Astronomy 10: The Sidereal Day

The Sidereal Day

Worth 20 points -- Due Friday, June 16th, 2000

In this project you will observe the progress of the stars over the course of several weeks. You will observe for yourself the difference between a solar (synodic) day and a sidereal one, and you will even predict the length of a sidereal year.

Unlike the other labs, this one would actually work out better by having some kind of building or tree blocking part of your sky. You could even do this lab from indoors, by looking out a window with a partial view of the sky. The critical thing about observing location is that you make each observation from the exact same spot. The time that you initially start this lab is unimportant (so long as it is at night and stars are clearly visible). But you will need to be able to repeat your observation at close to that same time.

Look into the sky and find a bright star that is about to pass some marker. For example, if you are looking out a window the star might just be moving pass a piece of the window frame. Or, if you are outside the star will just be about to pass by a branch of a tree, or a lamp post, or the corner of a building. The point is to be able to observe the star passing some unmoving marker. You could also use the place where the star rises, sets, or transits the meridian. These might be slightly more difficult to observe as precisely, however. Once you have found your marker, record the date and exact time (on a digital watch that is in good working order) that the star passes that marker. If you want to be really fancy, pick a constellation with many bright stars in it and record when all of those stars pass your marker. Sketch what your marker looks like with the star(s) passing it. Use the star maps in your textbook or computer planetarium programs to identify the star(s) and the constellation of which it(they) is(are) a member. Make sure that it is a star and not a planet that you are observing.

Now, repeat the exact same observations over the course of at least 3 weeks. You need not observe every single day, but you should make at least 3 observations on consecutive days, and you should observe on a minimum of 10 days. Be very careful to record the exact times that the stars pass by your marker. You will notice that they will not pass by at the same time everyday. We will discuss in class that the sidereal day (the time for a star to make one complete circuit in the sky) is different from the mean solar day. It is 4 minutes shorter than the solar day. So you will have to be at your observing location a little earlier every time from the last.

Along with your observations you will turn in a discussion of what you have observed and your interpretations of the observations. This need be no longer than 3 pages typed (double-spaced). In your discussion address the following questions:

  1. Based on your observations, what is the average time interval for a star to consecutively cross your marker? What is the name of this time interval?
  2. How does this time interval differ from the mean solar day?
  3. Why are they different?
  4. Which is the amount of time for Earth to spin on its axis exactly once?
  5. Calculate how long it will be before the star(s) you observed is(are) no longer above the horizon during the night.
  6. Calculate how long it will be before the stars(s) you observed cross the marker again at the exact same time as your original observation. What is this time interval called? What is its significance?

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