The moon, like Earth, is a sphere, and it is always half-illuminated by the sun. As the moon travels around Earth, we see more or less of the illuminated half. The moon’s phases describe how much of the moon’s disk is illuminated from our perspective.
New Moon: The moon is between Earth and the sun, and the side of the moon facing toward us receives no direct sunlight; it is lit only by dim sunlight reflected from Earth.
Waxing Crescent: As the moon moves around Earth, the side we can see gradually becomes more illuminated by direct sunlight.
First-Quarter: The moon is 90 degrees away from the sun in the sky and is half-illuminated from our point of view. We call it “first quarter” because the moon has traveled about a quarter of the way around Earth since the new moon.
Full Moon: The moon is 180 degrees away from the sun and is as close as it can be to being fully illuminated by the sun from our perspective. The sun, Earth and the moon are aligned, but because the moon’s orbit is not exactly in the same plane as Earth’s orbit around the sun, they rarely form a perfect line. When they do, we have a lunar eclipse as Earth’s shadow crosses the moon’s face.
Waning Gibbous: More than half of the moon’s face appears to be getting sunlight, but the amount is decreasing.
Last Quarter: The moon has moved another quarter of the way around Earth, to the third quarter position. The sun’s light is now shining on the other half of the visible face of the moon.
Waning Crescent: Less than half of the moon’s face appears to be getting sunlight, and the amount is decreasing.