|
A Closer Look: Moon Motions
What’s the difference between the Moon’s rotation
and its revolution?
The rotation of a planetary body refers to the length
of time it takes it to turn 360¾ on its axis, which is called “one
day.” The
length of one day on the Moon is 27.3 Earth days — that is, it takes
27.3 of our days for the Moon to rotate once. Revolution refers
to the time it takes an orbiting planetary body — called a satellite
in the Moon’s case — to orbit its planet. The time it takes
the Moon to orbit Earth is also 27.3 days. The length of the
lunar day is
thus identical to the length of its revolution around Earth.
As a result, the same side of the Moon always faces the Earth.
Therefore, relative
to Earth, the Moon has a "near side" and a "far side."
There
is evidence that the length of the lunar day has not always been
the same as the duration of the lunar orbit — in fact, for
a moon to rotate on its axis with the same frequency as it rotates
around its planet is quite rare. When the Moon first formed,
scientists believe
it rotated more rapidly, so that its day was shorter. Just as
the Moon creates tides on Earth by exerting gravitational pull
on the water, Earth
created “tides” on the Moon, although the tidal forces acted
on rock, not water. As the Moon turned on its axis, the tidal
bulge closer to Earth was pulled on by the Earth's gravity in
the opposite direction
of the Moon's rotation — this would be like trying to pull someone
who was turning away from you back toward you. Over time, scientists
believe that this constant gravitational pull on the Moon's tidal
bulge by the
Earth's gravity slowed down the Moon's rotation until the length
of the lunar day became equal to the length of the Moon's orbit
around Earth.
What causes the phases of the Moon?
Your observations of the Moon will
confirm that the Moon has a different appearance from night to
night during its orbit around
Earth. Why is this so? Because the Moon doesn't make any of its
own light, the
only light we see from it is reflected from the Sun toward Earth.
At any moment during the Moon's 27.3 day orbit around Earth,
half of the Moon
is lit by the Sun and half is in darkness, just as half of Earth
is lit by the Sun at any point during the day. The only part
of the Moon we can
observe from Earth, however, is its "near side," which is the
side of the Moon that faces Earth. The different appearances,
or phases, of the Moon occur because, during its orbit, the relative
positions of
the Moon, Earth, and Sun change, causing the amount of illumination
that can be seen from Earth to change.
During a new Moon, for
example, we are unable to see the Moon at all. When this happens,
the Moon is between Earth and the
Sun, and the entire far side of the Moon, which we can’t see, is
illuminated by light from the Sun. During these periods, the
near side of the Moon
dark and the Moon is positioned over the illuminated side of
the Earth. During a full Moon, the entire disk of the Moon is
visible and the Moon
is positioned over the dark side of the Earth, which is why it’s
visible at night. When this happens, Earth is between the Sun
and the Moon. The rest of the time we are able to see only a
portion of the Moon's “face.” Between
the new moon and the full moon, some portion of the near side
of the Moon is illuminated, and the remaining portion of the
near side is dark.
As a new cycle of the Moon begins, the new Moon goes
through
a crescent phase (less than half of the Moon is illuminated)
to the first-quarter stage (one-half of its face is illuminated).
The Moon continues through
a gibbous phase (more than half of the Moon is illuminated)
until the full moon. The second half of the cycle proceeds from full
moon through a gibbous phase to the third-quarter stage. Finally,
a crescent phase
occurs before the next new Moon occurs.
It is important to realize
that the phases of the Moon are not in any way caused by the
shadow of the Earth.
What causes an eclipse?
There are two types of eclipses: solar
and lunar. During a solar eclipse, the Moon's orbit around
Earth brings it directly between and
on the same plane with the Earth and the Sun. The apparent size
of the Moon in the sky is the same as the apparent size of
the Sun in the sky,
so the Moon is able to completely block the Sun. Because the
Sun and Moon have to be on the same side of Earth for a solar
eclipse to occur, there
are only solar eclipses during a new Moon. Most new moons do
not result in solar eclipses, however, because the Moon does
not orbit the Earth
in the same plane as the Earth orbits the Sun, so the alignment
necessary for a solar eclipse doesn't happen often.
During
a lunar eclipse, the Moon's orbit around Earth brings Earth directly
between the Moon and the Sun. Earth blocks the
Sun's light from the Moon, which casts a shadow over it. Because
the Sun and the Moon
have to be on opposite sides of the Earth for a lunar eclipse
to occur, there are only lunar eclipses during a full Moon.
Most full moons do not
result in a lunar eclipse because the Moon does not orbit Earth
in the same plane as Earth orbits the Sun, so the alignment
necessary for a lunar
eclipse doesn't happen often.
If the orbit of the Moon were on
the ecliptic (plane of the orbit of Earth around the Sun), each
lunar orbit would result in a
solar eclipse during the new Moon and a lunar eclipse during
the full Moon. The plane
of the Moon's orbit is at a 5° angle to the ecliptic, which usually
causes the Moon to pass above or below the Sun as seen from the
Earth, without an eclipse taking place.
|
prev:
a closer look intro
