Lunar Phases, Minor and Major
If you ever looked t the moon in the night sky, you probably notice when it's full or when it is'nt. However, you might not notice when it has a strange orientation of what it was before. For example, on the 3rd quarter of the month, the moon can have light on the left instead of on the right when looking at the 1st quarter ( 1st= Top of the diagram| 3rd= Bottom)
Well, it's of note when you do. This is because of our position in the solar system and the revolution of the moon around us. Every 30 days, the moon goes from a New Moon(0 days) to a Full moon (14 days) and finally back to New Moon (30 days)
However, every 7 days has a distinct minor or major name. In the first 7 days it is a 1st quarter. It's light is on the right (Cuzz it's getting bright) and on the 21st day mark it will be as if a reflection of the 1st quarter, light on the left (leaving)
This is because we see the light on the moon that has come from the sun in a certain position. The moon also infuences the tides we have, when the sun and moon are alligned it is high tide, and when it is in 1st/3rd quarter phases they will have low tide.
In conclusion, the moon's light comes from the sun, changes in between moon cycles, and we observe the one side we can see and see it as either gaining until full or lessening until only darkness remains.