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). In 2018, that full moon instant arrives on December 22 – less than one day after the December solstice – at 17:49 UTC. At U.S. time zones, that places the full moon instant at 12:49 p.m. Eastern Time, 11:49 a.m. Central Time, 10:49 a.m. Mountain Time, 9:49 a.m. Pacific Time, 8:49 a.m. Alaskan Time and 7:49 a.m. Hawaiian Time on December 22.
The solstice is December 21 at 22:23 UTC. So the solstice and full moon fall less than one day apart. The last time that happened was in 2010, and the next time will be 2029.
What does it mean? Only that – although the Northern Hemisphere has its longest winter night on December 21 – the lamp of a nearly full moon will light up the nighttime from dusk until dawn.
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.
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 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.
Bottom line: Full moon – when the moon is most opposite the sun for this month – happens on November 23, 2018 at 05:39 UTC; translate UTC to your time. That means that – for the Americas – the moon will appear fullest on the night of November 22.
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.
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.