Full moon – when is the next full moon?

There is a full moon when Earth lies between the Sun and the Moon, so the entire side of the Moon facing us is lit. The full moon rises around sunset, is highest at midnight and sets around sunrise – which is why it shines all night. Below you can see the upcoming full moons, calculated astronomically.

How often is there a full moon?

There is a full moon about every 29.5 days – roughly one full moon per month. Sometimes two full moons fall in the same calendar month; the second is popularly called a «blue moon». Between two full moons the Moon passes through new moon and the other moon phases.

Why does the full moon sometimes look bigger?

When the full moon is low over the horizon it can look unusually large. This is an optical effect known as the «moon illusion». When the Moon is also near Earth in its elliptical orbit, it is called a supermoon, which can appear slightly larger and brighter than usual.

See also the moon today with moonrise and moonset for your location, or the whole month in the moon calendar.

Why the full moon looks bigger near the horizon

A full moon just above the rooftops looks markedly larger than the same moon high in the sky a few hours later. Measured with a ruler it is not — it is in fact slightly smaller, because it is further away when it sits on the horizon. The effect is called the moon illusion, and it happens in perception rather than in the atmosphere.

The most widely accepted explanation is that the brain judges size relative to surroundings. At the horizon it has trees, buildings and landscape to compare against; high in the sky it has nothing. The illusion disappears if you view the moon through a tube, or bend down and look at it between your legs.

The colour, on the other hand, is real. Near the horizon moonlight travels through far more atmosphere, which scatters the blue away and lets red and orange through — exactly the mechanism that makes a sunset red.

The names, and the one that means something

Most of the traditional full moon names are folklore — wolf moon in January, worm moon in March, sturgeon moon in August. They are pleasant, they are not astronomy, and different cultures used entirely different sets.

One is different. The harvest moon, the full moon nearest the September equinox, is named for a real and useful effect. Normally moonrise slips about 50 minutes later each night. Around the September equinox at northern latitudes it slips by far less — as little as 20 to 25 minutes in Britain or Denmark — so for several consecutive evenings a bright moon comes up close to sunset, giving unbroken light from day into night. Before machinery and floodlights, that was several extra evenings of usable harvesting time.

The cause is geometry rather than luck. The delay in moonrise depends on the angle the moon's path makes with the horizon, and in autumn at northern latitudes that path lies unusually flat. A flat path means the moon's nightly eastward shift translates into only a small delay in rising time. In spring the same geometry works the other way and moonrise can slip by well over an hour a night. The hunter's moon, the following month, is the same effect one cycle later.

How much bigger is a supermoon, really

The moon's orbit is an ellipse, so its distance varies between roughly 356,500 km at perigee and 406,700 km at apogee — a difference of about 14%. When a full moon coincides with perigee, the disc is around 14% wider and up to about 30% brighter than a full moon at apogee.

Those numbers sound dramatic and mostly are not, for a simple reason: you never see the two side by side. A 14% difference in width is easy to measure and almost impossible to notice from memory, six months apart. Photographs comparing a supermoon with a micromoon are genuine, but they compress into one frame a comparison your eye never actually gets to make.

The moon illusion described above is a much larger perceptual effect than the supermoon is a real one — which is why a very ordinary full moon rising behind a skyline usually looks far more impressive than a supermoon overhead.

Why the full moon is high in winter and low in summer

A full moon sits opposite the sun in the sky, and that has a consequence worth knowing: it takes the path the sun will take six months from now.

So in December, when the midday sun barely clears the rooftops, the full moon rides high across the sky all night and floods the landscape with light — which, with snow on the ground, is bright enough to walk by comfortably. In June the reverse: the full moon skims low along the southern horizon, up for only a few hours, often orange from the thickness of atmosphere it is shining through. Northern midsummer full moons are low, brief and rarely spectacular, and this is why.

The same reasoning explains why a full moon rises at sunset and sets at sunrise, and why its time above the horizon is roughly the night length — the mirror of the day length for that date.

Does a full moon affect sleep?

This is the most-asked question about the moon, and the honest answer is that the evidence is thin and mixed. A handful of small studies have reported slightly shorter or later sleep around the full moon; larger studies have generally failed to replicate the effect, and the most plausible mechanism left standing is the obvious one — a full moon is bright, and bright light delays sleep.

What is not supported is any influence on behaviour, birth rates or mood. The tidal effect of the moon on a human body is vanishingly small; the moon raises tides in oceans because oceans are enormous, not because the force is strong. Tides themselves, on the other hand, genuinely do peak around full and new moon, when the sun and moon pull in line — the spring tides. For dates, see the list above, or the whole cycle in the moon calendar.

Frequently asked questions about the full moon

When is the next full moon?

See the upcoming full moons in the list above. The dates are calculated astronomically and updated continuously.

How often is there a full moon?

There is a full moon about every 29.5 days, roughly once a month.

What is a blue moon?

A blue moon is when two full moons fall in the same calendar month. It happens because the 29.5-day lunar cycle is slightly shorter than most calendar months.