The Starry Sky

Orion: Are these stars physically related? Green = Yes, Purple = No

•There are roughly 6000 or so stars visible to the naked eye in the sky or, 3000 since you can only see about one half of the sky.

•The stars are so distant that they appear only as points of light.

•We can detect color and brightness with the naked eye – but not much more.

The Ancient Observatory of Stonehenge

Unique Construction

•Pretend the stars are painted on a giant ‘bubble’ around the Earth known as the celestial sphere

The stars of a constellation only appear to be close to one another, they may be located at very different distances from us.

Stars are at different distances

Constellations

•Stars are grouped into “easily” recognized shapes called constellations.

•The sky is completely mapped by the constellations.

•There are 88 official constellations

Early Sky Map

Lots of art work

Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

Some sky maps leave out the boundaries ; size of star depends upon magnitude (how bright).

Constellations: Three interpretations of the Winter Constellation Orion:

Early The Sky Modern Maps

Star Names an example

alpha Orionis Betelgeuse beta Orionis Rigel delta Orionis Mintaka gamma Orionis Bellatrix epsilon Orionis Alnilan kappa Orionis Saiph

Bayer System Name

Stars are named by a Greek letter () according to their relative brightness within a given

constellation + the possessive form of the name of the constellation. Catalogues HD, ASO

Bayers’ System

Sagittarius the archer ( better known as the teapot an asterism)

Stars can be used to locate other stars and constellations. Star hopping

To find your directions in the sky, find the north star, put your back to the north star and you will be looking southward.

East will be on your left, with west on your right.

Facing due south you are you are looking along your meridian.

Diurnal (Daily) Motions• The Sun , Moon , Stars and Planets-

rise in east, set in the west. • These motions are all due to Earth’s

Rotation about Its Own Axis (1 Day = 24 hours (almost).

Celestial Motions

The Moon, and Planets have their own motion, which is from west to east moving counter clockwise around the Sun..

Annual (Yearly) Motions• Position of Stars, Constellations

– They rise 4 minutes earlier each night.– Different constellations are visible at

different times of the year .• Position of the Sun

– The position of Sun on horizon shifts back and forth and the peak height in the sky changes.

• Seasons– Length of a day, night, and the climate changes

over course of the year.•Earth orbits Around the Sun, one year = 365 Days (almost)Earth orbits Around the Sun, one year = 365 Days (almost)

Other ChangesPhases of the Moon (monthly)

Eclipses Sun or Moon becomes “blocked” from view during a year.

•Moon rises about 50 minutes later each night. Takes 29.5 days from New Moon to the next New Moon.Change in “Celestial Poles”The “North Star ( Polaris)” has not always been the North Star.

N

W

E

S

Horizon

ZenithLocal Sky

The Horizontal System

Zenith is a point on the celestial sphere directly above your head.

Celestial Meridian•A line from north to south, through your Zenith is called the Celestial Meridian.

Astronomical noon is when Sun is on the Meridian, not necessarily straight up.

.

•When objects are on the Meridian at their highest point in the sky, we say they are in transit at this point.

South Pole

Equator

North Pole

Meridian

Celestial Sphere

Celestial Sphere

NorthNorth SouthSouth

ZenithZenith

NadirNadir

MeridianMeridian

EastEast

WestWest

AltitudeAltitude

Altitude is measured from your horizon up to the celestial object in degrees.

Celestial Sphere

Celestial Sphere

NorthNorth SouthSouth

ZenithZenith

NadirNadir

MeridianMeridian

EastEast

WestWest AzimuthAzimuth

AltitudeAltitude

Azimuth is measured along the horizon from a point below the North Pole, which is zero degrees, to a point directly below the

object on the horizon.

North is 0 deg, East is 90 deg, South is 180 deg and west is 270

Horizon

N

W

E

S

NCPCEq

Zenith

Your Latitude

•For a geographic latitude of L degrees on the northern hemisphere, you will see the celestial north pole L degrees above the horizon.

Your Latitude = altitude of celestial pole

The

Equatorial

System

Celestial Co-ordinates• The altitude and azimuth of a star

change all the time, and depend on where you are.

We need co-ordinates “fixed” to the celestial sphere.

•Declination and Right Ascension

A star’s Dec & RA will change very little in our lifetime.

South Pole

North Pole

South Pole

Celestial Sphere

It seems as though there is a sphere of stars surrounding the earth.The Celestial Sphere is an extension of the Earth.

Earth

Earth

Celestial Equator

Places on the Earth are located by Latitude & Longitude

Declination (Dec)Declination (Dec)

North Celestial Pole (NCP)North Celestial Pole (NCP)

Celestial EquatorCelestial Equator

Declination (Dec) is analogous to latitude and is measured in degrees up and down from the celestial equator. (Max is +90 deg or –90 deg)

Right Ascension (RA)Right Ascension (RA)

North Celestial Pole (NCP)North Celestial Pole (NCP)

Celestial EquatorCelestial Equator

(zero of RA)(zero of RA)

(zero of Dec)(zero of Dec)

Right Ascension is analogous to longitude, but is measure in time units of hours, min and sec, from the zero point located at the vernal equinox. (0 hours to 24 hours)

15 deg = 1 hr

Right Ascension (RA)Right Ascension (RA)

Declination (Dec)Declination (Dec)

North Celestial Pole (NCP)North Celestial Pole (NCP)

Celestial EquatorCelestial Equator

(zero of RA)(zero of RA)

(zero of Dec)(zero of Dec)

Right Ascension & Declination With these two directions, any object can be locatedon the celestial sphere.

