NASA visualization shows the Moon’s phases in 2025 (NASA’s Goddard Space Flight Center)
Whether you’re an avid skywatcher or not, you’ve likely heard the phrase “once in a blue moon,” and while the term refers to a real astronomical occurrence, the novice skygazer may not be familiar with what a Blue Moon actually is.
There are two types of Blue Moons: monthly and seasonal – with the former gracing the sky more frequently.
Because the Moon’s cycle takes approximately 29.5 days to complete, a month will experience two full Moons roughly every two years. This phenomenon is known as a monthly Blue Moon.
A seasonal Blue Moon, on the other hand, occurs when one of the astronomical seasons – winter, spring, summer or fall – has four full Moons. When the phenomenon happens, the third full Moon of the season is designated as the seasonal Blue Moon.
Any month with 30 or 31 days has the potential to see a Blue Moon, but the highest likelihood occurs during the longer months.
January, March, May, July, August, October and December are the most common months for a Blue Moon to occur.
As the bright parts of the Moon appear to change shape during the month, each stage of the change is called a phase, and each phase carries its own name. (Image: NASA/Bill Dunford)
(NASA)
WHAT ARE THE PHASES OF THE MOON?
Due to the dates of the summer solstice and fall equinox, Blue Moons most often occur during the summer season.
A Blue Moon can also occur when a Supermoon or Micromoon happens, but the events are not inherently linked.
A Supermoon occurs when a full Moon is closest to Earth, while a Micromoon occurs when the Moon is at its farthest point from the globe.
According to statistics compiled by NASA, only about 25% of full Moons are Supermoons and less than 3% are Blue Moons.
SEE THE OBJECT HUMANS LEFT BEHIND ON THE MOON
The origin of the name remains unclear, with no definitive source of its beginning usage.
Some believe the term emerged after the historic eruption of Krakatoa in 1883, while others suggest it may be linked to the Farmer’s Almanac.
It’s important to note that the Moon doesn’t actually turn blue, and the occurrence of a Blue Moon is relatively common, which contrasts with the phrase “once in a blue moon,” typically used to indicate something rare.
If the needed atmospheric conditions align, particles of smoke or dust can scatter red light, making any Moon appear blue to the naked eye.
The last occurrence of a Blue Moon was August 19, 2024, when there were four full Moons during astronomical summer.
The next Blue Moon is set to occur on May 31, 2026, followed by events on May 20, 2027, August 24, 2029 and August 21, 2032.
This was the Full Moon of Halloween Night, October 31, 2020 which was also the smallest, most distant Full Moon of 2020 and was also a Blue Moon, as any Full Moon on the 31st of a month has to be, meaning it was the second Full Moon of the calendar month. Being a Full Moon, the bright rays emanating from recent impacts, particularly from large craters such as Tycho (bottom) and Copernicus (left), are prominent. The Moon was actually full about 12 hours earlier, so there is a slight phase here, with a terminator visible on the Moons eastern limb at right. This is a single exposure with the Astro-Physics 130mm EDF refractor with the 2x AP Barlow lens (for a focal length of 1600mm) and Canon 6D MkII camera at ISO 100, taken to be part of a pair with a perigee Full super Moon in six months. I developed this image for increased contrast and colour saturation to bring out the subtle colour differences in the mare areas. Taken though some high cloud.
(Photo by: Alan Dyer/VW Pics/Universal Images Group via Getty Images / Getty Images)