Night after night — and sometimes day after day — you’ll see the Moon take a slightly different shape in the sky. These different shapes are called the phases of the Moon, and they change with a regular pattern over a period of about 30 days.
The phases of the Moon are caused by the changing positions of the Moon, Earth, and the Sun. As the Moon goes around the Earth, different parts of it are illuminated by the Sun. But because the same side of the Moon always points toward Earth, most of the time some of the illuminated part (daytime on the Moon) faces away from us. And, some of the non-illuminated part (nighttime on the Moon) faces toward us. This creates the different phases of the Moon.
There are eight main Moon phases:
New Moon: This is when you can’t see the Moon easily at all, because the part that is illuminated by the Sun is on the side of the Moon pointing away from Earth. This phase happens when the Moon is between the Sun and Earth. Because the Moon’s orbit around the Earth is tilted by about five degrees, it’s usually either above or below the Sun. But sometimes it lines up directly with the Sun, and this creates a solar eclipse. On average, a solar eclipse happens between two and five times each year.
Waxing crescent: As it moves in its orbit, a small sliver of the Moon becomes visible. It takes a crescent shape because of the Moon’s roundness; we only see the edge of the part that’s illuminated by the Sun, and that edge is on a round object so it appears curved.
First quarter: This is when half of the Moon’s illuminated surface is visible from Earth. This is the equivalent of saying we are seeing one-quarter of the total Moon’s surface illuminated, hence the rather confusing name.
Waxing gibbous: When more than half of the Moon’s surface visible from Earth is illuminated, we call it a Gibbous Moon.
Full Moon: When the Moon is on the opposite side of the Earth from the Sun, we see the entire near-side of the Moon illuminated. For the same reason we don’t have solar eclipses with every new Moon, we don’t get lunar eclipses with every full Moon. The Moon's orbit around the Earth is tilted by about five degrees, meaning sometimes it’s below our shadow and sometimes it’s above it. But every once in a while things line up so that it does go through the Earth’s shadow, and we get a lunar eclipse.
Waning gibbous: After the full Moon, the illuminated portion of the Moon visible from Earth begins to wane, or decrease in size. During the waning gibbous phase, the Moon’s surface visible from Earth is still mostly illuminated.
Third quarter: This is when half of the Moon’s illuminated surface is visible from Earth. The illuminated part is the half of the near side of the Moon that was not illuminated at first quarter.
Waning crescent: The final phase of the Moon is the waning crescent, which happens when only a small sliver of the Moon is visible (on the far side of the Moon’s visible surface, compared to the waxing crescent) before it disappears into the new Moon phase once again.