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
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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