that the echo of the explosion should be detectable as radiation with atemperature of about 5 degrees above Absolute Zero. This radiation should permeate throughoutthe Universe. It would not be detected for over 30 years.

Using Hubble's Law and working backwards, they estimate that the age of the Universe is 2thousand million years. This figure is smaller than the age of the Earth as calculated bygeologists.

Alexander Friedman uses Einstein's equations of General Relativity to work out that there aretwo possible ends to the Big Bang Universe.

If the amount of matter in the Universe is above a certain critical level, then the expansion of theUniverse would eventually slow down and stop. The Universe would then contract with all thegalaxies and stars moving towards each other until they were back in a small area. This is known

as the Big Crunch.

If the amount of matter in the Universe is below the critical level, then the expansion wouldcontinue forever. Eventually the Universe would expand so much that galaxies would not bevisible to each other. Cold, dark, and isolated embers would be all that was left of the galaxies.

To distinguish between these two scenarios requires a knowledge of how quickly the Universe isexpanding compared to how much matter it contains. This problem would not be solved for 70years.


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1932

Karl Jansky discovers radio waves from space. He finds that these signals come

from the centre of the Galaxy, its position agreeing with Shapley's. Radio waves

open a new window to the Universe; a window that is not affected by gas and dust.

This marks the birth of Radio Astronomy.

1935

Otto Struve proves that invisible interstellar dust and gas exists by finding a

spectral line of Calcium.

He develops a new theory of planetary formation that is a normal part of stellar evolution ratherthan the rare stellar encounter ofJeans' model.

1938

Hans Bethe and Carl Weizscher work out the details of how the Sun produces

its energy. It is by nuclear fusion, converting Hydrogen to Helium. Every second

over 3 million tonnes of the Sun's matter is converted into energy.

1942

Harold Jones measures the Astronomical Unit (the distance between the Earth and

the Sun) to an accuracy of over 99.99%.

Walter Baade studies the stars in the Andromeda Galaxy. He discovers that there are twopopulations, each with different ages and chemical compositions. The Cepheids of eachpopulation have a slightly different Period-Luminosity Law. This discovery corrects thedistances to the galaxies as measured by Hubble.

The distance to the Andromeda Galaxy is tripled to over 2 million Light Years.These changes increase the age of the Universe to 6 thousand million years. This is longer thanthe geologists' estimate of the age of the Earth.

1948

Thomas Gold and Fred Hoyle suggest an alternative cosmology to explain the

expanding Universe. The Steady State Theory describes a Universe essentially

unchanging in space and time. As the Universe expands, new matter is created to

fill in the gaps left. There was no Big Bang.

Nobody can suggest how this new matter arises. For the idea to work a few hundred atoms would

need to be created per cubic kilometre every year.1950

Martin Ryle finds radio emissions from the Andromeda Galaxy. He finds that many

(but not all) galaxies give out radio waves. These radio galaxies tend to be more

abundant amongst the further galaxies rather than those nearby. Looking at great

distances implies looking at the past. This is the first hint that the Universe has

changed with time. If so, the Steady State Theory could not be correct.


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William Morgan studies the distribution of luminous hot blue stars in our galacticneighbourhood. He finds that they are arranged in parallel lines which mark out our Galaxy'sspiral arms. The arm that includes the Sun is called the Local Arm. Away from the centre is thePerseus Arm. Closer to the centre is the Sagittarius Arm.

These observations are later confirmed by studying the distribution and motions of glowing

nebulae. Using optical techniques, observations can only be made to a distance of about 10,000Light Years. This is only one third of the distance to the centre of the Galaxy. The Galaxycontains dust and gas which block out light from the very distant stars.

Hendrik van de Hulst uses radio telescopes to map the positions of clouds of Hydrogen. Thisallows the Galaxy to be mapped over a larger area. He finds another spiral arm outside thePerseus. Radio waves travel through gas and dust better than light does.

1958

Allan Sandage calculates the age of the Universe by studying distances to nearby

galaxies. His age is 13 thousand million years. This is older than the Earth and the

Sun. It is not as old as the oldest Globular Clusters.

1961

Yuri Gagarin becomes the first human being to orbit the Earth.

1963

Maarten Schmidt studies a group of radio objects that appear to be stars. These

"stars" are shown to have very large Red Shifts. This indicates that they are further

than most galaxies. They are labelled as "quasi-stellar objects" (or, more commonly,

Quasars).

Quasars are mysterious objects: highly luminous and very small. The nearest Quasar (called

3C273) is at a distance of 2 thousand million Light Years. This is over 800 times further than theAndromeda Galaxy. It shines with the luminosity of 100 normal galaxies! Its brightness varies inperiods of about a month so it must be small compared to a galaxy. 3C273 has been estimated tohave a diameter of over 750,000 million kilometres. This is a million times smaller than ourGalaxy or 4800 times the distance between the Sun and the Earth.

No Quasars are found in the regions of space near our Galaxy. They are now considered to bevery young and active galaxies.

