This evening there is an eclipse of the moon. It is at a time for easy viewing on the east coast of the United States. So, if the clouds are few, this should be a great show for all of us. Since the earth has to get between the sun and the moon for a eclipse to occur, it can only happen at full moon, which occurs at 10 pm, September 26, 1996. There are about two eclipses of the moon each year. Why not at every full moon? Because usually the moon is above or below the plane of the earth's orbit around the sun.
There are about 29.5 days between full moons, but it takes the moon about 27.3 days to go around the earth once. This is because from full moon to full moon, the moon has to go around the earth more than once since the earth is also going around the sun. The moon has to go around the earth about 1 and 1/12 times betwween full moon and full moon.
The lenght of time of the longest eclipse was used in ancient Greece to figure out how far away the moon is from the earth. Since it was known that the moon takes 27.3 days to go around the earth once, and that during an eclipse the moon is going through the earth's shadow. For example, if it takes four hours for the moon to go through the earth's shadow then the diameter of the earth is 4 hr/27.3 days of the circumference of the moon's orbit around the earth. Since C = 2*pi*r then r, the distance from the earth to the moon is equal to C divided by 2*pi. Since C = 27.3*24/4 * earth's diameter or 163 earth diameters, the distance from the earth to the moon is 163/6.3 earth diameters or 27.3 * earth diameter. Once Eratosthenes figured the earth's diameter to be 8,000 miles this distance could be calculated to be 220,000 miles, which isn't bad for such rough calculations.
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