Supermoon
What is a supermoon?
A supermoon is a full moon that lands near perigee, the closest point of the Moon’s orbit. The orbit is an ellipse: the distance swings between roughly 357,000 and 407,000 km, about 14% end to end, so a full moon at the near end looks a little bigger and meaningfully brighter than one at the far end. “Supermoon” is not an astronomer’s term and has no official definition — which is why the supermoon list on this site names the threshold it uses and prints every full moon’s distance beside it, so you can see the choice being made.
What you would actually notice
Side by side, the year’s closest full moon is about 14% wider than its most distant one — a coin next to a very slightly larger coin. From memory, a month apart, you would struggle to call it. Brightness is the real difference: light grows with the area of the disc, so the closest full moons run around 30% brighter than the farthest. And the famous giant moon sitting on the horizon is a different thing entirely — the phase is real, but the horizon-hugeness is an illusion your brain adds at any distance.
A coincidence of two cycles
The Moon passes its closest point every single orbit, so a supermoon is not a rare approach — it is a coincidence of timing, full phase and closest approach landing together. The cycle of phases (about 29.5 days) runs slightly longer than the orbit’s own close-far cycle, so the full moon slowly slides around the ellipse, and roughly every fourteen months a run of two or three full moons lands near the close end.
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The tides notice more than you do
The Moon’s tide-raising grip strengthens faster with closeness than its brightness does. A full moon is already a spring-tide alignment; a full moon at perigee stacks the two, and those “perigean spring tides” run measurably higher — they are the king tides that make coastal news. If flood warnings follow a supermoon by a day or two, that is the same coincidence of cycles, read from the beach.
See it live
On the hub: Moon