What causes the phases of the moon?
The phases of the moon are caused by the changing angles (or relative positions) of the Earth, moon, and Sun. The length of time from New Moon to New Moon is called the lunar month or synodic period of the Moon, which is 29.53 days.
One half of the moon is always illuminated by the Sun. During the phases of the moon, we see both the sunlit portions, and the shadowed portions. The shadowed portions, at various “stages” are given names, which we know as the phases of the moon.
In the diagram below, the Earth is at the center and the Sun’s rays are coming from the right side of your screen. The dotted lines represent your line of sight when looking at the moon at eight different phases during its revolution around the earth
During the full moon, the Earth, moon, and Sun are in approximate alignment, just as the new moon, but the moon is on the opposite side of the Earth, so the entire sunlit part of the moon is facing us. The shadowed portion is entirely hidden from view (this is the so-called “dark side of the moon”… more on that later).
The new moon (right side of the image) occurs when the moon is positioned between the Earth and Sun. The three objects are in approximate alignment. The entire illuminated portion of the moon is on the back side of the moon, the half that we cannot see.
The first quarter and third quarter moons (usually called the “half moon“), occurs when the moon is at a 90 degree angle with respect to the Earth and Sun. Thus, we see half of the moon illuminated and half of the moon in shadow.
Other phases of the moon include the crescent moon, gibbous moon, waxing moon, and waning moon. “Crescent” refers to the phases where the moon is less than half illuminated. “Gibbous” refers to phases where the moon is more than half illuminated (it is between the quarter and full moon, and more than half of the moon can be seen). “Waxing” essentially means “growing” or increasing in illumination, and “waning” means “shrinking” or decreasing in illumination. Therefore, a waxing moon is when the illuminated portion of the moon we see gets larger each night until it is a full moon. A waning moon is when the illuminated portion of the moon appears to be getting smaller every night until it is a new moon
Bonus Moon Fact 1: Why do we always see the same side of the moon?
The same side of the Moon always faces the Earth because the Moon’s orbital period is equal to its rotational period. Basically, the moon spins on its axis at the same rate that it revolves around the Earth.
Bonus Moon Fact 2: Is there really a dark side of the moon?
The “dark side of the moon” is not always dark. The back side of the Moon is only fully dark when we are seeing a full moon from Earth. The back side has phases just like the side facing Earth. When the Moon is at the phase called “new”, it is fully lit on the back side.
Bonus Moon Fact 3: What is a blue moon?
A second full moon in one month is usually called a blue moon. The saying “Once in a blue moon” refers to something that doesn’t happen often – like a blue moon. Incidentally, depending on your definition, a blue moon occurs once every 2 years and 5 months, or once every 2 years and 9 months. Not that rare!
Incidentally, if you were on the moon looking back at the Earth, you’d see the “phases of the Earth”. If you look up at Venus, you can see the same phases as well. One can only imagine the night time view from other planets in our solar system where there are more than a single moon.