A full moon is only truly full for an instant, and rarely completely
We talk about a full moon as if it lasted all night, yet the phase peaks at a single moment, often while the Moon sits below the horizon. And because its orbit is tilted about five degrees, the disc is almost never perfectly lit, except on the nights it slips into Earth's shadow during a lunar eclipse.
Full moon happens when Earth lies between the Sun and the Moon, so the whole near side is sunlit. The cycle from one full moon to the next averages about 29.53 days, but the Moon's uneven orbit can shift a given full moon by up to thirteen hours either side of the average. Time zones add more wobble, which is why printed calendars can disagree by a day. Almanacs therefore give the exact instant, usually in Coordinated Universal Time.
That tilted orbit also explains why lunar eclipses are not monthly events. Usually the full moon passes just above or below Earth's shadow; only when it falls near one of the two points where its path crosses Earth's orbital plane does it turn coppery red, coloured by sunlight bent and filtered through our atmosphere. That lines up roughly every six months. For astronomers, full moon is an awkward time: shadows vanish from the lunar surface, and the glare drowns out many stars.
Cultures have long organised time around it. Buddhists celebrate Vesak on the full moon of the month of Vaisakha, marking the Buddha's birth, enlightenment and death, and Hindus observe various Purnima days. Old month names were once names of moons; the monk Bede, writing in 725, matched Old English lunar months to their Roman equivalents.
Folklore links the full moon with sleeplessness, werewolves and madness, which is where the word lunatic comes from. Psychologists find no strong evidence for any behavioural effect. In December 2000 the British Medical Journal published two studies of dog bites side by side: one from a Bradford hospital found bites twice as common at full moon, while one from Australian hospitals found them less likely.
Source: Full moon
