When Is the Blue Moon in 2021 and How Often Does It Happen?

The next full moon will grace the skies on the night of August 21-22, and will be what is known as a blue moon.

But what is a blue moon and how often do they occur?

Full moons happen when the moon is located opposite the sun, with the Earth directly in between. This alignment will happen next at 8:02 a.m. ET on Sunday, August 22.

At this time, the moon will reach peak illumination, appearing like a perfect circle in the sky. While observers in North America will not be able to see the moon at this exact moment because it will already have set below the horizon, our natural satellite will appear full to the untrained eye on the night before and after this peak.

The full moon in August is commonly referred to as the sturgeon moon because Native American tribes once considered this a prime time for catching the giant fish in the continent's lakes. Because this year's sturgeon moon is the third of four full moons in the Northern Hemisphere's summer—and the Southern Hemisphere's winter—it is also known as a "blue moon."

There are actually two different definitions of a blue moon. The one coming up later this month is a "seasonal blue moon." This refers to the third full moon in an astronomical season that has four full moons.

The other definition of blue moon is a "monthly blue moon." This definition refers to the second full moon in a calendar month with two full moons.

The time between full moons is roughly 29.5 days, but most calendar months are slightly longer. This discrepancy means that most months only have one full moon, but every so often two will appear in the same month.

Both types of blue moons come around roughly every two or three years, although monthly blue moons are slightly more frequent. The next seasonal blue moon will appear on August 19, 2024, according to TimeAndDate.com.

These definitions have nothing to do with the color of the moon. Sometimes, however, the moon does actually appear blue, although this phenomenon is uncommon and only occurs under specific conditions.

The moon can appear blue when the atmosphere above a given location is filled with ash or dust particles of just the right size, according to SpaceWeather.com These particles can scatter red light being reflected off the moon in such a way that it looks blue in appearance.

Thus, in incredibly rare circumstances, it is possible that a seasonal or monthly blue moon may actually appear blue in color from some locations.

A full moon
The moon above Barcelona, Spain, on March 29, 2021. The August full moon will be a blue moon. Aristos Georgiou