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PHASES OF THE MOON
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The Moon is
the second brightest object in the sky after the Sun and
is one of the most familiar objects to human beings. However, unlike the
Sun, which always appears round, the Moon's shape changes as the nights go
by. One night it will be a full bright disk (a Full Moon)
and several nights later, it will be half a disk. Look at it a few nights later
again and it may appear to be the shape of a banana, or a crescent if you want
to use its proper term. Eventually, the Moon will not even be visible before
going on to be crescent-shaped again, half full and then completely full. The
image to the left shows what the Moon should look like tonight.
The different
shapes of the Moon are known as its phases and each phase has a name. As the
Moon is such a familiar and easy-to-observe object, its phases have been studied
for centuries. We now know a great deal about the Moon's phases, when they occur
and what causes them.
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The reason why the Moon goes through phases is because of its
changing position in relation to the Sun and
Earth.
Earth is a planet which travels around, or
orbits, the Sun. It takes just over 365 days
(a year) for Earth to complete one orbit.
But, while
Earth is orbiting the Sun,
the Moon orbits Earth. It takes the Moon
just over 27 days to complete one orbit of Earth
(27.3 days to be precise).
As the Sun is a star, it generates its own
light. It is the only object in the Solar System to produce light.
All other objects in the Solar System - the planets and their moons,
the asteroids and
comets - reflect light shone on them from the
Sun. This means that only the sides of the objects receiving
light from the Sun will be lit up. It is only
possible for half of a round object, like a planet or most larger
moons, to be lit up by the Sun. And this is
the simple reason why there is day and night on
Earth. The half of Earth where it is
day is the side that is receiving light from the
Sun. The half of
Earth where it is night is the side that
isn't receiving light from the Sun.
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Diagram showing how
day and night is caused by light from the Sun.
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So, when you look at the
Moon or even a planet in space, you are seeing the part of it that is
reflecting light from the Sun which is basically the
side facing the
Sun. As the Moon moves around
Earth on its 27 day journey, the Moon always receives the same amount of
sunlight but what you can see of it changes. For example, when the Moon is
positioned so that it is in between the Sun and
Earth, you will be unable to see it because the lit side of the Moon is
on the opposite to what is visible from Earth. It is
also difficult to see because it will usually only be in a viewable position
during day on Earth. This is known as a New Moon. As
the Moon moves around Earth, you are gradually able
to see more of it. When the Moon is getting bigger each night, it is
"waxing". After about 14 days, the Moon is positioned so that
Earth is in between it and the
Sun. The means that, when viewed from
Earth, the lit side is completely visible. The Moon shines as a full
disk and is now known as a Full Moon. As its
journey continues, the visible side of the Moon begins to shrink again
(waning) until it returns to its original position in between
Earth and the
Sun and becomes a New Moon again. This is 29.5
days after it was originally in the same phase. The reason it takes a couple
of days longer than it takes for the Moon to completely orbit Earth (27.3
days) is because Earth has also moved. Remember that Earth is constantly
moving around the Sun, so for the Moon to be in the same position as it was
originally to produce the same phase, it needs to be at the same angle in
relation to the Sun and Earth, so needs to travel a little further than a
complete orbit. The image below
shows the Moon's cycle as it orbits Earth, viewed as
if you are looking down on the objects from above. It shows what the Moon
would look like when viewed from
Earth
when it is in the positions shown.
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As mentioned
above, a New Moon occurs when the Moon is in between Earth
and the Sun. Sometimes, when the Moon is new, it is
positioned so that it actually blocks the view of the Sun
from Earth. The results in an eclipse. By an amazing
coincidence, the Moon is 400 times smaller than the Sun,
but it is 400 times closer to Earth than the Sun is. This
means that, when viewed from Earth, the two objects
actually appear to be the same size. This means that when the Moon is exactly in
front of the Sun, it completely obscures the
Sun's disk and can turn day into night. This event is known as a total solar
eclipse. See this page for more information.
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It is
also sometimes possible for Earth to block
sunlight to the Moon. This happens when the Moon is in its
Full Moon phase. When this happens, Earth simply casts a shadow on
the Moon. It can either be partially or totally eclipsed. During a total
lunar eclipse, the Moon doesn't appear totally dark as light from the
Sun
is refracted through Earth's atmosphere which
reflects dimly on the Moon and gives it a dull brownish colour as shown
in the image below.
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