What Is a Blue Moon (and a Supermoon)?

"Blue moon" and "supermoon" get used a lot, often loosely. Neither is an official astronomical term, but both describe real, predictable events. Here is what each one actually means.

A blue moon has two definitions

There are two common meanings, and both are about timing, not colour:

• Monthly blue moon — the second full moon in a single calendar month. Because the cycle of phases is about 29.5 days and most months are a little longer, occasionally two full moons squeeze into one month. The second one is the "blue moon".
• Seasonal blue moon — the older definition: the third full moon in a season that has four (a season usually has three). This is the original sense the term came from.

Either way, a blue moon looks exactly like any other full moon.

So why "blue"?

The phrase is just an idiom for rarity — "once in a blue moon". The moon can genuinely appear bluish, but only when the air carries smoke or volcanic ash with particles of a specific size that scatter away red light. That is a rare atmospheric effect and has nothing to do with the calendar-based blue moon.

What is a supermoon?

The moon’s orbit is not a perfect circle — it is slightly oval, so the moon’s distance from Earth varies. The closest point is called perigee and the farthest is apogee. When a full moon happens near perigee, we call it a supermoon. It can appear roughly 14% larger and 30% brighter than a full moon at apogee.

In practice the size difference is hard to spot with the naked eye, because you have nothing to compare it against in the sky. The extra brightness is easier to notice. A supermoon is also why the famous "moon illusion" — the moon looking huge near the horizon — gets so much attention, though that illusion happens at every full moon, super or not.

How often do they happen?

Monthly blue moons come roughly every two to three years. Supermoons are more frequent — typically three or four full moons a year qualify, since several full moons each year land reasonably close to perigee. Occasionally the two coincide, producing a "super blue moon".

You can see the dates of every upcoming full and new moon — and spot when two full moons fall in the same month — with the Moon Phase tool. New to the cycle? Start with moon phases explained.