Supermoon
What is a supermoon?
A supermoon is a full moon near the Moon’s closest approach to Earth. It looks slightly larger and brighter than a full moon near the farthest part of the orbit. There is no single official distance cutoff.
Try the question
How much difference does Moon distance make?
Move from the closest to the farthest part of the orbit. Compare apparent width and brightness side by side.
Watch for: The size change is subtle; the brightness change is easier to measure.
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.