1. Some Basic Facts 12 Constellations of the Zodiac: groups of stars
in an 18 degree wide belt around the Ecliptic -apparent path of the Sun in the Sky among the stars
No physical relationship among stars in a constellation, except apparent 2-D location
Ancient Greeks  Geocentric Model
Modern (Copernicus)  Heliocentric Model
9  8 Planets: 4 inner Terrestrial and 4 outer Jovian (Pluto is neither)

2. Relative Sizes in astronomy From very small to very large (meters)

3. Some Essential Numerical Figures
Radius of the Earth = 6500 Km
Speed of light – 300,000 Km/sec
Astronomical distances are so large that we use the speed of light to measure them
Mean Earth-Sun Distance – 150 million Km
= 1 Astronomical Unit (AU) = 8.3 Light Minutes
1 Light Year (Ly) = 9.5 trillion Km = 63,240 AU

4. The Milky Way 100,000 Lys Across

5. Stellar Constellations
Connect bright stars to discern some shape

6. Ancient Figures and Constellations

7. The Orion Constellation

8. The North Star (Polaris)

9. The Summer Triangle

10. Winter Triangle of Bright Stars

11. The Distance Scale

12. the farther the object, smaller the a
LINEAR AND ANGULAR SIZE OF OBJECTS
 angle subtended by the object at the observer;
the farther the object, smaller the a

13. Angular size of moon = 30’
1 Degree = 60’ (minutes) = 60 x 60 = 3,600 ‘’ (seconds)
What is the angular size of the Sun? How large does the Sun appear ?

14. Angular ‘distance’ between stars
While angular distance can be measured by observations, actual distances are
difficult to measure (What do we need?)

15. Orbital and angular motion of the Earth
The Earth moves one degree in its orbit around the Sun each day. Why?

18. Distance Measure in Astronomy: The Parallax Method
Parallax is the change in
angle due to motion
Circle = 360o (degrees)
1 degree = 60’ (minutes)
1 minute’ = 60” (arcseconds)
1 AU 90
Measure of distances in angles:
The distance d of an object that
makes an angle of 1” as the Earth
moves to opposite sides of the
Sun d d a a
d (pc) = 1 / a
1 parsec (pc) = 3.26 Light Years (Ly)
Object at a distance of 1 pc

19. Stellar and Astronomical Distances
1 parsec (pc) = 3.26 LY = 205,000 AU
The stars are very far away
Nearest Star Alpha Centauri  4.3 LY, more than 1 pc ! The parallax angle a is less than one arcsecond (“)
That’s why the Greeks could not see the stars move
Galaxies have been seen up to more than 10 billion Lys away

20. Night Sky Exposure Geocentric or Heliocentric ?

21. Earth’s rotation and the Sky

22. Daily Rotation of the Earth and Stars

23. Annual Revolution of the Earth around the Sun and position of stars

24. Location of Heavenly Objects
How do you locate places on the Earth?
Latitude and Longitude
Latitude: angle measured from the Equator (0o), up or down, N-S
Longitude: angle measured from the Prime Meridian, E-W, 0o – 180o
How would you find location in mid-ocean ?
First rule of navigation: Lookup angle of Polaris  Latitude
How do you find the longitude? Clock ?

25. Celestial Map and Celestial Coordinates
Analogous to
Latitude and
Longitude on
The Earth –
Measured in
Degrees
Celestial Poles
And Equator –
Extension of the
Earth’s poles
And equator
Celestial Equator
is the extension
of the Earth’s
Equator up to
the CS

26. Ecliptic and the Celestial Equator
Ecliptic is the apparent
Path of the Sun on the
Celestial Sphere
Autumn Equinox
Summer Solstice
The Ecliptic and the
Celestial Equator
Intersect at
Vernal (Spring) and
Autumn Equinoxes
At an angle of 23.5o
to each other
Vernal Equinox
Winter Solstice

27. THE CELESTIAL SPHERE: Coordinates and Map of Objects in the Sky
Star
Declination d:
“celestial latitude”
Right Ascension a:
“celestial longitude”
Star at (a,d)
“celestial coordinates”
Vernal Equinox:
Position of Sun
In the Sky
on the first day
of spring;
Day = Night
a = 0

28. Apparent Rotation of Celestial Sphere