Because light takes time to travel across space, Quasars show that the early Universe wasdifferent in the past. The Universe is therefore changing in time; it is an evolving Universe. Thiscontradicts the Steady State Theory.

Arno Penzias and Robert Wilson discover a Universal Background Radiation coming from alldirections equally. This is an effect of the Big Bang predicted by Gamow. The heat producedduring the explosion should have cooled down to a temperature of a few degrees above AbsoluteZero.

The new radiation indicates a temperature around 3 degrees above Absolute Zero. Thisphenomenon cannot be explained by the Steady State Theory.


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The Big Bang Theory is now accepted by most scientists. Speculation begins about how theUniverse will end. Will it expand forever or will it eventually contract back to nothingness? Thisdepends on the amount of matter in the Universe.

1965

Roger Penrose shows that very massive stars could collapse in on themselves. In

theory, they could form an object with a gravity so high that even light could not

escape from them. These objects are called Black Holes and are dismissed by most

scientists. Black Holes have the peculiar property of absorbing matter but never

allowing any to escape. As the matter approaches the Black Holes, it would radiate

huge amounts of energy as it becomes compressed by the gravitational forces.

At the centre of a Black Hole, there would be an object with an infinite density and zero size.This is called a Singularity. As bizarre as they sound, Singularities are not precluded by theGeneral Theory of Relativity.

Stephen Hawking shows that if the Theory of Relativity is correct, then the Universe would

have begun as a Singularity rather than as Lematre's "cosmic egg". At the time of the Big Bang,the Singularity would have exploded and the Universe would have come into being. Space, time,and energy would have been created and would expanded together. The original state wouldhave had an extremely high temperature. As the temperature dropped, matter would form out ofthe energy and eventually, stars and galaxies would have formed out of the matter.

1969

Neil Armstrong and Edwin Aldrin become the first human beings to step on

another world, the Moon.

1973

Paul Richards accurately measures the spectrum of the Universal Background

Radiation. He finds that it agrees with theoretical predictions for the Big Bang.

The abundances of various isotopes of certain elements within galaxies also agree withtheoretical predictions for the Big Bang.

1975

Gustav Tammann refines the age of the Universe from Sandage's work. His

figure of 18 thousand million years is older than the oldest known objects in the

Universe. It would later be shown to be an over-estimate.

1977

R Brent Tully,J Richard Fisher and others develop several new distance

yardsticks with which to measure the size (and age) of the Universe. These are

described briefly below.

The luminosity of a spiral galaxy is related to the properties of a particular radio emission in itsspectrum.

The apparent light smoothness of elliptical galaxies is related to their distance.


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Distant galaxies that give off X-rays affect the Universal Background Radiation lying betweenthem and us in a way dependent on the distance.

Distant Quasars passing close to a large galactic mass may have their light bent. This producesdouble or multiple images of the Quasar. There is a relation between the angle of the bending,the time between light variation of the Quasar to be repeated in the duplicate images, and the

distance to the Quasar.1979

Alan Guth studies the early history of the Universe in terms of particle physics. He

suggests a reason why the Universal Background Radiation appears to be so

uniform. This leads to the development of Inflationary Big Bang theories.

The idea is that the early expansion of the Universe was very rapid for a short while beforesettling down to the rate seen today. These theories explain several points in the Big BangTheory. However, there is no observational evidence for them.

1980

Margaret Geller and others discover structure in the Universe. The galaxies arearranged in groups, clusters, clouds and superclusters.

Our galaxy is a member of a group (the Local Group) consisting of about 20 galaxies in a regionthat is 5 million Light Years in diameter. Our galaxy, The Andomeda Galaxy and a third (calledM33) are all large spirals. The Andromeda galaxy is the dominant member of the group with 400thousand million stars. Our galaxy and M33 contain about 100 thousand million stars.

The spirals have a number of satellite galaxies. The Andromeda Galaxy has two ellipticalcompanions. Our Galaxy has five companions. Two are irregular galaxies called the MagellanicClouds. These are visible in the Southern Hemisphere and resemble detached portions of theMilky Way. Three are small almost-spherical ellipticals hidden behind the Galactic centre. The

rest of the galaxies of the group are small.The Local Group is on the edge of a cloud of galaxies called the Coma-Sculptor Cloud. This isabout 25 million Light Years across. This cloud is part of the Virgo Supercluster. Thissupercluster contains over 1000 galaxies that are mainly elliptical. The centre of the VirgoSupercluster is 60 million Light Years away from our Galaxy. Our Galaxy appears to be movingtowards the centre of the Virgo Supercluster at a speed of 600 kilometers per second.

1983

Andrei Linde suggests that an Inflationary Universe would be perfectly balanced

between its rate of expansion and the amount of matter it contains. Such a Universe

would carry on expanding forever.

1992

From satellite observations, George Smoot finds temperature variations (of the

order of 10-5 degrees) in the Universal Background Radiation. These "wrinkles" could

explain why the Universe is clumpy with groups of galaxies rather than being

perfectly smooth.

