The daily rising and setting of the sun and stars is due to the earth's rotation about its axis. The apparent motion of the sun, from east to west across the sky during the day, is familiar to us all. But have you ever noticed that the stars complete a similar motion at night? Probably not, unless you have gone out and spotted your favorite constellation then returned a few hours later to look for it again. To calculate how long it takes the earth to spin once on its axis (its period of rotation), you will measure how fast stars appear to move overhead.
Observations
1 ) First use Ursa Major as a Guide to the Constellations and go outside to find a star that lies near the celestial equator (the celestial equator is the imaginary line that runs around the sky 90 down from the north star).
2) Whichever star you decide to use, be sure to record its name.
3) Find a place from which to make your observations. This place should be located so that your star appears to be balancing on some structure, like a building, a telephone pole, or electric power lines. Make sure that this structure is rigid, not something flimsy like a tree or a branch, which can be blown about by the wind. Also make sure that this structure is far enough away that your star still appears to balance on it if you take a small step to the left or right. We'll call this structure your ``reference point''. Make sure that your spot is one that you will be able to relocate accurately about a week later. It would be ideal if you could mark your spot with a stone and not have it be moved.
4) When you have your star balancing on your reference point,
record the date and time (to the nearest minute).
5) Come back to your spot an hour later (exactly where you were before) and relocate your star. Measure how many degrees it has moved from its original position on your reference point. Record this result, along with the time to the nearest minute.
5) The angular velocity of the star in the sky during your observation is:
6) Calculate w in units of degrees/minute.
7) Now that you have the angular velocity of stars in the sky, calculate how long it would take for a star to travel 360 degrees, or all the way around the sky.
8) Since the star is ``moving'' because of the earth's rotation, this period is the amount of time it takes for the earth to spin once.
Answer the following Questions
1. What is the common name for the earth's rotational period?
2. The earth's rotational period is 23 hours 56 minutes, but you probably didn't get this exact answer. Why not?