How to understand moon phases: 4 tips!

Moon phases seem mysterious … until they don’t

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 sun always illuminates half of the moon. That means the moon has a day side.

As seen from our earthly vantage point, as the moon orbits around Earth once each month, we see varying fractions of its day side: the changing phases of the moon.

How can you understand moon phases? Look below for 4 tips. They’ve helped many people learn to see the moon in a whole new way!

Tip #1. When you see the moon, think about the sun

After all, it’s the sun that’s illuminating and creating the day side of the moon. So moon phases depend on where the moon and sun are in space.

But don’t 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

Tip #2. The moon rises east and sets west, every day.

It has to. The rising and setting of all celestial objects is due to Earth’s continuous daily spin toward the east under the sky.

So, for example, when you see a thin crescent moon in the west after sunset, it’s not a rising moon. It’s a setting moon. Earth is always carrying you — on your special spot upon the world — toward the east, or toward the sunrise direction. So that crescent moon in the west afater sunset must have risen earlier in the morning. It rose soon after the sun and followed the sun across the sky all day, lost in its glare.

Tip #3. One month = one orbit of the moon

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. And it’s a motion in front of the fixed stars. So, if you just glance at the moon one evening – and see it again a few hours later – you might notice it has moved westward. That noticeable, westward motion is caused by Earth’s spin toward the east.

You can detect the moon’s motion in orbit, too. The moon moves in orbit toward the east (the sunrise direction), the same direction Earth spins. The moon’s orbital motion is less noticeable than its apparent motion caused by the spinning Earth. It’s easiest to catch the moon’s motion in orbit from one day to the next.

And — if you’re looking for the moon’s eastward motion in orbit from one day to the next — it’s easiest if you watch at the same time each day or night.

From Earth, the moon appears to be moving on the inside of a circle of 360 degrees. And 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 on the dome of Earth’s sky.

Tip #4. The moon’s moves in orbit toward the east.

Each day, the moon moves another 12 to 13 degrees toward the east on the sky’s dome. That means — here’s an important key — Earth has to rotate a little more time each day, to bring you around to where the moon is in space.

So 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.

And when the moon appears in a different part of the sky to us on Earth’s surface, it’s also in a different part of the sky with respect to the sun. So you see varying fractions of the moon’s day side. Voila! Moon phases!

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

More moon phases from the EarthSky Community

Bottom line: The moon is a world in space just as Earth is and 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.

Here are the names of all the full moons.

Visit Sunrise Sunset Calendars to make your custom moon phases, moonrise-moonset and sunrise-sunset calendar.