Moon Phases

What is a full moon?

Full moon in white and gray with black background.
View at EarthSky Community Photos. | Gizaw Legesse in Addis Ababa, Ethiopia, captured this photo of the full moon on October 20, 2021. Gizaw wrote: “Tonight, the moon is 100% full on the Addis Ababa sky.” Thank you, Gizaw!

Full moon always opposite the sun

A full moon is opposite the sun in its orbit around Earth. Its sunlit side is entirely visible from Earth. The moon appears full to the eye for two to three nights. However, astronomers regard the moon as full at a precisely defined instant, when the moon is exactly 180 degrees opposite the sun in ecliptic longitude.

The moon, the Earth and the sun are aligned. Earth surrounds the sun in a big circle. The moon surrounds the Earth in a smaller circle.
At full moon, the sun, Earth, and moon are aligned in space, with Earth in the middle. The moon’s day side – its fully lighted hemisphere – directly faces us. Chart via John Goss/ EarthSky.

It’s that feature of a full moon – the fact that it’s opposite the sun as viewed from Earth – that causes a full moon to look full and round.

Earth in space with moon, viewed from directiy above north pole.
In the linked animation you can see the moon revolving around Earth. Note that the full moon is on the opposite side of the Earth from the sun. Not to scale. Image via Wikipedia Commons. Click here to see animation.

See the moon phase for every day in 2022 on EarthSky’s lunar calendar. Order yours before they’re gone!

Bright full moon with a woman and a girl watching it.
View at EarthSky Community Photos. | Radu Anghel in Parjol, România caught what he called “a family photo with the supermoon of July 13, 2022.” Thank you, Radu! Supermoons don’t appear bigger to the eye than other full moons. But they do appear brighter!

Why does a full moon look full? Remember that half the moon is always illuminated by the sun. That lighted half is the moon’s day side. In order to appear full to us on Earth, we have to see the entire day side of the moon. That happens only when the moon is opposite the sun in our sky. So a full moon looks full because it’s opposite the sun.

That’s also why every full moon rises in the east around sunset – climbs highest up for the night midway between sunset and sunrise (around midnight) – and sets around sunrise. Stand outside tonight around sunset and look for the moon. Sun going down while the moon is coming up? That’s a full moon, or close to one.

Just be aware that the moon will look full for at least a couple of nights around the instant of full moon.

When is the moon full?

Want to know the instant of full moon in your part of the world, as well as the moonrise and moonset times? Visit Sunrise Sunset Calendars, remembering to check the moon phases plus moonrise and moonset boxes.

Read more: What are the full moon names?

Often, you’ll find two different dates on calendars for the date of full moon. That’s because some calendars list moon phases in Coordinated Universal Time, also called Universal Time Coordinated (UTC). And other calendars list moon phases in local time, a clock time of a specific place, usually the place that made and distributed the calendars. Translate UTC to your local time.

Why no lunar eclipse every full moon?

If a full moon is opposite the sun, why doesn’t Earth’s shadow fall on the moon at every full moon? The reason is that the moon’s orbit is tilted by 5.1 degrees with respect to Earth’s orbit around the sun. At every full moon, Earth’s shadow sweeps near the moon. But, in most months, there’s no eclipse.

Why there’s no lunar eclipse every full moon

Earth at center with moon's orbit crossing the ecliptic line twice.
The moon’s orbit tips 5 degrees relative to Earth’s. Eclipses only occur when the moon crosses the ecliptic during a full or new moon. Image via Wikimedia Commons (CC BY-SA 4.0).

Read more about the moon’s phases

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

New moon
Waxing crescent moon
First quarter moon
Waxing gibbous moon
Full moon
Waning gibbous moon
Last quarter moon
Waning crescent moon

Bottom line: A full moon looks full because it’s opposite the sun. Its lighted face is turned entirely in Earth’s direction.

Read more: Top 4 keys to understanding moon phases

Posted 
January 1, 2022
 in 
Moon Phases

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Deborah Byrd

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