Earth’s Place in the Universe

Lunar Cycles

Like all moons, Earth’s moon orbits its planet. Sometimes we see a full moon. Other times only a small crescent. And for a night or two each month, we see no moon at all. How much moon we see from Earth depends on where the moon is in relation to the sun and the earth.

As the crescent of the moon grows bigger, we call that a waxing moon. After the full moon, the surface that we can see grows smaller. This is called a waning moon. When the moon is dark from Earth’s view it is called a new moon. At that time, the sunny side of the moon is facing away from Earth.

As the moon circles Earth, our planet is also rotating on its axis. That makes the moon seem to rise and set in the sky, but it’s really Earth that’s turning away from the moon. And even though the moon seems to glow in the sky, it doesn’t give off any light or heat of its own. Instead, that beautiful glow is actually the sun’s light reflected off the moon’s surface.

Lunar cycles

Lunar Month

The moon orbits around Earth every 27.3 days. Each rotation of the moon is called a lunar month. During each orbit, the moon goes through different cycles. Moon cycles are also the basis for the months in our calendars. The months in our familiar 12-month year are not quite the same as lunar months. They are longer than moon cycles so that one year equals the time it takes the earth to go around the sun.

Some societies use a lunar calendar where each month is the length of one moon orbit of earth. However, 12 of those 27.3- day moon orbits don’t quite add to up a year. So lunar calendars need to add an extra month every few years, like the leap year in a 12-month calendar.

Written by Laura McCamy

Edited by Beth Geiger, MS Geology

Illustrated by Renee Barthelemy