1. Astronomy 305 Exam Topics Exam I – Star Charts
Exam II –Coordinate Systems and Telescopes
Exam III – Planets, Comets and Asteroids
Exam IV – Stars and Deep Sky Objects

2. Coordinate Systems 17 - Horizontal Coordinates
18 - Equatorial Coordinates
19 - Ecliptic Coordinates
20 - Galactic Coordinates
21 - Converting decimal degrees to °, ’, ”
22 - Converting Degrees to Hours
23 - Converting between Coordinate Systems
24 - Converting Right Ascension to Hour Angle
25 – Equatorial to Horizontal Coordinate Conversation

3. Celestial Coordinate Systems
These are coordinate systems with two angular measurements:
Geographical
Horizontal
Equatorial
Ecliptic
Galactic

4. Geographical System Latitude (f) Longitude (L)
the angle of a city above the equator
Longitude (L)
the angle around the equator from the Prime Meridian

5. 17 - Horizontal System Altitude (a) Azimuth (A)
the angle of a star above the horizon
Azimuth (A)
the angle around the horizon from the north point; measured eastward

6. 18 - Equatorial System Declination (d) Right Ascension (a)
Hour Angle (H)
the difference between the right ascension of your meridian and the right ascension an object

7. 24 – Converting between a and H
LST = Local Sidereal Time
LST is the same as the right ascension of the meridian on your star charts
H = LST – a
Examples:
What is the hour angle of Regulus at 11:00 PM tonight?
What is the hour angle of Sirius at 8:00PM tonight?

8. Hour Angle S H O R I Z O N Celestial Meridian d H Celestial Equator
90°- f S
H O R I Z O N

9. Celestial Meridian d
Celestial Equator a 
90°- f S
H O R I Z O N

10. 19 - Ecliptic System Ecliptic Latitude (b) Ecliptic Longitude (l)
the angle of a star above the ecliptic
Ecliptic Longitude (l)
the angle around the ecliptic from the vernal equinox
measured eastward
labeled along the ecliptic on the star charts
Obliquity (e)
the angle between the ecliptic and the celestial equator and is approximately 23.5 degrees

11. 20 - Galactic System Galactic Latitude (b) Galactic Longitude (l)
the angle of a star above the galactic plane
Galactic Longitude (l)
the angle around the galactic plane
measured eastward from the galactic center

12. Examples Problem For all of the planets find… Hint: Use Appendix 11
their ecliptic longitudes
the host constellation
their equatorial coordinates
Hint: Use Appendix 11

13. Planet Positions Planet l a d Constellation Mercury Venus Mars Jupiter
Saturn
Uranus

14. Planet Positions Planet l Visible? Rise Time Set Time Mercury Venus
Mars
Jupiter
Saturn
Uranus

15. Example Problems
Calculate the altitude and azimuth of Capella for 11:00PM tonight.

16. 25 - Equatorial to Horizontal
Step 1
Find a, LST and H is decimal hours.
Step 2
Convert H to degrees.
Step 3
Convert d to decimal degrees.

17. Step 4 Step 5 Step 6 Find sin a = sin d sin f + cos d cos f cos H
Find a
Step 6
Find

18. Step 7 Step 8 Step 9 Find A’ If sin H is negative, A = A’
If sin H is positive, A = 360° – A’
Step 9
Find a and A in degrees, minutes and seconds.

19. More Examples
What are the coordinates of Jupiter on May 5, 2005 at 8:00PM? b l a d
LST H A

20. Reading Assignment Chapter 2 Chapter 3 Chapter 16 A Tour of the Sky
The Monthly Sky Maps
Chapter 16
Telescopes and Binoculars

21. More Examples What are the coordinates of Venus tonight at 7:00pm? b l
LST H A

22. 25 - Equatorial to Horizontal
Step 1
Find a, LST and H is decimal hours.
Step 2
Convert H to degrees.
Step 3
Convert d to decimal degrees.

23. Step 4 Step 5 Step 6 Find sin a = sin d sin f + cos d cos f cos H
Find a
Step 6
Find

24. Step 7 Step 8 Step 9 Find A’ If sin H is negative, A = A’
If sin H is positive, A = 360° – A’
Step 9
Find a and A in degrees, minutes and seconds.

25. More Examples
Suppose that the Moon occults the star Antares on Feb 6th. Find the following for the Moon: a d b l

26. Ecliptic Coordinates Step 1 Step 2 Step 3 Find a and d.
Find a and d in decimal degrees.
Step 3
Find sinb = sind cose – cosd sine sina

27. Step 4 Step 5 Step 6 Find b. Find y = sina cose + tand sine
Find x = cosa

28. Step 7
Find l.

29. More Examples On what day does Vega cross the meridian at 8:00 PM?
What is the maximum altitude of the Sun for today in Nacogdoches?
At approximately what time does the Sun cross the meridian today?
What is the maximum altitude of the Sun over one year?