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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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Moon
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