The Lunar phases that we have observed for millennia occur because the Moon does not always turn its daylight side towards us. When the Moon is between the Earth and the Sun, we cannot see the Moon as the sunlit side is the far side. This is the New Moon. (Note, it is incorrect to call the far side of the Moon 'the dark side' as it does receive sunlight). This is also the point when a Solar eclipse could occur; if the bodies are aligned the Moon will pass directly in front of the Sun, allowing Solar flares etc to be observed.
As the Moon phases progress towards being full, the Moon is said to be waxing. A slender crescent (waxing crescent) appears in the Western sky just after Sunset and disappears within a few hours after the Sun goes down. From night to night the crescent broadens and the Moon sets later. A week or so after the new Moon half of the near side is observable until approximately midnight. This is called the first quarter.

The edge of any celestial body that is the geometric edge of its sphere is called the limb. The Western curved edge at first quarter is the limb. The Eastern edge which looks like a straight line, (but is in fact a circle viewed edge on) is called the terminator. The terminator marks the boundary between the Moon's day and night.

After the first quarter Moon, the terminator seems to bulge out, so that more and more of the Lunar surface is illuminated by sunlight. This is known as the waxing gibbous stage.

Approximately a two weeks after the new Moon we will see the full Moon, when we can see the complete near side face as a bright white disk. The Earth is now between the Sun and the Moon. The full Moon rises as the Sun sets, is up all night and sets at sunrise.

The Moon will then be seen waning back towards the new Moon as it completes its orbit around the Earth. Waning gibbous; last quarter; then waning crescent and finally the new moon again.

This cycle takes 29.5 days, a period defined as the synodic month. Synodic means the period of revolution of the Moon with respect to the Sun. This is the Lunar phase period.

The Moon takes 27.3 days to move around the barycentre - the centre of gravity of the Earth Moon system. That is, it takes the Moon 27.3 days to complete one revolution of the Earth. This cycle is called the sidereal month, meaning the period of revolution with respect to the stars.

However, because the Earth is also orbiting the Sun the period between successive full Moons is not 27.3 days but 29.5 days (or roughly one calendar month). The word month has the same origin as the word moon.

The Moon's orbit is not circular, (more of a wobbly ellipse). The Moon also moves faster when nearer the Earth. This results in the amount of axial spin and orbital position becoming out of step, which results in a Moon wobble. This means that the Moon is able to show 59% of its surface to an Earth bound observer (this is called Libration), although actually only a maximum of 50% at any moment in time. There is 41% of the Lunar surface that we will never see from Earth - the "Far side".

Rotation

We regularly receive questions on the subject of the rotation of the Moon. We all know that the Earth rotates once every 24 hours, but many people assume that the Moon does not rotate as it always shows us the same face.

Actually, the Moon does rotate about its own axis. Its rotation is in sync with its orbit, that is it rotates once with every orbit. This means that the Moon can rotate and always show the same face to the Earth, but it also means that it not appear to rotate.

If the Moon did not rotate, it would eventually show all of its surface to the Earth.

The Lunar Eclipse

During an eclipse the shadow of the Earth is cast on to the Moon, this shadow is called the umbra. The area of expanding shadow is called the penumbra.The Earth's atmosphere refracts light in to the umbra. As our atmosphere is selective in its absorption of sunlight, more red than blue light is present in the umbra. Therefore the lunar eclipse appears a lovely copper-red color.

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