What Does a Full Moon Mean?

A full moon is the moment the Moon reaches the far side of Earth from the Sun, so sunlight strikes the face turned towards us almost head on and the entire disc appears lit. It is one instant rather than a night, and it comes round roughly every 29.5 days.

What is actually happening

The Moon makes no light of its own. Every part of it you have ever seen is reflected sunlight, and one half of the Moon is lit at all times. The phases are nothing more complicated than how much of that lit half happens to be pointing our way as the Moon travels round its orbit.

At full we are looking straight at the lit half. The line-up is close but never exact, and the inexactness matters. The path the Moon follows is tilted by about five degrees against the path Earth takes around the Sun, so in most months the Moon slips above or below the shadow Earth throws behind it. When it does pass through that shadow, the result is a lunar eclipse, which is why an eclipse of the Moon can only ever occur at full moon and still does not occur most months.

You can check the geometry with your own eyes. A full moon comes up as the Sun goes down, stands highest around the middle of the night and sinks as morning arrives. It is the only phase that is in the air all night long, and a moon riding high overhead at teatime is not a full one, whatever the calendar claims.

Why it looks full for three nights

The exact instant of fullness is timed to the minute, and it will often fall while the Moon is below your horizon or the sky where you live is bright. What you actually see are the nights on either side of it. Nobody can tell a disc lit to 98 per cent from one lit to 100 per cent by eye, so the Moon reads as full on three evenings running and people cheerfully disagree about which was the real one.

That same precision explains why listings disagree. A fullness timed at 01:20 on a Thursday by the universal clock is Wednesday evening in New York and Thursday lunchtime in Tokyo. Neither entry is wrong. They are the same instant written down on different local clocks, which is why a calendar is only useful if it is set to your own part of the world.

The sign every full moon lands in

Because the Moon has to stand opposite the Sun in order to be full, it is always in the sign opposite the one the Sun is passing through. While the Sun is in Taurus, the full moon of that month falls in Scorpio. While the Sun is in Sagittarius, the full moon falls in Gemini. The rule never varies, because it is geometry rather than convention.

This is why a full moon is always labelled with a sign other than the one whose season you are living in, a detail that confuses a lot of people the first time they meet it. It also gives every full moon the same underlying shape in the tradition: a matched pair of opposite signs occupied at the same time, one holding the Sun and one holding the Moon.

What the tradition reads into it

Where a new moon is treated as a starting point, the full moon has for a very long time been read as a peak: the stage at which whatever was set going becomes visible, arrives, or shows what it was worth. The reading follows the picture overhead fairly directly. Everything is lit, nothing is hidden, and the two brightest objects in the sky are pulling from opposite ends of it.

In practice the date gets used as a marker for four things:

• Finishing. Clearing the last of something already under way, rather than opening a new file on it.
• Seeing plainly. Information that had been missing is often described as surfacing around a full moon, so it is treated as a good moment for an honest look at where something stands.
• The pull of two demands. Sun and Moon at opposite points is read as the familiar squeeze between two genuine needs, work against home, other people against yourself, with the task being to hold both rather than to choose.
• Letting go. The commonest modern ritual use, running from writing down a habit and burning the paper to simply saying out loud what you are finished with.

These are meanings laid over a dependable natural rhythm, not effects anyone has demonstrated. What they are good for is the prompt. A date that turns up every month and asks you to review what you started is worth something whether or not the Moon has any hand in the answer.

Supermoons, blue moons and harvest moons

The orbit of the Moon is an oval rather than a circle, so its distance from us changes over the course of a month. A full moon at the near end of that oval looks about 14 per cent wider and roughly 30 per cent brighter than one at the far end, and that is what gets called a supermoon. The catch is that you cannot see the difference without a photograph of the other kind held beside it. On its own, in an empty sky, every full moon looks like a full moon.

A blue moon is a naming quirk with no colour in it at all: the second full moon to fall inside a single calendar month, which comes round every two or three years because 29.5 days fits inside most months with several days to spare. A harvest moon is the full moon nearest the September equinox in the northern hemisphere, and it earned its name honestly, since around that date it rises soon after sunset for several evenings in a row and handed farmers extra light to finish cutting by.

Does it change how people behave?

The claims repeated most often, more births, more accidents, more sleepless nights, have been tested many times over and the results come back weak and contradictory. There is no settled evidence for a lunar effect on human behaviour, and anyone telling you otherwise is well ahead of what has been shown.

Two plainer things are going on. A full moon genuinely floods the night with light, and for most of human history that was the difference between travelling after dark and staying where you were, which is reason enough for the date to have carried weight long before anyone wrote a horoscope. Then expectation finishes the job: once you know tonight is the full moon, a restless night gets filed under the full moon, while an identical restless night a fortnight earlier was simply a bad night.