Top 4 keys to understanding moon phases

How can you understand what causes the various phases of the moon? Four tips, here.

Top view of half-lit Earth and half-lit moon with lines between dark and light sides aligned.

Click here to see animation. As seen from the north side of the moon’s orbital plane, the Earth rotates counterclockwise on its rotational axis, and the moon revolves counterclockwise around Earth. Not to scale. Image via Wikimedia.

EarthSky’s lunar calendar shows the moon phase for every day in 2021. Order yours before they’re gone!

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

Why does the moon seem to change its shape every night? It’s because the moon is a world in space, just as Earth is. Like Earth, the moon is always half illuminated by the sun; the round globe of the moon has a day side and a night side. And, like Earth, the moon is always moving through space. As seen from our earthly vantage point, as the moon orbits around Earth once each month, we see varying fractions of its day and night sides: the changing phases of the moon. How can you understand moon phases? Here are four things to remember.

1. When you see the moon, think of the whereabouts of the sun

2. The moon rises in the east and sets in the west, each and every day

3. The moon takes about a month (one moonth) to orbit the Earth

4. The moon’s orbital motion is toward the east

Three rows showing moon from thin crescent to full round circle to thin crescent again.

Moon phase composite via Fred Espenak. Read more about this image.

1. When you see the moon, think of the whereabouts of the sun. After all, it’s the sun that’s illuminating and creating the dayside of the moon.

Moon phases depend on where the moon is with respect to the sun in space. For example, do you see which moon phase is being shown in the first illustration above? The answer is, it’s a full moon. The moon, Earth and sun are aligned with Earth in the middle. The moon’s fully illuminated half – its dayside – faces Earth’s night side. That’s always the case on the night of a full moon.

Don’t just take our word for it. Go outside. No matter what phase of the moon you see in your sky, think about where the sun is. It’ll help you begin to understand why the moon you see is in that particular phase.

Diagram of moon's path in the sky from rising to setting.

Earth’s daily spin causes the moon – like the sun – to rise in the east and set in the west each day. Image via Martin Clebourne’s article Where is the Moon?

2. 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 daily spin beneath the sky.

Just know that – when you see a thin crescent moon in the west after sunset – it’s not a rising moon. Instead, it’s a setting moon.

At the same time, though …

3. The moon takes about a month (one moonth) 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.

This is a slower, less noticeable motion of the moon. It’s a motion in front of the fixed stars. If you just glance at the moon one evening – and see it again a few hours later – you’ll notice it has moved westward. That westward motion is caused by Earth’s spin.

The moon’s own orbital motion can be detected in the course of a single night, too. But you have to watch the moon closely, with respect to stars in its vicinity, over several hours.

The moon’s eastward, orbital motion is easiest to notice from one day (or night) to the next. It’s as though the moon is moving on the inside of a circle of 360 degrees. The moon’s orbit carries it around Earth’s sky once a month, because the moon takes about a month to orbit Earth.

So the moon moves – with respect to the fixed stars – by about 12 to 13 degrees each day.

Diagram showing moon at two locations labeled '13 degrees per day.'

The moon’s orbital motion carries it eastward in Earth’s sky. Image via cseligman.com.

4. The moon’s orbital motion is toward the east. Each day, as the moon moves another 12 to 13 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 times of the moon cause our companion world to appear in a different part of the sky at each nightfall for the two weeks between new and full moon.

Then, in the two weeks after full moon, you’ll find the moon rising later and later at night.

We have more details on individual moon phases at the links below. Follow the links to learn more about the various phases of the moon.

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

Plus, here are the names of all the full moons.

Finally, here are the dates and times of 2021 moon phases.

Orbital view of Earth's straight horizon vertically on left with crescent moon standing out to its right.

Earth and moon, via NASA.

Bottom line: The moon is a world in space just as Earth is. Half of it is always illuminated by the sun. As the moon orbits Earth, we on Earth’s surface see varying fractions of its lighted face, or day side. These are the changing phases of the moon. Four tips to understanding moon phases, here.

Deborah Byrd