news sciences: Why does a full moon look full?

This is last year’s full Beaver Moon. That’s a Northern Hemisphere name for the full moon in November. It’s from John Entwistle Photography, who was at the Twin Lights Lighthouse, Jersey Shore, New Jersey. Read more about full moon names.

Full moon falls on November 23, 2018 at 05:39 UTC. That means that – for the Americas – the night of November 22 will feature this month’s fullest moon.

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.

So 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.

A full moon is opposite the sun. We see all of its dayside. Illustration via Bob King.

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.

A full moon normally passes above or below Earth’s shadow, with no eclipse. Illustration by Bob King.

Full moon and Earth’s shadow on the morning of March 2, 2018, via Eliot Herman in Tucson, Arizona.

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

Man and moonset by Martin Marthadinata in Surabaya, East Java, Indonesia.

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.

Check out EarthSky’s guide to the bright planets.

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