Rotation of the Sky

Summary: To draw maps of the night time sky and observe:

• 1) the diurnal rotation of the celestial sphere
• 2) the seasonal rotation of the celestial sphere

Needed Supplies: A notebook, colored pencils or pens, clear skies, a place to view the sky.

Due Date: October 3 (Part I), October 31 (Parts II and III).

Part I: Diurnal Rotation of the Celestial Sphere

On any one night between the first day of class and the due date, you sketch the night time sky three times. On the night when you observe, your three sketches must be made at intervals of at least one hour between each sketch and at least three hours from the first to the last. You should make these observations as soon as possible. You could observe, for example, at 7:30 PM, 8:30 PM and 10:00 PM.

Note: this lab does not require the lab facility. While you are welcome to perform it during lab time, be aware that you may want to take advantage of clear nights when you have them to use the telescopes. Additionally, even if your assigned lab night is clouded out, we will still expect you to complete this lab as long as any night has been clear enough to do it.

Procedure:

1. Pick one location where you have a good view of the sky. The best location will have a view toward the eastern or western horizon. You will make all three sketches from the same location.

2. Draw a reasonably careful sketch of the horizon (trees, buildings, landmarks) sufficient for using this sketch to locate the stars in the sky relative to objects on the horizon.

3. Using your sketch of the horizon, draw on this same sketch as many stars as you can, trying to mark their locations carefully, with relative distances above the horizon and between the stars as realistic as possible. You will probably want to use the lab crossbows or fist-and-finger rules to insure a good drawing.

4. (Say) one hour later. Using a different colored pencil or pen, on the same sketch, draw as many of the same stars as you can, marking their locations relative to the horizon as carefully as possible.

5. (Say) two hours later. Using a third colored pencil or pen, on the same sketch, draw as many of the same stars as you can, marking their locations relative to the horizon as carefully as possible.

6. Answer the following questions:

   • (a) Describe what appears to have happened to the sky, based on your observational results.
   • (b) What caused this effect to occur?
   • (c) What fundamental property of the Earth could you measure from these observations?

Part II: Seasonal Rotation of the Celestial Sphere

On each of at least three different evenings before the due date, you will draw one simple sketch of the night time sky. Your observations must be made at approximately the same time each evening. Consecutive observations must be separated by at least 7 days (unless you make more than three observations) and the first and last observations must be separated by at least three weeks.

Procedure:

1. Do the same procedure as for Part I, changing only the time between your observations from a few hours to at least 7 days.

2. Answer the same questions as for Part I

Part III: Measurement of the Length of a Sidereal Day

A day can be defined in several different ways, each is useful for particular applications. The mean solar day lasts 24:00:00 hours exactly, by definition. This is the average time between two passages of the Sun on the meridian (South for us). We will now determine how long it takes for a star (other than the Sun) to complete one (apparent) rotation in the sky.

Additional Needed Supplies: A fairly steady watch (not noticeably slow or fast over about a week).

Procedure:

1. You will need to do some "scouting" before you can do this project. Pick a fairly bright star (not a planet!) in the southern half of the sky. You need to find a way of sighting to a fixed spot in the sky where the star will pass. Your sighting method must be reliable and repeatable over a few days. You could, for example, use the East side of a campus building oriented along the North-South direction (most campus buildings have this orientation). Or you could note the time when the star rises (sets) above (below) a rooftop as seen from a very specific location (say, seated in front of your window).

2. Sighting along the wall (for example), note the time when the star meets the plane of the wall to the nearest 1/2 minute or so. Keep a record of the dates, time, the star you chose, location and method of sighting, and direction of observations.

3. Return to the same spot several days later and repeat the measurement (it will be a bit earlier than the first time). Repeat the measurement more than once if weather permits.

4. Answer the following questions:

   • (a) Using your records, determine the duration in hh:mm:ss of a sidereal day. How does it compare with the mean solar day?
   • (b) Why are they different?
   • (c) How does your result compare with the accurately determined value for the sidereal day (see textbook)?
   • (d) In your opinion, which factors may explain the difference?
   • (e) How do you relate your measurement with the seasonal shift of the constellations in the night sky?