Blue Moon and Supermoon — What the Names Mean

News stories often call a full moon a “blue moon” or a “supermoon.” Both names are about timing and distance, not about how the Moon looks at first glance. This page explains what each name means, using NASA’s own explanations.

Blue moon — the monthly meaning

The meaning most people use today is the monthly one. A monthly blue moon is the second full moon in a single calendar month.

The Moon’s cycle is 29.5 days. That is just a bit shorter than the average length of a calendar month. Sooner or later, a full moon falls on the 1st or 2nd of a month, with enough days left for a second full moon on the 30th or 31st. NASA says monthly blue moons always occur in the last two or three days of the month.

Blue moon — the seasonal meaning

The older meaning is the seasonal one. A season — spring, summer, fall, or winter — usually has three full moons. When a single season has four full moons instead of the usual three, the third moon in that lineup is called a blue moon.

Past observers used the name this way. NASA notes that in a season with four full moons instead of three, the third full moon used to be — and sometimes still is — called “blue.”

How the monthly meaning started

NASA says the monthly definition is recent, and calls it modern folklore. The modern definition sprang up in the 1940s.

In those days, the Maine Farmer’s Almanac offered a definition of blue moon that many astronomers struggled to understand. In 1946, Sky & Telescope published an article called “Once in a Blue Moon.” The author, James Hugh Pruett, cited the 1937 Maine almanac and wrote that the second full moon in a month is called a blue moon. NASA says that reading was not correct, but it could be understood — and so the modern blue moon was born.

How often blue moons happen

NASA says a second full moon in the same month happens every two to three years. NASA also describes blue moons as occurring roughly once every two and a half years.

Blue moons are uncommon but not extremely rare. NASA says only 6 percent of full moons are blue moons, counting both the seasonal and monthly kinds.

Does a blue moon look blue?

No. NASA is clear that the name is about the full moon’s timing, not its color. Most blue moons look pale gray and white, just like the Moon on any other night.

On rare occasions, the Moon really can appear blue. NASA says tiny particles in the air — typically of smoke or dust — can scatter away red wavelengths of light, causing the Moon to appear blue.

NASA’s examples need big events. A truly blue-looking Moon usually requires a volcanic eruption, such as Krakatoa in 1883. NASA also lists reports after the eruptions of El Chichon in 1983, Mt. St. Helens in 1980, and Mount Pinatubo in 1991. Forest fires can do the same thing, because their smoke can hold particles of just the right size.

What is a supermoon?

A supermoon is a full moon that happens when the Moon is at or near perigee. Perigee is the Moon’s closest point to Earth in its orbit.

The Moon travels around Earth in an elliptical orbit — an elongated circle — not a perfect circle. Each month, the Moon passes through perigee, its closest point, and apogee, its farthest point. NASA gives perigee as about 226,000 miles (363,300 km) from Earth, and apogee as about 251,000 miles (405,500 km), reached during every 27-day orbit.

NASA notes that “supermoon” is not an official astronomical term. It is typically used for a full Moon that comes within at least 90 percent of perigee. NASA also describes a supermoon as a full Moon that occurs within a day or so of perigee.

How much bigger and brighter is a supermoon?

NASA says the Moon can look up to 14 percent bigger at perigee than at apogee. At its closest point, the full Moon can appear up to 14 percent bigger and 30 percent brighter than the faintest Moon of the year, which occurs when the Moon is farthest from Earth.

You probably will not notice a big difference in size. NASA compares the size difference to the difference between a quarter and a nickel. Because the Moon is a bit closer than usual, it will appear a bit brighter than usual.

How often supermoons happen

NASA says supermoons happen three to four times a year, and they always appear consecutively. About 25 percent of all full moons are supermoons.

Through most of Earth’s orbit around the Sun, perigee and the full Moon do not overlap. NASA also notes that because a supermoon is at its closest approach to Earth, it can cause higher tides than usual.

When a moon is both

Sometimes a supermoon is also a blue moon. NASA calls that combination a super blue moon. NASA says the time between super blue moons is quite irregular — it can be as much as 20 years — but in general, 10 years is the average.