The Moon Supermoon
Supermoon
Another name for a perigee full Moon
A full Moon that falls near perigee, and so looks larger and brighter.
- Perigee distance
- About 356,500 km
- Apogee distance
- About 406,700 km
- Difference in apparent size
- About 14%
01 / Overview
Meet Supermoon
The Moon’s orbit is an ellipse, with perigee near 363,000 km and apogee near 407,000 km. A full Moon close to perigee is about 14% wider and some 30% brighter than one at apogee. That is what the press calls a supermoon: a popular label rather than an astronomical term.
Can the eye tell?
Looking at one full Moon on its own, you cannot really tell: the sky offers nothing to compare against, and a 14% difference in diameter only shows side by side. What actually makes the Moon look enormous is the moon illusion near the horizon, which has nothing to do with distance.
Are the tides bigger?
At a perigee full Moon the tidal pull of Sun and Moon adds up, and the range does run slightly above average — a perigean spring tide. The increase is usually a few centimetres to a few tens of centimetres, and without a storm surge on top it makes no practical difference.
02 / Key data
Core parameters
- Perigee distance
- About 356,500 km
- Apogee distance
- About 406,700 km
- Difference in apparent size
- About 14%
- Difference in brightness
- About 30%
- Recurrence
- Three or four times a year
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- Frame the Moon with a building or a ridge; only then does the size difference read.
- Shoot soon after moonrise to use both the moon illusion and the landscape.
- One frame at perigee and one at apogee with the same gear makes the clearest comparison.
04 / Sources