Observations

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.

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.

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:

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where w is the angular velocity you found in the earth's rotation part of the lab.

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.



Michael Bolte
Sun Feb 6 21:49:28 PST 2000