1. The Night Sky
Astronomy

2. What are you seeing at Night?
When you look at the night sky, your looking out through a layer of air slightly greater than 100 km thick.
Our planet is one of many objects spread throughout a vast universe.
Because the earth rotates on its axis, the sky appears to revolve around us in a daily cycle.
The sun and the stars appear to “rise” in the east and “set” in the west.

3. The Stars
Constellations – way of grouping stars together in a certain pattern in the night sky.
Ancient constellations were named for heroes, gods or mythical beasts.
Different cultures grouped and named constellations differently.
There are 88 official constellations.
Of the ancient constellations, 48 are still used today.
Examples: Andromeda, Aries, Cancer, Leo

4. The Stars
Asterisms – are less formally defined groupings of stars that are part of bigger constellations.
Examples: The “Big Dipper” is an asterism that is part of the larger constellation “Ursa Major”
Important Note: While stars in constellations and asterisms appear close together in the night sky, most stars that make up these groupings are not physically close to each other at all. The stars are frequently very far apart and even moving in different directions!
The only thing stars in a group have in common is that they lie approximately in the same direction as viewed from earth.

5. Naming Stars
In addition to naming groups of stars, ancient astronomers named the brightest individual stars.
These names had mythical origins. Ex: Betelgeuse, Sirius
Today stars are named to designate their brightness in a constellation. Astronomers use the Greek Alphabet.
The Brightest star gets the letter alpha (a) followed by the possessive form of the constellation. The next brightest star would be designated as a beta (b) star.
a Canis Majoris – (Sirius) Brightest star in Canis Major
b Canis Majoris – Second Brightest star in Canis Major
g Canis Majoris – Third Brightest star in Canis Major

6. Names of some Favorite Stars
While naming stars according to their brightness and constellation is helpful for cataloging, certain star names are used because people identify these stars with their names.
Polaris – North Star
Sirius – Brightest Star in the sky (other than our sun)
Betelgeuse – Bright red star in Orion
Regel – Bright Blue Star in Orion
Alpha Centauri – Nearest star to our sun.
Vega – Bright Star overhead

7. The Brightness of Stars
Magnitude Scale – system astronomers used to measure the brightness of stars.
First appeared in the writings of Ptolemy ( 140 A.D.) but was probably used earlier.
Lower on the scale indicates a brighter star. Early astronomers said the brightest stars were “First Magnitude” stars. Those that were fainter were “Second Magnitude” Stars. The listed all stars seen with the naked eye up to the “Sixth Magnitude”
Stars fainter than the 6th Magnitude need to be seen with a telescope.

8. The Brightness of Stars
The Magnitude Scale has been updated over the years to give more precision in the brightness of stars.
Chort (q Leonis) was a magnitude 3 star but now it is 3.34
Some bright stars are less than one. Vega (Alpha Lyrae) = 0.04
Some really bright stars use negative numbers
Sirius (a Canis Majoris) = -1.47
Our Sun = The Full Moon = Venus = -4.4
The faint end of the magnitude scale has been extended as well. Limit of Hubble Space telescope = +30

9. Apparent Visual Magnitude
Apparent Visual Magnitude (mv) describes how stars look from the view of the earth
The human eye cannot detect infrared or ultraviolet light so these are not included on this scale.
This scale only tells how bright a star is from the view of the earth.
This scale does not worry about how far the star is away
Since brightness is subjective, astronomers use flux to measure the light energy from a star that hits one square meter in one second.

10. Magnitude and Intensity
Flux measures the intensity of starlight!
Astronomers use the following formula to relate the Intensity of the light from a star and its brightness on the magnitude scale.
IA/IB = (2.512)(mB-mA)
This equation can rearranged to solve for the magnitude difference.
mB – mA = 2.5 log(IA/IB)
These equations are based on astronomers defining two stars differing in five magnitudes (brightness) have an intensity difference of 100

11. The Celestial Sphere
Astronomers know that the stars are scattered through space at different distances but it is helpful to look at the sky as a “starry sphere” enclosing the earth.
The sky appears to move westward around the earth each day due to the eastward rotation of the earth.
Astronomers measure angular distance across the sky as angles and express them as degrees, arc minutes (1/60 of a degree) and arc seconds (1/60 of an arc minute). The same units are used to measure the angular diameter.
What we see of the sky depends on where on the earth we are. The northern hemisphere sees a different night sky than the southern hemisphere.

12. Precession of the Earth
Precession – when a rotating object is spinning and the object is not lined up with its axis, the object will sweep out the shape of a cone as it spins.
The Earth does not spin upright on its axis. The Earth is tipped 23.5o away from its axis. This tilt causes the Earth’s precession in its orbit and completes one cycle every 26,000 years.
This precession causes a different view of the night sky over a long period of time.
Currently , Polaris is the star over the north pole but in about 12,000 years Vega will be over the north pole.

13. The Cycles of the Sun
The Earth rotates on its axis approximately every twenty-four hours.
The Earth makes one revolution around the sun every days.
The sun moves across the sky against a background of stars.
The apparent path of the sun against the background of stars is called the ecliptic.
The glare of the sun does not allow us to see the stars behind it.

14. The Cycles of the Sun
The sun moves in front of constellations throughout the year.
In January - “the sun is in Sagittarius”
In March – “the sun is in Aquarius”
Since the Earth takes days to go around the sun, the sun appears to circle the sky in the same period.
The sun travels across the ecliptic in days
The sun moves westward across the sky (so do the stars)

15. The Seasons
Because the Earth spins on an axis that is tilted 23.4o from the perpendicular to its orbit, we have the seasons.
This tilt of the axis causes the sun to move across the northern sky in the spring and summer and across the southern sky in the fall and winter.
Vernal Equinox – “green” combined with “equal amount of daylight and darkness” occurs in the spring.
Summer Solstice – “Sun” is relatively “stationary” in the sky causing more daylight in the summer.
There is an Autumnal Equinox in the fall and a Winter Solstice in the winter.

16. The Seasons
The change in the seasons is a result of the differing amounts of solar energy between the Earth’s Northern and Southern Hemispheres.
In our summer (the Northern Hemisphere) we are getting more direct sunlight for longer periods of time. Therefore it is warmer.
In our winter the sunlight hits at an angle and is more spread out. Therefore it is cooler.
Note the seasons are reversed in the southern hemisphere.

17. The Motion of the Planets
The Planets do not produce their own light. We see them because of the light reflected from the sun.
Mercury, Venus, Mars, Jupiter and Saturn are visible to the eye and look like stars going across the ecliptic.
Uranus and Neptune need to be seen with a telescope.
Outer planets move slowly eastward along the ecliptic but the further a planet is away the slower it moves.
Venus and Mercury move differently because they are closer to the sun than the earth. They sometimes move eastward and sometimes move westward but stay close to the sun along the ecliptic.

18. Influences on Earth’s Climate
The changes in the motion of the earth can have a significant effect on the climate on the earth.
We do not notice these gradual changes because they occur over thousands of years.
Scientists have evidence of at least four Ice Ages (probably more)
Currently we are in a warming period of receding glaciers.
Milankovitch Hypothesis – says that small changes in the earth’s orbit, precession and inclination affect earth’s climate and can cause an ice ages.