Bob the Alien's Tour of the Solar System
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Phases of the Moon


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 nearby 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. 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.


Crescent Moon
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 opposite to the side 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.

Phases of the Moon




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.

Lunar eclipse diagram
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.
Lunar eclipse



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