Moon Phases

What is a full moon?

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January 1, 2023

Full moon: Full Harvest Moon. — View at EarthSky Community Photos. | Cathy Adams of St. Stephen, New Brunswick, Canada, captured this image of the full moon on September 10, 2022 and wrote: “While our moon looks white or yellow, it is actually many different colors depending on the composition of the rock in each area. I am working on bringing those natural tones out. First try is with the Harvest Moon last night.” Thank you, Cathy!

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.

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. Is the sun going down while the moon is coming up? Then 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.

Collage of full moons. Center has smallest and largest moon comparing size surrounded by full moons. — View at EarthSky Community Photos. | Niveth Kumar created this cool composite image and wrote: “The moon orbits Earth on an elliptical path. As a result, the distance between the moon and Earth varies throughout the month and the year. The point on the moon’s orbit closest to Earth is called the perigee (supermoon) and the point farthest away is the apogee (micromoon). Here’s a comparison of the size of our moon’s closest and farthest throughout the year, represented along the 12 full moons of 2022.” Thank you, Niveth.

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.

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, the 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.

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January 1, 2023

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Moon Phases

Deborah Byrd

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About the Author:

Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Prior to that, she had worked for the University of Texas McDonald Observatory since 1976, and created and produced their Star Date radio series. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. In 2020, she won the Education Prize from the American Astronomical Society, the largest organization of professional astronomers in North America. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.