1995


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The Hubble Space Telescope surveys the distant parts of the Universe. By doing so,

it is looking into the past.

It is found that spiral and elliptical galaxies are generally stable and unchanging. Irregulargalaxies are active and changing. Even when the Universe was only 30% of its current age,galaxies had already formed. It appears that star formation was more active when the Universewas only 50% of its current age.

1996

Bruno Leibundgut observes a time delay in the way distant supernovas decay.

This is another verification that the Universe is expanding.

Carlos Frenksimulates the early history of the Universe on a supercomputer to try andreproduce the wrinkled structure of the Universe discovered by Smoot. The results only work ifthe expansion of the Universe increases with time.

1998

J Richard Gott finds that Clusters and Superclusters of galaxies are linked to formfilaments. They form "walls" or "sheets" up to 1,000 million Light Years long and

enclosing enormous voids. The Universe on the large scale has the appearance of a

sponge.

The Universe resembles fractals produced by mathematical Chaos Theory. This has led tospeculation that this structure may have been caused by random quantum fluctuations during thevery early phase of the Universe.

Saul Perlmutter and his team complete a study of Supernovae (exploding stars) in othergalaxies. The luminosity of these stars can be calculated by studying the way their brightnessfades. The study looks at stars out to a distance of 7 thousand million Light Years. The results

indicate that the expansion of the Universe is increasing.Brian Schmidt confirms that the expansion of the Universe was 15% greater when the Universewas half its current age. There is speculation of a repulsive force present on the large scale. Thisleads to ideas about the existance of dark energy.

2000

Observations of the variation of temperature in the Universe indicate that it will

expand for ever (i.e it is flat and open).

2001

A type ofsub-atomic particle, the Nutrino, is shown to oscilate between three types

and to have mass. This mass, although small, could have an effect on the largescale structure and evolution of the Universe.

New ideas, called M Theory, may explain the origin of the Big Bang as the collision of 11dimensional spaces.

2002, 2006 KryssTal

This essay is dedicated to Patrick Moore and Isaac Asimov.
http://www.krysstal.com/subatomic.htmlhttp://www.krysstal.com/subatomic.html


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Unanswered Questions What is the exact mechanism of the Big Bang?

How did the smooth Universe of the Background Radiation evolve into thewrinkled sponge Universe?

What part do quasars play in the evolution of galaxies?

Is there life elsewhere in the Universe?

Are there really other Universes?

Is there a purpose to the Universe? Is this a religious question?

We Are Stardust

The Evolution of Stars

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What are stars? How do they affect ourexistence?

The answers are startling.

Read on for a surprising story that touchesall the sciences.

What is a Star?

We see many points of light on a clear, dark, moonless night. Some are bright, others barelyvisible. If we look carefully we can make out colours. Thebright starVega (overhead in theNorthern Hemisphere summer) is white; Capella (overhead in the winter) is yellow; Regulus (of
http://www.krysstal.com/brightest.htmlhttp://www.krysstal.com/brightest.html


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Leo) is blue, Arcturus (a springtime star) is orange; Antares (from the Greek "rival of Mars" -brightest star is Scorpius) is, of course, red.

At first glance it may seem a difficult thing to find out anything about the stars. But by using thegas laws and the laws of thermodynamics it is possible to find out about their physicalconditions. Using atomic physics and the laws of radiation (like light) we can find out about their

chemistry.Although stars are very distant, we are lucky to have one very close to us: the Sun. The Sun is atypical star.

Light (travelling at 300,000 km per sec) goes seven times around the Earth in one second. Fromthe Sun it takes just 8.3 minutes to reach us (from a distance of 149 million km). The nearest starafter the Sun is so far away that light takes over 4 years to travel that distance. If this hugedistance is expressed in kilometres, it would mean nothing to most people. We say that thenearest star is over fourlight years away. A light year is the distance light travels in one year.

I will leave it to the reader to calculate that distance in kilometres or miles!

The Chemistry of the Universe

The Universe is composed of 85% Hydrogen (H), 14% Helium (He) and 1% of everything else.That "everything else" includes the Carbon (C) of life, Oxygen (O), Silicon (Si) that makes uprocks, Iron (Fe), Uranium (U): in fact all the other 90 or so chemical elements in existence.

Clearly, the Earth is not typical of this composition. The Earth is mainly rock and metal.However, if we look at the stars, their chemistry is different. The Sun, as previously mentioned,is a typical star. It has a mass 300,000 times that of the Earth. Its composition is 85% H, 14% He,and 1% (everything else: C, Si, N, P, O, Na, Fe, Ni, K, etc).

The sun is a huge (109 Earth diameters) ball of glowing gas. It is surrounded by bits left overafter its formation. The Earth and the other planets that orbit the Sun, and their moons compose a

mass of about 400 times that of the Earth, or more graphically, about 0.1% the mass of the Sun.In other words, we can describe the Sun to be composed of a huge sphere of glowing gas madeup mostly of Hydrogen and Helium with a scattering of more solid material going around it.

This is what a typi

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