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How do you measure the moon from the distance?
Question Date: 2005-12-08
Answer 1:

The moon was first measured by a Greek astronomer named Aristarchus of Samos (310-230 BC). He measured the diameter of the Moon during a lunar eclipse, when the full moon was passing through the shadow of the earth.

Aristarchus timed how long the Moon took to travel through Earth's shadow and compared this result with the time required for the moon to move a distance equal to its diameter. He found that the shadow was about 8/3 the diameter of the Moon. Aristarchus knew that it takes for the moon 28 days to go around the Earth, which means 360 degrees.

Like Aristachus method, there are some more methods that measure the moon in a way relative to the earth; this means that measuring the earth, it is possible to measure the moon. On the other hand, in 1969, the Apollo 11 astronauts put a reflector on the moon (on the Sea of Tranquility), in order to run experiments that make possible the measurements of the moon's rotation. This project is named Lunar Laser Ranging Experiment and has been running since 25 years ago. The laser reflector consists of 100 fused silica half-cubes, called corner cubes, mounted in a 46-centimeter square aluminum panel. Each corner cube is 3.8 centimeters in diameter. The lunar ranging involves sending a laser beam through an optical telescope; the beam enters the telescope where the eye piece would be, and the transmitted beam is expanded to become the diameter of the main mirror, and then bounced off the surface toward the reflector on the moon.

Once the laser beam hits a reflector, scientists at the ranging observatories use extremely sensitive filtering and amplification equipment to detect the return signal, which is far too weak to be seen with the human eye. Even under good atmospheric viewing conditions, only one photon is received every few seconds.

From the ranging experiments, scientists know that the average distance between the centers of the earth and the moon is 385,000 kilometers.

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