Ecliptic vs. Celestial Equator

The Earth’s axis of rotation is inclined to its orbital plane by 23.50

Celestial Equator

With no tilt, the planets would all seem to be located somewhere on the Celestial Equator, straight out from the Earth’s equator.

The planets are revolving around the Sun, but they would appear to be located along the equator.

The Sun does not move, the Earth’s motion makes the Sun appear to move.

Because the Earth is tilted 23 1/2 degrees, the Sun & Planets move in a different location, as seen from Earth.

The Ecliptic is the path of the Sun, Moon & Planets revolving around the Sun as seen from Earth.

0 RA where the Sun on the Ecliptic crosses the CE on its way upward

The position of Sun on horizon shifts back and forth, so that the peak height in sky changes

Finding North• The big dipper does not set from most of the USA.• The two end stars point to Polaris which happens to be

near the celestial pole• The big dipper acts like a big clock: It goes once round • in 1 a day

Star Trails at our location Looking north you’ll see the stars circle around Polaris.

Looking at stars rising roughly east you’ll see them rise upward, go to highest point then start downward setting in the west.

Looking toward the south you’ll see stars rise in the south east, go to the highest point and set in the southwest, a shorter arc across the sky.

At the North PoleZ

NCP

North Pole•If you were at the north pole of the Earth, Polaris would be overhead.

All stars would go round the north celestial pole never setting: all are circumpolar

What you see depends on your latitude

At the Equator

NS

W

E

Z

NCP

Equator

Stars that never set , and go round and round the pole are called circumpolar stars.90 deg – your latitudeis the Declination of the limit of circumpolar stars.

For Atlanta, 90 deg- 33 deg 45 min = 56 deg 15 min. So stars southward of this point are not circumpolar.

The Seasons

Inclined Pole Causes SeasonsEarth’s equator

is inclined against the

ecliptic by 23.5º.

Remember the Sun does not move, the Earth moves.

June(Summer Solstice)

March & September(Equinoxes)

December(Winter Solstice)

The change in position of the rising sun.

23.5 -23.50

1 KW/m2 1 KW/m2

1 m2 2 m2

The Tropic of Cancer & Capricorn

Measuring angles

Angular distanceBasic unit is the degree 1°1° split into 60’ minutes of arc1’ split into 60’’ seconds of arc1° is 3600” seconds of arc

• The easiest way to estimate relative angular distances on the sky without any equipment is to use your outstretched hand like an angular ‘ruler’

Outstretched Hand as an Angular Ruler

The Earth and its Motions

Earth’s rotation is causing the day/night cycle.

The Suns motion, which is really the movement of the Earth around the Sun, causes the Sun to appear to move through the same constellations at certain times of the year.

CONSTELLATION TRADITIONAL ACTUAL (2000) DAYS

Capricornus Dec. 22 - Jan. 21 Jan. 21 - Feb. 16 26

Aquarius Jan. 22 - Feb 21 Feb. 16 - Mar. 11 24

Pisces Feb. 22 - Mar. 21 Mar. 11 - Apr. 18 38

Aries Mar. 22 - Apr. 21 Apr. 18 - May 13 25

Taurus Apr. 22 - May 21 May 13 - Jun. 22 40

Gemini May 22 - Jun. 21 Jun. 22 - Jul. 21 29

Cancer Jun. 22 - Jul. 21 Jul. 21 - Aug. 10 20

Leo Jul. 22 - Aug. 21 Aug. 10 - Sep. 16 37

Virgo Aug. 22 - Sep. 21 Sep. 16 - Oct. 31 45

Libra Sep. 22 - Oct. 21 Oct. 31 - Nov. 23 23

Scorpius Oct. 22 - Nov. 21 Nov. 23 - Nov. 29 6

Ophiuchus - Nov. 29 - Dec. 18 19

Sagittarius Nov. 22 - Dec. 21 Dec. 18 - Jan. 21 34

There are actually 13 constellations along the ecliptic

Precession

The Sun’s gravity is doing the same to the Earth.

The resulting “wobbling” of the Earth’s axis of rotation around the vertical takes about 26,000 years, and is called precession.

Gravity is pulling on a slanted top Wobbling around the vertical.

Except for presently being the pole star, there is nothing special about Polaris.

Because of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years.

12,000 years from now, it will be close to Vega in the constellation Lyra.

It will be closest to Polaris ~ A.D. 2100.

Messier CatalogueMessier Catalogue• Charles Messier compiled a list of

approximately 100 diffuse objects that were difficult to distinguish from comets.

Star Clusters Nebula Galaxies

Deep Sky Object Catalogs Messier Catalog

– e.g. M1, M2, M3, etc.

New General Catalog– e.g. NGC 7293

Index Catalog• - e.g. IC302

•Solar day (noon-to-noon = 24 hours) is rotation period with respect to the Sun 4 minutes longer than the true rotation period of the Earth with respect to the stars called the sidereal day.•Roman Julian Calendar year = 365 d + leap yr•Problem with Julian Calendar average yr is 365.25 d, 11 minutes too long !

• By the 1500’s, the time of Pope Gregory, the calendar was ahead of astronomical time keeping by more than two weeks

Gregorian-Julian Reform: Calendar Century years not divisible by 400 are ordinary years, not leap years (Example: 1700 AD was not a leap year, but 2000 AD was)

SFA Star Chart 1

Thanks to the following for allowing

me to use information from their

web site :

Nick Stobel

Bill Keel

Richard Pogge

John Pratt

NASA, JPL, OSHO