Name: Partner(s): Day/Time: Version: SN

# Coordinate Systems with a Computer Planetarium Program

Instructions are given for Starry Night Pro, however this activity can be done with almost any planetarium program capable of showing the coordinates of an object. The Intro. to Starry Night may be useful if you have never used the program before.

## Part 1: alt-az

Your GSI will give you two stars, one north of the equator and one south of the equator. Record their names in table 1 and table 2.

Stop the time flow and set the planetarium program to display the stars tonight at 8 PM. You may want to turn off daylight under Local View in the Options tab. Find and Center on the star that is north of the equator. Record the star's altitude and azimuth as position 1 in table 1.

Turn on the meridian (Alt-Az Guides - Meridian under Guides in the Options tab.) Change the time flow to 1 minute and run time forward until the star transits (is on the meridian.) Record the altitude and azimuth under position 2 for the first star.

Go back to 8 PM and find the star that is south of the equator. Record the altitude and azimuth for position 1 in table 1. Center on this star and run time forward again until it transits. Record the altitude and azimuth for position 2 for this second star.

Table 1: Alt-Az positions
 Star name Pos'n Altitude (deg) Azimuth (deg) 1 2 1 2

Turn and face south and set the sky in motion. Observe the motions of the stars. Turn and face north, set the sky in motion, and observe the stars' motions again. Using the table and these observations, andswer the following questions.

1. Does the altitude of most of the stars remain constant or change throughout the night?  Does the azimuth?

2. What object(s) has/have (roughly) the same alt-az all the time?  What are its coordinates or their range of coordinates?

3. What is the range of altitude? Does this depend on the location? Explain your answer.

4. What is the range of azimuth?

## Part 2: RA and dec

Now you will observe the same 2 stars just like you did in part 1, but this time measure their Right Ascension and declination. Record your results in table 2.

Table 2: RA and dec
 Star name Position RA (hours) Dec (degrees) 1 2 1 2
1. Does the RA of most of the stars remain constant or change throughout the night? Does the dec?

2. What object(s) has/have (roughly) the same RA and dec all the time?

3. What is the possible range of RA in hours visible from Ann Arbor? Why do we use hours instead of degrees?

4. What is the possible range of dec visible in Ann Arbor?