• Moon

    The Moon is the only natural satellite of Earth: orbit: 384,400 km from Earth
    diameter: 3476 km mass: 7.35e22 kg Called Luna by the Romans, Selene and Artemis
    by the Greeks, and many other names in other mythologies. 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. Due to its size and composition, the Moon is sometimes classified as a
    terrestrial "planet" along with Mercury, Venus, Earth and Mars. 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. But the Earth is not completely fluid, either.

    The Earth's rotation carries the Earth's bulges get slightly ahead of the point
    directly beneath the Moon. This means that the force between the Earth and the

    Moon is not exactly along the line between their centers producing a torque on
    the Earth and an accelerating force on the Moon. This causes a net transfer of
    rotational energy from the Earth to the Moon, slowing down the Earth's rotation
    by about 1.5 milliseconds/century and raising the Moon into a higher orbit by
  • about 3.8 centimeters per year. (The opposite effect happens to satellites with
    unusual orbits such as Phobos and Triton). The asymmetric nature of this
    gravitational interaction is also responsible for the fact that the Moon rotates
    synchronously, i.e. it is locked in phase with its orbit so that the same side
    is always facing toward the Earth. Just as the Earth's rotation is now being
    slowed by the Moon's influence so in the distant past the Moon's rotation was
    slowed by the action of the Earth, but in that case the effect was much
    stronger. When the Moon's rotation rate was slowed to match its orbital period
    (such that the bulge always faced toward the Earth) there was no longer an
    off-center torque on the Moon and a stable situation was achieved. The same
    thing has happened to most of the other satellites in the solar system.

    Eventually, the Earth's rotation will be slowed to match the Moon's period, too,
    as is the case with Pluto and Charon. Actually, the Moon appears to wobble a bit
    (due to its slightly non-circular orbit) so that a few degrees of the far side
    can be seen from time to time, but the majority of the far side (left) was
    completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959.
    (Note: there is no "dark side" of the Moon; all parts of the Moon get
    sunlight half the time. Some uses of the term "dark side" in the past
    may have referred to the far side as "dark" in the sense of
    "unknown" (eg "darkest Africa; but even that meaning is no longer
    valid today!) The Moon has no atmosphere. But evidence from Clementine suggested

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