Enjoying EarthSky? Subscribe.

125,452 subscribers and counting ...

By in
| Moon Phases on Oct 05, 2013

Understanding moon phases

Why does the moon seem to change its shape every night? Remember that the moon is a world in space – with a day side and a night side.

Why does the moon seem to change its shape every night?

The answer is the moon is a world in space, just as Earth is. Like Earth, it’s always half illuminated by the sun. In other words, the round globe of the moon has a day side and a night side. From our earthly vantage point, as the moon orbits around Earth, we see varying fractions of its day and night sides. These are the changing phases of the moon.

As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.

Photo credit: Glass House

Waxing Crescent
First Quarter
Waxing Gibbous
Full Moon
Waning Gibbous
Last Quarter
Waning Crescent
New Moon

… and here are the names of all the full moons.

The primary key to understanding moon phases is to think about the whereabouts of the sun. After all, it’s the sun that’s illuminating and creating the day side of the moon. Moon phases depend on the sun. They depend on where the moon is with respect to the sun in space.

Another key to understanding moon phases is to remember that, like the sun and all the planets and stars, the moon rises in the east and sets in the west each and every day. It has to. The rising and setting of all celestial objects is due to Earth’s continuous spin beneath the sky.

Also, remember that the moon takes about a month (one “month”) to orbit the Earth. Although the moon rises in the east and sets in the west each day (due to Earth’s spin), it’s also moving on the sky’s dome each day due to its own motion in orbit around Earth. The moon’s orbital motion can be detected in front of the stars from one night to the next. It’s as though the moon is moving on the inside of a circle of 360 degrees. Thus the moon moves about 12 degrees each day.

And remember that the moon’s orbital motion is toward the east. Each day, as the moon moves another 12 degrees toward the east on the sky’s dome, Earth has to rotate a little longer to bring you around to where the moon is in space. Thus the moon rises, on average, about 50 minutes later each day. The later and later rising time of the moon causes our companion world to appear in a different part of the sky at each nightfall for about two weeks. Then, in the couple of weeks after full moon, you’ll find the moon rising later and later at night.

A composite of various moon phases by EarthSky Facebook friend Jacob Baker.  He wrote:

A composite of various moon phases by EarthSky Facebook friend Jacob Baker. He wrote: “This is a composite of 8 separate photos taken over the past summer showing some various phases. All shots taken with my Meade 114mm reflector, 2X Barlow, CanonT3 at Prime Focus.”

Bottom line: This post explains why the moon waxes and wanes in phase, and gives some keys to understanding moon phases. It also provides links to descriptions of the various phases of the moon.