The moon’s orbit is an ellipse, not a circle. At perigee it is about 356,500 km away; at apogee about 406,700 km. That is a swing of roughly 14%, and it means the moon’s apparent size in the sky genuinely does vary through the month.
A supermoon is a full moon that happens to fall near perigee. There is no official astronomical definition — the term was coined by an astrologer, Richard Nolle, in 1979 — but the common working threshold is a full moon within about 360,000 km of Earth. On this site that is the threshold used, and it produces one to three supermoons in a typical year: three is the usual maximum, and some years, such as 2026 and 2027, have only one.
How much bigger, really
A perigee full moon is about 14% wider and around 30% brighter than an apogee full moon. Those numbers sound substantial, and in a side-by-side photograph they are obvious.
In the sky they are almost impossible to notice. You never see the two side by side — they are six months apart — and the human visual system is poor at judging absolute angular size without a reference. Someone who did not know a supermoon was scheduled would be unlikely to spot one.
The comparison that helps: hold a coin at arm’s length, about 70 cm away. To grow by the 14% that separates a perigee full moon from an apogee one, you would have to bring it roughly 9 cm closer — a change you would notice immediately with the coin in your hand, and one you have no way to judge on an object you only ever see alone against an empty sky.
What actually makes a moon look huge
When a full moon looks enormous, you are almost certainly seeing the moon illusion — the well-documented effect that makes the moon appear far larger near the horizon than overhead. It is a perceptual effect occurring entirely in the brain; the moon’s measured angular size is very slightly smaller near the horizon, not larger.
You can break the illusion instantly. Look at a low, apparently gigantic moon, then bend over and view it upside down between your legs, or look at it through a cardboard tube that hides the horizon. The moon shrinks back to normal.
Tides
Supermoons do produce measurably higher tides, called perigean spring tides. The effect is real but modest — typically a few centimetres above a normal spring tide. It becomes newsworthy only when it coincides with a storm surge, which is a matter of weather rather than of the moon.