Why the Moon Isn't Always the Same Distance Away

Quick answer: The Moon's orbit is an ellipse, not a circle, so its distance from Earth swings between roughly 356,500 km at perigee (closest) and 406,700 km at apogee (farthest) over the course of each 27.3-day orbit — a difference of about 14%. Check today's exact Earth–Moon distance live.

Like every other orbit in the solar system, the Moon's path around Earth is an ellipse rather than a perfect circle. The average Earth–Moon distance is often quoted as 384,400 kilometres, but "average" masks a real and fairly large monthly swing: at perigee the Moon can come as close as about 356,500 kilometres, while at apogee it recedes to roughly 406,700 kilometres.

That 50,000-kilometre difference — more than the Moon's own diameter times fourteen — is large enough to be noticeable, both to careful observers and to anyone comparing photographs of a full moon taken months apart.

A supermoon is simply a full moon (or new moon) that happens to coincide closely with perigee. Because the Moon is genuinely closer to Earth at that point in its orbit, it appears up to about 14% larger in diameter and up to roughly 30% brighter than a full moon at apogee — sometimes called a "micromoon." The effect is real but modest; most people can't reliably tell a supermoon apart from an ordinary full moon without a side-by-side comparison, since the eye and brain are poor at judging the absolute size of a disc in the sky.

To complicate things further, the Moon's orbit is itself slowly rotating — a phenomenon called apsidal precession, which takes about 8.85 years to complete a full cycle. Combined with the gravitational tug of the Sun, this means even perigee distances vary slightly from one month to the next: some perigees bring the Moon closer than others, which is why headlines periodically describe a "biggest supermoon in years."

On top of the monthly perigee-apogee cycle, the Moon is very gradually spiralling away from Earth, by about 3.8 centimetres per year, as tidal interactions transfer angular momentum from Earth's rotation to the Moon's orbit. Over human timescales this is imperceptible, but over hundreds of millions of years it measurably lengthens both the day and the month.