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The Moon, of course, has been known since
prehistoric times. It is the second brightest
object in the sky after the Sun. As the Moon
orbits around the Earth once per month, the
angle between the Earth, the Moon and the Sun
changes; we see this as the cycle of the Moon's
phases. The time between successive new moons is
29.5 days (709 hours), slightly different from
the Moon's orbital period (measured against the
stars) since the Earth moves a significant
distance in its orbit around the Sun in that
time. |
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Moon in Hawaii |
Moon in the desert |
Moon in Alaska |
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The
Moon was first visited by the Soviet spacecraft
Luna 2 in 1959. It is the only extraterrestrial
body to have been visited by humans. The first
landing was on July 20, 1969 (do you remember
where you were?); the last was in December 1972.
The Moon is also the only body from which
samples have been returned to Earth. In the
summer of 1994, the Moon was very extensively
mapped by the little spacecraft Clementine and
again in 1999 by Lunar Prospector.
The gravitational forces between the Earth and
the Moon cause some interesting effects. The
most obvious is the tides. The Moon's
gravitational attraction is stronger on the side
of the Earth nearest to the Moon and weaker on
the opposite side. Since the Earth, and
particularly the oceans, is not perfectly rigid
it is stretched out along the line toward the
Moon. From our perspective on the Earth's
surface we see two small bulges, one in the
direction of the Moon and one directly opposite.
The effect is much stronger in the ocean water
than in the solid crust so the water bulges are
higher. And because the Earth rotates much
faster than the Moon moves in its orbit, the
bulges move around the Earth about once a day
giving two high tides per day. (This is a
greatly simplified model; actual tides,
especially near the coasts, are much more
complicated.) |
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