Blue Moon: What It Is & When the Next One Is
A blue moon is an extra full moon under one of two calendar definitions. It can be the second full moon in a calendar month or the third of four full moons between an equinox and the following solstice.
Despite the name, the Moon won’t normally look blue. A blue moon is about timing, not color.
When is the next blue moon?
The next blue moon reaches its full phase on May 20 at 6:59 a.m. EDT, or May 20 at 3:59 a.m. PDT. It qualifies as a seasonal blue moon because it is the third of four full moons between the March equinox and the June solstice.
The fact sheet above shows the corresponding time and classification for your location. For a monthly blue moon, your time zone can determine whether the event qualifies at all.
Like any full moon, a blue moon will appear nearly full on the evenings before and after the exact phase. The event doesn’t create a sudden change in the Moon’s appearance.
Upcoming blue moons
What is a blue moon?
There are two accepted ways to define a blue moon. Both arise because the 29.5-day cycle from one full moon to the next doesn’t fit neatly into our months, seasons, and calendar years.
- Monthly blue moon: The second full moon in one calendar month. Does time zone matter? Yes—the full phase can fall on different local dates.
- Seasonal blue moon: The third full moon in an astronomical season containing four. Does time zone matter? No—the season and full phase are defined by the same global instants.
Monthly blue moon
Most calendar months are slightly longer than one lunar cycle. If a full moon falls near the beginning of a month, there may be enough time for another one near the end.
Because a monthly blue moon depends on a local calendar date, a full moon can be the second of the month in New York but fall on the previous date somewhere farther west—so make sure to check in your local time zone.
Seasonal blue moon
Astronomical seasons run from one equinox or solstice to the next and normally contain three full moons. Every few years, one season fits four, and the third is called the seasonal blue moon.
Why the third instead of the fourth? Calling the extra moon the third allowed the season’s final full moon to keep its usual place in the old naming sequence.
How often does a blue moon happen?
Monthly and seasonal blue moons each occur about once every two or three years. They don’t follow a simple every-other-year pattern because neither the calendar year nor the astronomical seasons contain a whole number of lunar cycles.
February can’t contain a monthly blue moon because even in a leap year it is shorter than the average 29.5-day full-moon cycle. In some years February has no full moon at all, leaving January and March with two each—a much rarer arrangement sometimes called a double blue moon.
Why is it called a blue moon?
The phrase “once in a blue moon” described something rare or long delayed before either modern calendar definition appeared. In 1937, the Maine Farmers’ Almanac used blue moon for the third full moon in a season containing four—an unusual naming rule that was clarified decades later.
The monthly definition came from amateur astronomer James Hugh Pruett’s interpretation of that rule in a 1946 issue of Sky & Telescope. A contemporary note in Nature mentioned his article alongside reports of Moons that really did appear blue. The new meaning then spread through radio, reference books, and popular culture.
Does a blue moon actually look blue?
Usually, no. A monthly or seasonal blue moon looks like an ordinary full moon and may appear yellow or orange near the horizon for the same reason sunsets look warm in color.
Very rarely, smoke or volcanic material in Earth’s atmosphere contains particles that scatter red light more strongly while allowing more blue light through. Observations collected after the 1883 Krakatoa eruption included blue, green, and other unusual appearances of the Sun and Moon, sometimes months after the eruption.
That optical effect has nothing to do with the calendar. A Moon can appear blue during any phase if the atmospheric conditions happen to be right.
How to see a blue moon
Use the moonrise time and direction in the fact sheet to find a clear view of the horizon. No special equipment is required, and the Moon will remain visible for most of the night if the weather cooperates.
If the blue moon is also a supermoon or blood moon, those terms describe its distance from Earth or an eclipse happening at the same time. The blue-moon label itself won’t change what you see.
When moon names stack up
- A super blue moon is both a blue moon and a supermoon.
- A blue blood moon is a blue moon that occurs during a total lunar eclipse.
- A super blue blood moon satisfies all three descriptions at once.
The stacked names can sound like separate astronomical events, but each part answers a different question: where the full moon falls in the calendar, how close it is to Earth, and whether it passes through Earth’s shadow.