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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. The Moon's Effect on Tides
  4. What Is a Micro Moon?
  5. How Can Full Moon Be in the Daytime?
  6. Is a Blue Moon Blue?
  7. The Moon's Orbit
  8. The Far Side of the Moon
  9. What Is a Black Moon?
  10. What Are Moonbows?
  11. Full Moon Names
  12. Taking pictures of the Moon

Moon index

Moonrise & Moonset Times

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