Blue Moon — May 20, 2027 (Seasonal)

What this countdown tracks

The clock counts down to the seasonal Blue Moon of Thursday, May 20, 2027 — the third full moon of an astronomical season that contains four full moons, and the rarer of the two Blue Moon definitions.

About this celestial event

A seasonal Blue Moon is the original definition, codified in the Maine Farmers' Almanac in the early 20th century. An astronomical season — between solstice and equinox or vice versa — usually contains three full moons. About every 2.5 to 3 years, a season contains four; the third of these four is the seasonal Blue Moon. The 2027 northern-hemisphere astronomical spring (March 20 to June 21) contains four full moons (March 22, April 20, May 20, June 19), making the May 20 full moon the seasonal Blue Moon.

This is a different definition from the more common "second full moon in a calendar month" version (the calendar Blue Moon). Both definitions are now in general use, but the seasonal definition is older and is the original meaning of the phrase "once in a blue moon."

The Moon will not actually appear blue. The colour name has multiple folk-etymological origins — possibly from the Old English belewe, meaning "betray," referring to a "betrayer moon" that disrupted the calendar count of full moons in an ecclesiastical year — and has only loosely been associated with the rare reddish-blue tint produced by volcanic ash or wildfire smoke at unusual times.

What to expect

The May 20, 2027 Blue Moon reaches full phase visible all night from rise to set. The Moon will appear entirely normal — silver-grey with a yellow or orange cast near the horizon. Best viewing is moonrise on the evening of May 20 from an open eastern horizon, and through the night with the Moon high in the south.

Because seasonal Blue Moons are rarer than calendar Blue Moons, they tend to attract more attention from astronomy clubs and educators. Major observatories run Blue Moon evenings.

Past blue moons

• May 31, 2026 — calendar Blue Moon (second full moon of May 2026)
• August 31, 2023 — calendar Blue Moon (and supermoon)
• August 22, 2021 — seasonal Blue Moon (third of four full moons in summer 2021)
• October 31, 2020 — Halloween calendar Blue Moon, first since 1944
• March 31, 2018 — second of two calendar Blue Moons in 2018
• January 31, 2018 — Super Blue Blood Moon (calendar Blue Moon, supermoon, lunar eclipse)
• August 21, 2013 — seasonal Blue Moon (third of four full moons in summer 2013)

How to observe

NASA's Daily Moon Guide, EarthSky, Sky & Telescope and timeanddate.com publish moonrise/set times for thousands of cities. Stellarium and SkySafari give precise local timings. The Virtual Telescope Project usually streams Blue Moon events over Rome. No equipment is needed; the Blue Moon is enjoyed naked-eye.

Related countdowns

The May 2027 seasonal Blue Moon follows the Blue moon May 2026 (calendar variety) by exactly one year. Pair with the Eta Aquariid meteor shower 2026 and June solstice 2027 on the late-spring sky calendar.

FAQ

When is the May 2027 Blue Moon? Thursday, May 20, 2027 — the third full moon of the four-full-moon spring 2027 astronomical season. Why is this called a 'seasonal' Blue Moon? Because it follows the original Maine Farmers' Almanac definition: the third of four full moons in a single astronomical season. Will the Moon look blue? No. The "Blue Moon" name is a calendar quirk, not a description of the Moon's colour. How often do seasonal Blue Moons happen? Roughly every 2.5 to 3 years; the next after May 2027 is in August 2032.