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Earths Moon
The Moon is the planet Earth's
only usual satellite. It has no formal name other than "The
Moon" although it is some times called as Luna, to distinguish
it from the generic "moon". Its symbol is a crescent.
The terms lunar, selene / seleno-, and cynthion (from the
Lunar deitie Selene and Cynthia) refer to the Moon (aposelene,
selenocentric, pericynthion, etc.).The average space from
the Moon to the Earth is Orbit.
The
Moon makes a complete orbit about once every 28 days.
Each hour it orbits round the stars by an amount roughly
equal to its angular diameter, or by about 0.5°.The
Moon varies from most satellites of other planets in
that its orbit is shut to the plane of the ecliptic
and not in the Earth's equatorial plane.
The two most familiar
ways of ruling the table are: the side real month being
the time it takes to make a complete orbit with respect
to the stars, about 27.3 days; and the synodic month
being the time it takes to reach the same phase, about
29.5 days. These differ because in the mean time the
Earth and Moon have both orbited some distance around
the Sun. |
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The gravitational pull that
the Moon exerts on Earth is the cause of tides in the sea.
The tidal flow period or the phase, is synchronized to the
Moon's orbit around Earth. The tidal bulges on Earth, caused
by the Moon's gravity, are accepted ahead of the apparent
position of the Moon by the Earth's rotation, in part because
of the friction of the water as it slides over the ocean bottom
and into or out of bays and estuaries. As a result, some of
the Earth's rotational momentum is gradually being transferred
to the Moon's orbital momentum, resulting in the Moon slowly
receding from Earth at the rate of approximately 38mm per
year. At the same time the Earth's rotation is gradually slowing,
the Earth's day thus lengthens by about 15 µs every
year. A more detailed discussion follows in the section titled
Earth & Moon.
The Moon is in synchronous
rotation, meaning that it keeps the same face turned to the
Earth at all times. This synchronous rotation is only true
on average because the Moon's orbit has definite eccentricity.
When the Moon is at its perigee, its rotation is slower than
its orbital motion, and this allows us to see up to an additional
eight degrees of longitude of its East (right) side. Conversely,
when the Moon reaches its summit, its rotation is faster than
its orbital motion and reveals another eight degrees of longitude
of its West (left) side. This is called longitudinal librations.
Because the lunar orbit is also inclined to the Earth's equator,
the Moon seems to oscillate up and down (as a person's head
does when nodding) as it moves in celestial latitude (declination).
This is called latitudinal librations and reveals the Moon's
polar zones over about seven degrees of latitude. Finally,
because the Moon is only at about 60 Earth radii distance,
an observer at the equator who observes the Moon throughout
the night moves by an Earth diameter sideways. This is diurnal
librations and reveals about one degree's worth of lunar longitude.
Earth and Moon orbit about
their bary center, or general center of mass, which lies about
4700 km from Earth's center (about 3/4 of the way to the surface).
Since the bary center is located below the Earth's surface,
Earth's motion is more regularly described as a "wobble".
When viewed from Earth's North Pole, Earth and Moon rotate
counter-clockwise about their axes; the Moon orbits Earth
counter-clockwise and Earth orbits the Sun counter-clockwise.
It may seem inquisitive that
the proclivity of the lunar orbit and the tilt of the Moon's
axis of rotation are shown as varying considerably. One must
be reminded here that the orbital inclination is measured
with respect to the primary's equatorial plane (in this case
the Earth's), and that the axis of rotation's tilt is measured
with respect to the normal to the satellite's orbital plane
(the Moon's). For most planetary satellites, but not for the
Moon, these conventions model physical reality and the values
are therefore stable. The plane of the lunar orbit maintains
an inclination of 5.145 396° with respect to the ecliptic
(the orbital plane of the Earth), and the lunar axis of rotation
maintains an inclination of 1.5424° with respect to the
normal to that same plane.
The lunar orbital plane processes quickly (i.e. its intersection
with the ecliptic rotates clockwise), in 6793.5 days (18.5996
years), mostly because of the gravitational perturbation induced
by the Sun. During that period, the lunar orbital plane thus
sees its inclination with respect to the Earth's equator (itself
inclined 23.45° to the ecliptic) vary between 23.45°
+ 5.15° = 28.60° and 23.45° - 5.15° = 18.30°.
Simultaneously, the axis of lunar rotation sees its tilt with
respect to the Moon's orbital plane vary between 5.15°
+ 1.54° = 6.69° and 5.15° - 1.54° = 3.60°.
Note that the Earth's tilt reacts to this process and itself
varies by 0.002 56° on either side of its mean value;
this is called nutation.
The points where the Moon's
orbit crosses the ecliptic are called the "lunar nodes":
the North (or ascending) node is where the Moon crosses to
the North of the ecliptic; the South (or descending) node
where it crosses to the South. Solar eclipses occur when a
node coincides with the new Moon; astral eclipses when a node
coincides with the full Moon.384, 403 kilometers (238,857
miles). The Moon's diameter is 3,476 kilometers (2,160 miles).The
first manmade object to land on the Moon was Luna 2 in 1959,
the Foremost photographs of the otherwise undetectable far
side of the Moon were finished by Luna 3 that same year, and
the first people to land on the Moon came aboard Apollo 11
in 1969.
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