Supermoon, blue moon, blood moon: what they are, and the next dates

Supermoon: a full moon near perigee

The Moon's orbit is an ellipse: its distance from Earth swings between about 356,000 and 407,000 km each month. A full moon that falls near the closest point, the perigee, looks up to 14% wider and about 30% brighter than one at the farthest point, the apogee. That is a supermoon. The word was coined by an astrologer in 1979 and adopted by the press decades later; astronomers simply say perigee full moon.

There is no official threshold, which is why lists disagree. The strictest common rule counts a full moon closer than 360,000 km; Fred Espenak's widely used tables count any full moon within 90% of that month's closest approach, which admits a few more. The table gives the strict reading; the next paragraph gives the other.

Espenak's rule also admits the full moons of 3 January 2026 and 24 November 2026, both a little over the line; most newspaper lists follow it, which is why they announce three or four supermoons a year where the strict rule finds one or two.

One caveat, worth stating: side by side, the difference would be obvious; on its own in the sky, a supermoon looks like a full moon. The impression of a giant Moon comes from seeing it near the horizon, and that is an illusion available at every full moon. The opposite case, a full moon at apogee, is sometimes called a micromoon: the blue moon of 31 May 2026 was one, the farthest full moon of its year.

Blue moon: a matter of calendars

The Moon does not turn blue. A blue moon is a calendar accident, and there are two rules. The popular one, born of a mistake in a 1946 Sky & Telescope article, calls blue the second full moon in a calendar month: it happens every two to three years, and depends on your time zone when the instant falls near midnight. The older rule, from the Maine Farmer's Almanac, calls blue the third full moon of a season that holds four instead of three.

• Last monthly blue moon: 31 May 2026. Next: 31 December 2028, which is also a total lunar eclipse, a blue moon and a red one on the same night.
• Next seasonal blue moon: 20 May 2027, the third of the four full moons of spring 2027.

Genuinely blue-tinted moons exist, and they are rarer than either kind of calendar blue moon: they follow volcanic eruptions or great forest fires, when particles of just the right size in the air scatter red light and let the blue through. Krakatoa in 1883 turned the Moon blue for months.

Blood moon: a total lunar eclipse

A blood moon is a total lunar eclipse, when the full moon passes entirely through the Earth's shadow. It does not go dark: the Earth's atmosphere bends the light of every sunrise and sunset on the planet onto the Moon at once, and the disc turns copper, orange or deep red for an hour or more. Unlike a solar eclipse, it is visible from the whole night side of the Earth, safely, with no equipment.

• Last total lunar eclipse: 3 March 2026.
• Coming eclipses: partial on 28 August 2026, then three penumbral ones in 2027 (20 February, 18 July, 17 August) that only shade the Moon slightly, then partial on 12 January and 6 July 2028.
• Next total lunar eclipse, the next blood moon: 31 December 2028.