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My Astronomy Class - Measuring Lunar Motion

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Celestial mechanics is often a difficult topic to teach - and to understand! But this activity will help you out a lot. It is also very simple, requiring only a ruler and a clock. The essence of the lab relies upon the fact that almost all adults have about the same reach - and that a ruler held at arm's length shows 1 cm = 1 degree in the sky. While this isn't perfect, it works well enough for most people.

The idea here is to measure the Moon's distance from the horizon in degrees several times before moonset; then use the distance traveled and time taken to find the Moon's speed as it travels across the skies.

My students will compare their results to the Earth's rotational speed (0.25 degrees per minute) and the Moon's orbital speed (0.009 degrees per minute). You should find that your own results are a lot closer to the first answer than the second - proving that most of what you see at Moonset is the Earth's rotation, not the Moon's orbital motion!

Let me know how you get on!


Tracking Lunar speed.doc

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