The Moon

Phases and Appearance

Phases

Casual observation of the Moon over several nights reveals that not only does its position change when viewed at the same time each night but the shape or 'Phase' (the fraction of its surface visible to us) changes too. The change in nightly position is due to its movement around the Earth and the change in phase due to the angle it makes between the Earth and the Sun - which of course is changing as it moves around the Earth in its orbit.

Don't confuse the nightly change of position with the change in direction that takes place over several hours during a single night. This latter movement is due to the rotation of the Earth on its axis and the Moon and stars appear to move together as the Earth rotates. The Moon does move amongst the stars but it takes night to night observations rather than hour to hour observations to make it really obvious.

When the Moon is in the same part of the sky as the Sun we cannot see it, since the Sun's light drowns it out. But, as the Sun - Earth - Moon angle increases as it orbits the Earth, we begin to see a little more of its surface illuminated each night. When the Moon is finally opposite to the Sun in the sky we see the full face of the Moon illuminated. These two opposite positions are known as 'New Moon' and 'Full Moon' respectively and are shown in the diagrams below. These diagrams show the Moon in it's orbit around the Earth as viewed from above the North Pole of the Earth. The side panels show the phase of the Moon as seen from the Earth for that particular point in the Moon's orbit.

The New Moon Phase

The Full Moon Phase

So, as the Moon progresses in its orbit from the New Moon position the phase gradually grows showing more of the surface each night. The illumination progresses from the right hand side (if you're in the Northern Hemisphere - Left hand side if not) of the Moon as we see it. This is known as the Waxing part of the cycle. There are various named stages within this waxing part of the cycle; for example, when the Moon makes a 90 degree angle to the Sun and Earth, then half of the visible surface is illuminated. This is known as 'First Quarter' phase since the Moon is a quarter of the way around in its orbit relative to the New Moon position.

However, between the New Moon and First quarter phases less than half of the surface is illuminated and this is known as the Crescent phase or more specifically 'Waxing Crescent'. Between the First quarter and Full Moon phases the surface is more than half but less than fully illuminated. This is known as the 'Gibbous' phase (specifically Waxing Gibbous).

The Waxing Crescent Phase

The First Quarter Phase

The Waxing Gibbous Phase

As the Moon moves on past the Full Moon position the Sun - Moon - Earth angle begins to decrease again and so does the amount of illuminated surface. This half of the cycle is known as the Waning part. When the angle becomes 90 degrees in this part of the cycle then, again, only half of the surface is illuminated but this time it's the left hand side (if you're in the Northern Hemisphere) that's illuminated as seen by us. This phase is called Third Quarter or Last Quarter as the Moon has reached the last quarter of its orbit relative to the New Moon position.

As in the Waxing part of the cycle we have Waning Gibbous where the Moon is between the Full Moon and Last Quarter and also 'Waning Crescent' when the Moon is between Last Quarter and New Moon.

Contrary to some popular beliefs the phases of the Moon are not caused by the shadow of the Earth falling on the Moon. Although when the Sun Earth and Moon are in alignment at Full Moon the Earth's shadow can fall on the Moon. This is called a Lunar Eclipse. It doesn't cause a total blackout of illumination such as to cause a phase like appearance but does make the otherwise bright surface appear much darker.

The whole cycle from New Moon to New Moon takes 29.5 days. So, approximately, there's a week between New Moon and First Quarter, First Quarter and Full, Full and Last Quarter and Last Quarter and New Moon again. This is quite handy as it helps you to plan when to see which phase and knowing the phase helps to determine where it will be in the sky relative to the Sun and therefore the best time to view it, as we'll see shortly.

The whole phase cycle can be seen in the video below. This video shows a Northern Hemisphere bias.

Appearance

Apart from the changing shape of the Moon throughout the Lunar Cycle it can also be seen with the naked eye that the Moon is not completely featureless. The famous 'Man in the Moon' face is perhaps the best example of these darker areas that make up the face.