Supermoons explained (and why they're overhyped)
About 7% wider than an average full Moon. No eye can tell. Go and look anyway.
4 min read · Published 24 August 2026
Before the detail
A supermoon is a full Moon near perigee, the closest point of the Moon's orbit. It looks roughly 7% wider and 15% brighter than an average full Moon, and about 14% wider than one at apogee. Those differences are real and essentially undetectable by eye without a side-by-side. The term isn't astronomical. An astrologer coined it in 1979 and it has no formal definition.
The actual numbers
The Moon's orbit is an ellipse, so its distance swings between roughly 356,500 km at closest perigee and 406,700 km at furthest apogee.
A full Moon at perigee looks about 14% wider and 30% brighter than one at apogee. Against an *average* full Moon the figures are around 7% and 15%.
Those are real differences, and they're below the threshold anyone notices without a direct comparison. You can't compare, because you can't see two full Moons side by side. Every report of a supermoon looking enormous is something else, described below. Frankly, it's a press release. Go and look anyway, because the horizon Moon is always good.
| Date | Perigee distance | Gap between full and perigee |
|---|---|---|
| 24 December 2026 | 356,641 km | 7 hours |
| 22 January 2027 | 357,273 km | 15 hours |
Those two are the standouts of this period, both within a day of exact perigee. The December 2026 one falls seven hours from perigee, which is about as close a coincidence as the cycle produces.
What people are seeing
The Moon illusion: the Moon appears dramatically larger when near the horizon than when high overhead. This effect is enormous compared with the supermoon difference, and it's entirely a perceptual illusion.
The Moon is the same angular size in both positions. You can verify it: photograph it low and high with the same lens and measure, or hold a small object at arm's length to compare. It's measurably identical, and marginally *smaller* at the horizon because you're slightly further from it.
Explanations for the illusion are still debated in perceptual psychology, with the presence of foreground objects providing false distance cues being the leading account. What isn't debated is that it's an illusion.
So when someone reports a spectacular supermoon, they almost certainly saw a normal full Moon rising behind buildings or trees. Which is a lovely sight, and has nothing to do with perigee.
Supermoons are a press release. Go and look anyway, because the horizon Moon is always good.
What to note
- Tides. Perigean spring tides (when a perigee full or new Moon coincides with alignment) are measurably higher than usual, typically by a few centimetres to tens of centimetres. This is a real, measurable effect and counts for coastal flooding when it combines with a storm surge.
- Brightness for observing. A perigee full Moon is the worst possible night for deep-sky observing. If you are planning a session, it's a night to avoid instead of seek.
- Photography. The size difference is invisible to the eye but does show up in a measured photograph. Comparing a perigee and apogee full Moon at identical focal length is a satisfying project.
- A micromoon, the apogee opposite, gets almost no coverage, which tells you the term is about marketing instead of astronomy.
What it does not do
There is no evidence that supermoons trigger earthquakes, affect human behaviour, influence births, or cause anything beyond slightly higher tides.
The tidal force difference is small in absolute terms, the Moon's gravitational effect on the solid Earth is real but the perigee variation adds little. Studies looking for correlations with seismic activity haven't found effects that hold up.
The behaviour claims are older than the term and equally unsupported. Studies of hospital admissions, birth rates and police incidents across lunar phases consistently find no effect, and the belief persists largely through confirmation bias, memorable nights get attributed to the Moon and unmemorable ones don't.
So should you go and look?
Yes, but go and look at any full Moon rising. The rising is the spectacle, the illusion does the work, and it happens every month.
If a supermoon headline gets you outside looking at the sky, that's a net positive and there's no need to be sour about it. What makes the view striking is the horizon, the foreground and the atmosphere reddening it, not the extra 7%.
What is a supermoon?
A full Moon occurring near perigee, the closest point of its elliptical orbit. It appears about 7% wider and 15% brighter than an average full Moon, and 14% wider than one at apogee.
Can you see the difference in a supermoon?
Not by eye. A 7% difference in width is below the threshold anyone notices without a side-by-side comparison, which is impossible since you cannot view two full Moons at once. It does show up in measured photographs.
Why does the Moon look huge near the horizon?
The Moon illusion, a perceptual effect unrelated to distance. The Moon is the same angular size low or high (marginally smaller at the horizon, in fact) and photographs confirm it. Foreground objects providing false distance cues is the leading explanation.
Do supermoons cause earthquakes or affect behaviour?
No. Studies looking for correlations with seismic activity, hospital admissions, birth rates and police incidents have not found effects that hold up. The only measurable consequence is slightly higher perigean spring tides.
Natalie Hasselbank