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MOON PHASES - OVERVIEW*
Lunar phases are the result of our seeing the illuminated half of the Moon from
different viewing geometries. They are not caused
by shadows of the Earth on the Moon that occur during a lunar eclipse. The Moon
exhibits different phases as the relative
geometry of the Sun, Earth and Moon change,appearing as a full moon when the Sun
and Moon are on opposite sides of the
Earth, and becoming invisible as a new moon (also named dark moon) when they are
on the same side. The phases of full
moon and new moon are examples of syzygies, which occur when the Earth, Moon and
Sun lie (approximately) in a straight line.
The time between two full moons (or between successive occurrences of the same
phase) is about 29.53 days (29 days,
12 hours, 44 minutes) on average. This synodic month is longer than the time it
takes the Moon to make one orbit about
the Earth with respect to the fixed stars (the sidereal month), which is about
27.32 days. This difference is caused by the fact
that the Earth-Moon system is orbiting about the Sun at the same time the Moon
is orbiting about the Earth.
The actual time between two syzygies is variable because the orbit of the Moon
is elliptic and subject to various
eriodic perturbations, which change the velocity of the Moon. It might be
naively expected that once every month
when the Moon passes between Earth and the Sun during a new moon, its shadow
would fall on Earth causing a
solar eclipse. Likewise, during every full moon, we might expect the Earth's
shadow to fall on the Moon,
causing a lunar eclipse.
We do not observe a solar and lunar eclipse every
month because the plane of the Moon's orbit around the Earth is tilted
by about 5 degrees with respect to the plane of Earth's orbit around the Sun.
Thus, when new and full moons occur, the
Moon usually lies to the north or south of a direct line through the Earth and
Sun. An eclipse can only occur not only when
the Moon is either new or full, but also when it is positioned at (or very
nearly at) the intersection of Earth's orbit plane about
the Sun and the Moon's orbit plane about the Earth (that is, at one of its
nodes). This happens about twice per year, and so
there are between 4 and 7 eclipses in a calendar year. Most of these are quite
insignificant; major eclipses of the Moon or
Sun are rather rare and newsworthy events.
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