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 whole day side of the moon. That happens only when the moon is opposite the sun in our sky. A full moon is a moon opposite the sun. That’s 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.
Technically speaking, the moon is full at the instant it’s 180 degrees from the sun in ecliptic longitude. Want to know the instant of full moon in your part of the world, as well as the moonrise and moonset times? Click here, remembering to check the moon phases plus moonrise and moonset boxes.
Or you can just stand outside 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.
The moon looks full for at least a couple of night around the instant of full moon.
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 or 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. Click here to translate UTC to your local time.
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 titled 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.
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.
Bottom line: Full moon – when the moon is most opposite the sun for this month – happens on September 25 at 2:52 UTC; translate UTC to your time. Plus, why a full moon looks full.
Check out EarthSky’s guide to the bright planets.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. 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. 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.