If you are familiar with the night sky, you have probably noticed that there are different constellations visible in the summer and winter. This seasonal change is due to the earth's revolution around the sun. In this section, you will again follow the motion of a star, but this time you will use it to measure the earth's period of revolution.
Observations
1) To measure the earth's period of revolution, you will make another measurement on the same star you used before, but a few days later. If you were to return to your spot at the same time a few days after your first measurement, your star would appear to have moved westward relative to your reference point. You could then determine the star's motion in degrees per day by measuring how far your star had moved. For this lab, however, we'll use a slightly more accurate method.
2) A few days, say three to ten, after you've completed the observations
of the star in the earth's rotation part of the lab, return to
your spot. This should be exactly the same spot as before, but
arrive about 45 minutes earlier. Note that your star is just east
of your reference point. Wait at your spot until your star is
again balancing on your reference point. When this happens, record
the date and time to the nearest minute.
3) The star came to balance earlier than it did the first time.
In other words, if you waited until the same time of night as
when you made your first measurement, the star would be further
to the west by an amount: where w is the angular velocity
you found in the earth's rotation part of the previous experiment.
4) Calculate this shift. Now you know the westward shift of the
star in degrees and the number of days it took to make this shift.
Use these values to calculate a new angular velocity, v, in degrees/day,
due to the earth's motion around the sun. How many days would
it take for the star to move around 360 degrees? This is the earth's
period of revolution around the sun.
Answer the following questions
1. What is the common name for the earth's
period of revolution?
2. The earth's period of revolution is 365.25 days, but you probably
didn't get this answer exactly. Why not?