Our Moon is trapped in synchronous rotation with Earth. That means that wherever it may be in its orbit around us, it is always showing us the same face. However, the proportion of that face that is illuminated by the Sun changes throughout the lunar cycle; in fact if you observe the Moon for an hour, you will see the shadow terminator (the line between the illuminated and shadow sides) moving. As you can see from my lunar photo collage above, the amount of light we see on the surface ranges from a thin crescent right up to fully illuminated. So why and how does this occur?
The diagram below shows the relative positions of the Moon and Earth during the lunar cycle, so it will help you understand my explanations a little better.
First of all it important to remember that the Moon is a sphere, and at any one time half of that sphere is illuminated by sunlight.But its relative position to us here on Earth will determine how much of that illuminated portion is visible to us. Picture in your mind a football with a torch shining at it; no matter where you stand, half of that ball will still be illuminated, but where you stand will determine how much of that light you see. The same thing happens with the Moon.
Let’s start off at the beginning of the lunar cycle with a New Moon. This is when the Moon is located in between the Sun and the Earth; the whole of the illuminated side is facing away from us, so we cannot see any of the
illuminated portion from here; therefore the Moon is invisible from Earth during this phase. If you read the article I wrote about eclipses, you will recall that it is during a New Moon that a solar eclipse may occur, but everything needs to line up perfectly in order for an eclipse to take place and that doesn’t happen every month because the Moon’s orbit is slightly tilted. People often incorrectly refer to the far side of the Moon as the dark side; during a New Moon the whole of
the far side is bathed in sunlight, so remember that “the Dark Side of the Moon”
is a Pink Floyd album name, not a statement of fact!
Following on from the New Moon phase, the illuminated portion will slowly begin to increase from the right hand side of the Moon each night. This is when the Moon is said to be “waxing”. During this time you will see a slim Waxing Crescent which is visible in the western sky after sunset, setting not long after the Sun. This crescent grows day by day until it reaches First Quarter phase, approximately 7 days after New Moon. At this point, it looks like the right hand half of the Moon is illuminated so it sometimes referred to as a “Half-Moon”. A half Moon rises in the east at around noon, and sets in the west at around midnight.
The Moon then continues to wax day by day, at this point being called a Waxing Gibbous Moon. Approximately 14 days after New Moon, we have a Full Moon. A Full Moon rises in the East at exactly the same time as the Sun sets in the West, and it remains visible all night long. When the Moon is full, it is half way round its orbit of Earth and is on the opposite side of Earth than the Sun. This means the face we see if fully illuminated by sunlight, but the far side is now entirely dark. Once again, if you remember my eclipse article, you will recall that this is when a lunar eclipse may occur.
Moon Phase Graphic Image taken from www.moonconnection.com/moon_phases
The Moon then begins to wane, and this is when see the light moving slowly inwards from the right hand edge as the shadow grows each day. This is once again a Gibbous phase, but this time it is a Waning Gibbous Moon. Approximately 21 days after New, we reach Last Quarter, where once again half of the lunar surface appears to be illuminated, but this time it is the left hand half instead of the right. A last quarter Moon rises in the east at about midnight and sets again in the west at noon.
The Moon once again reaches a crescent phase but this time a Waning Crescent. The illuminated portion gets smaller each day and you can see a Waning Crescent Moon in the East shortly before sunrise. At 28 days the Moon is once again back to New Moon and the whole cycle begins again.
Because the Lunar month is only 28 days long, we sometimes get more than one Full Moon in a calendar month. If we get a second Full Moon within the same calendar month, it is called a Blue Moon.