If you look at the Moon during different days of the month, then you might notice that the moon looks a little different every day. Why do we see these different phases of the Moon? There are few different reasons why notice different phases of the Moon.
The phases of the Moon depend on the moon’s position compared to the Earth and Sun. Remember that the moon revolves around the Earth. As the moon goes around the Earth, half of the moon is always illuminated by the Sun. Meanwhile, the other half of the moon is always in darkness. Sometimes we see the parts being illuminated, and sometime we do not.
The moon looks bright because we see sunlight reflecting off of it. Depending on the position, we can only see fractions of the illuminated surface. For instance when the Moon is between the Earth and the Sun, the side of the moon facing Earth is not illuminated by the Sun. Therefore, to us humans on Earth, the moon is dark and we call this a New Moon.
As the Moon then moves away from the Sun, we begin to see more of the surface illuminated. The Moon then appears brighter and fuller as we see the sun reflecting and shining on its surface. In these phases, the Moon looks like it is growing. When the Moon appears half full, it has then reached its First Quarter. Then, when the Earth is positioned between the Moon and the Sun, we see a Full Moon.
Naturally, the Moon then starts to get smaller and smaller as it moves back towards the sun. It reaches Last Quarter when it looks empty. Finally, it goes dark and returns to a New Moon.
The lunar cycle, which means it is going from one New Moon to another New Moon, takes about 30 days to complete. That means all the phases happen once a month.
The Moon’s shape does not change, and it cannot cast light by itself. Instead, we only see a change in shape because we can only see the parts of the Moon that are being lit by the Sun. The parts we do not see are simply in darkness or shadow.
The phases of the Moon are easy to understand once you realize and remember that the phases are dependent upon the location of the Sun, Moon, and Earth. The phases occur because the Sun lights different parts of the Moon as the Moon revolves around the Earth. That means, the reason we see different phases of the Moon here on Earth is because we only see the parts of the Moon that are being lit up by the Sun.