1.2 The Sky

1.2THE SKYConstellationsA constellation is simply a grouping, or pattern, of stars.

In ancient times, constellations only referred to the brightest stars appearing to form groups.

They were believed to represent great heroes and mythological figures.

Today, they are well defined regions of the sky, regardless of the presence of bright stars.

ConstellationsThere are 88 official constellations we study today.

In addition to those, the sky contains a number of less formally defined groupings called asterisms.

Example 1: The Big Dipper is a well-known asterism of the constellation, Ursa Major (The Great Bear).

ConstellationsExample 2: The Great Square of Pegasus (includes stars from both the Pegasus and Andromeda constellations).

ConstellationsThe stars of constellations only appear to be close together. This is often known as the projection effect.

They may actually be located at very different distances from us.

Constellations and Their Stars

Naming StarsIn addition to naming groups of stars, ancient astronomers gave individual names to the brightest individual stars.

They are named by a Greek letter (, , ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation:

Example: brightest star in constellation Canis Major is alpha Canis Majoris ( Canis Majoris).

Brightest star in a constellation is usually alpha (), the second brightest beta (), and so on in descending order.Naming StarsIn the constellation, Orion, however, that is not the case. Orionis is actually brighter than Orionis.

Orionis = Betelgeuse Orionis = RigelBetelgeuseRigelAlnitakAlnilamMintaka

Brightness of StarsAstronomers measure the brightness of stars using what is known as the magnitude scale.

Ancient astronomers divided this scale into 6 classes.

The brightest: 1st magnitude stars, those fainter were known as 2nd magnitude stars, etc. in order of decreasing brightness down to 6th magnitude (faintest visible to the unaided human eye).

Sometimes, stars are so bright, they extend into the negative numbers.

Example: Sirius (brightest star in the sky) = -1.47

We also extend the faint end of the scale for stars extremely faint to us, requiring us to use a telescope.

These numbers are known as apparent visual magnitude (mv), describing how bright stars look to human eyes observing from Earth.

The actual brightness of a star from a standard distance (10 parsecs we will discuss later) is known as the absolute visual magnitude (Mv).10Brightness of Stars

Magnitude and IntensityFlux is a measure of the light energy in Joules (J) from a falling on one square meter per second.

Defines the intensity of starlight.

If 2 stars have intensities IA and IB, the ratio of their intensities is IA/IB.

Astronomers have defined the magnitude scale so two stars differing by 5 magnitudes have an intensity ratio of 100.

Therefore, stars only differing by 1 magnitude must have an intensity ratio of 5100, which equals 2.512.

The light of one star is 2.512 times more intense.Intensity Ratio Formula IA IB

= 2.512(m m )A BMagnitude and IntensityExamples:

Two stars differ by 6.32 magnitudes. What is their intensity ratio?

Two stars differ by 5.52 magnitudes. What is their intensity ratio?

337161Magnitude and IntensityWhat if we know the intensity ratio and want to find the magnitude difference?Difference In Magnitude Formula IA IB

(

)

mB mA = 2.5 logExample:

Light from the Sun is 24.2 times more intense than light from the North Star, Polaris. What is the difference in magnitude between the two stars.

3.46The Celestial Sphere

The Celestial Sphere

The Celestial SphereThe distance between two stars on the celestial sphere can only be given as the difference between the directions in which we see the stars.

Therefore, they are measured as angles, either in:

degrees (o):Full circle = 360oarc minutes ():1o = 60arc seconds ():1 = 60The Celestial SphereFrom geographic latitude l (Northern hemisphere), the north celestial pole is l degrees above the Northern horizon.

From geographic location l (Southern hemisphere), the south celestial pole is l degrees above the Southern horizon.

Celestial equator culminates 90 - l above the horizon.

The angular distance from the horizon to the celestial pole always equals your latitude l the basis for celestial navigation.

l90o lThe Celestial Sphere

Apparent MotionLooking north from mid-northern latitudes, some stars appear to circle around the north celestial pole.

Apparent MotionConstellations that never rise or set are called circumpolar constellations.

Below are the 5 we see from our latitude in the Northern hemisphere:

Ursa Major

Ursa Minor

Cepheus

Cassiopeia

Draco

The Sun and Its MotionsEarth has 3 distinct motions:

Rotation the turning, or spinning, of a body on its axis.

Revolution the motion of a body, such as a planet or moon, along its orbit around some point in space.

Precession the wobbling of a body around its axis of rotation.

RotationThe main result of Earths rotation (W to E) is the change from day to night and back again; rotates @ 1000 mph.

Axis of rotation tilted about 23.5

Synodic vs. Sidereal DayWe can measure Earths day in 2 ways:

Synodic (Solar) one complete rotation with respect to the Sun: 24 hours

Sidereal one complete rotation with respect to distant stars: 23 hours, 56 minutes

RevolutionEarths revolves around the Sun in a very slight elliptical orbit. Revolving at an average speed of 67,000 mph.

Average distance of 93 million miles.

Due to its slightly elliptical orbit, the distance will vary:

Perihelion Earth is closest to the Sun (91 million miles in January).

Aphelion: Earth is farthest from the Sun (95 million miles in June).

Annual Motion of the SunDuring the day, the Sun appears to move across the sky once it rises in the East until it eventually sets in the West.

Over the course of a year, the Sun appears to move East against the background of the stars.

The apparent path of the Sun against the background of the stars is called the ecliptic.

If the sky were a great screen, the ecliptic would be the shadow cast by Earths orbit.

Stars are present during the day (not visible due to Sun).

Certain constellations appear in the ecliptic path at different times of the year (sun is blocking them) and are known as the zodiac constellations.

Recently, astronomers have pointed out the original zodiac constellations may be outdated since they were created 3000 years ago and the Earth has moved slightly due to precession.

Believed that the Sun moves into a new constellation approx. every 2160 yrs.Zodiac Constellations

Zodiac Dates

Seasons on EarthEarths seasons are caused by 2 factors only:

Earths revolution around the Sun during the time span of 365.25 days

Tilted axis of approximately 23.5

The amount of solar energy Earths Northern and Southern hemispheres receive at different times of the year continuously changes during a revolution.

Seasons on EarthWinter SolsticeDecember 21SDR 23.5 S (Tropic of Capricorn)10 hrs. day, 14 hrs. night

Spring (Vernal) EquinoxMarch 20SDR Equator12 hrs. day, 12 hrs. night

Summer SolsticeJune 21SDR 23.5 N (Tropic of Cancer)14 hrs. day, 10 hrs. night

Fall (Autumnal) EquinoxSeptember 22SDR Equator12 hrs. day, 12 hrs. night

Seasons on EarthThere is only a varying angle of incidence of the Suns rays from season to season.

We receive more energy from the Sun when its shining on Earths surface at a steeper angle of incidence.