Home   Sun & Moon   Moon   Blue Moon

When Is the Next Blue Moon?

There are two definitions of a Blue Moon in astronomy; both are a type of Full Moon. If the moon actually looks blue, it's caused by a rare type of dust in the atmosphere.

Illustration image

If the Moon looks blue, it is caused by dust.

When volcanoes erupt, the dust particles can cause the Moon to appear blue.


The term 'once in a Blue Moon' means that that something is very rare. But just how rare, depends on your definition.

In astronomy, a Blue Moon is a Full Moon, which doesn't quite fit in with the months in our calendar. However, there are 2 completely different ways of calculating which Full Moon is a Blue Moon.

2 Different Definitions

  1. Blue Moon = The 3rd Full Moon in an astronomical season with 4 Full Moons
    (versus the normal 3)
  2. Blue Moon = The 2nd Full Moon in a month with 2 Full Moons.

Find Time & Date

On our Moon Phase Pages, you'll find your local and worldwide times and dates for both of the Blue Moon definitions, along with Moon Phases and lunations, Supermoons, and Black Moons, which are similar definitions for the New Moon.

Defining the Original Blue Moon

The original astronomical definition is that a Blue Moon is the 3rd Full Moon in an astronomical season with 4 Full Moons. A normal year has 4 astronomical seasons - spring, summer, fall (autumn), and winter - with 3 months and normally 3 Full Moons each.

When 1 of the astronomical seasons has 4 Full Moons, instead of the normal 3, the 3rd Full Moon is called a Blue Moon.

Blue Colored Moon

Astronomical Blue Moons happen either once every 2 to 3 years or so, depending on which of the definitions you apply.

A Moon that actually looks blue, however, is a very rare sight. The Moon, full or any other phase, can appear blue when the atmosphere is filled with dust or smoke particles of a certain size; slightly wider than 0.7 micron. The particles scatter the red light, making the Moon appear blue in color. This can happen for instance after a dust storm, a forest fire, or a volcanic eruption.

Eruptions like on Mt. Krakatoa, Indonesia (1883), El Chichon, Mexico (1983), Mt. St. Helens (1980) and Mount Pinatubo (1991) are all known to have caused blue moons.

Are Red Moons Rare?

Red Moons, which can be caused by other sizes of dust particles or by Total Lunar Eclipses, are much more common than Blue Moons.

Topics: Astronomy, Moon, Calendar, Months

Next Blue Moons:

2018Jan 312nd Full Moon in a month
2018Mar 312nd Full Moon in a month
2019May 183rd Full Moon in season

Blue Moons can vary by time zone. Dates above are based on the local time in Washington DC. Change location


Moon Phases In Your City

The Moon

  1. What Is a Supermoon?
  2. The Moon Phases
  3. What Is a Micro Moon?
  4. Is a Blue Moon Blue?
  5. The Moon's Orbit
  6. What Is a Black Moon?
  7. What Are Moonbows?
  8. Full Moon Names
  9. Taking pictures of the Moon

Moon index

Moonrise & Moonset Times

You might also like

Illustration showing all the different Moon phases next to one another

The Lunar Month

A lunar month is the time it takes the Moon to pass through all of the Moon phases, usually measured from a New Moon the next New Moon. more

Illustration of the Moon's position in space in relation to Earth and the Sun at New Moon.

The New Moon

In modern astronomy, the New Moon is when the Sun and Moon are aligned, with the Sun and Earth on opposite sides of the Moon. The New Moon also has important cultural and religious significance worldwide. more

First Quarter Moon in spring (Northern Hemisphere)

First Quarter Moon

The First Quarter Moon is a primary Moon phase when we can see exactly half of the Moon's surface illuminated. If it is the left or right half, depends on where you are on Earth. more

Waning Crescent Moon against a black night sky

Waning Crescent Moon

During the Waning Crescent Moon phase, the illuminated part of the Moon decreases from a semicircle at Third Quarter until it disappears from view entirely at New Moon. more