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.
The term 'once in a Blue Moon' means that that something is very rare. But just how rare, depends on your definition.
2 Different Definitions
- Blue Moon = The 3rd Full Moon in an astronomical season with 4 Full Moons
(versus the normal 3)
- Blue Moon = The 2nd Full Moon in a month with 2 Full Moons.
Find Time & Date for Both
Defining the Original Blue Moon
The correct, original 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.
Next Blue Moons:
|2016||May 21||3rd Full Moon in season|
|2018||Jan 31||2nd Full Moon in a month|
|2018||Mar 31||2nd Full Moon in a month|
Blue Moons can vary by time zone. Dates above are based on the local time in Washington DC. Change location
All about the Moon
- The Moon Phases
- What Is a Supermoon?
- What Is a Micro Moon?
- Is a Blue Moon blue?
- The Moon's Orbit
- What Is a Black Moon?
- What are Moonbows?
- Full Moon Names