What Causes the Phases of the Moon?

The phases of the moon are caused by the location of the moon in relation to the Earth. In other words, as the moon rotates around the Earth, sun hits it in different angles and therefore, this causes the moon to undergo different phases. Based on the time of the month, the side of the moon that we see will experience more sunlight on it and then less sunlight.

The Phases of the Moon

When the moon "starts" its cycle of phases, it is known as a New Moon. This means that the Earth sees the side of the moon that is receiving no sunlight because it is directly in line with the sun. Therefore, the moon is blocking out a bit of the sun. This new moon comes off as black or highly invisible to the eye.

The next stage is known as a waning crescent. This occurs when the moon is 45 degrees from where it started or ends at the new moon. For example, forty five degrees after leaving the new moon, the moon is in the waning crescent section. This means that a quarter of the moon is lit from the sun. However, the waning crescent section on the return towards the new moon is also at 45 degrees.

At 90 degrees from where it originally started, the moon enters the half moon stage. Here, half of the moon is illuminated via the sun and therefore, humans on Earth are able to see it.

At 135 degrees, the moon enters what is known as the waning gibbous stage. This is when nearly all the moon is filled; however, there are still some portions of the moon that are black due to the sun not reaching them.

Finally, at 180 degrees from where it originally started at a new moon, the moon enters the stage known as a full moon. The full moon is one of the most well known stages because the entire moon is visible. The sun is hitting the side of the moon humans see entirely and therefore, it appears as a "full moon." However, a day after this, it leaves that phase and begins to head back towards a new moon. It enters the waning gibbous stage, then the half moon stage, followed by the waning crescent stage and finally, the new moon all over again.