Supermoons explained (and why they're overhyped)
About 7% wider than an average full Moon, which is not a difference any eye can detect.
4 min read · Published 24 August 2026
The short answer
A supermoon is a full Moon occurring near perigee, the closest point of the Moon's orbit. It appears roughly 7% wider and 15% brighter than an average full Moon, and about 14% wider than a full Moon at apogee. Those differences are real and essentially undetectable by eye without a side-by-side comparison. The term is not astronomical — it was coined by an astrologer in 1979 and has no formal definition.
The actual numbers
The Moon's orbit is elliptical, so its distance varies between roughly 356,500 km at closest perigee and 406,700 km at furthest apogee.
A full Moon at perigee appears about 14% wider and 30% brighter than one at apogee. Against an *average* full Moon the figures are around 7% and 15%.
Those are genuine differences and they are also below the threshold at which anyone notices without a direct comparison. You cannot compare, because you cannot see two full Moons side by side. Every reported observation of a supermoon looking enormous is something else, described below.
| 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 actually 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 is 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 is measurably identical, and marginally *smaller* at the horizon because you are 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 is not debated is that it is 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 genuinely lovely sight, and has nothing to do with perigee.
What is genuinely worth noting
- 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 matters 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 is a night to avoid rather than 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 rather than 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 have not 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 do not.
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 is a net positive and there is no need to be sour about it. It is worth knowing that what makes the view striking is the horizon, the foreground and the atmosphere reddening it — not the extra 7%.
Common questions
- 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 actually 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.