The term “Blue Supermoon” describes a spectacular but uncommon event, representing the simultaneous occurrence of three distinct lunar phenomena: a Full Moon, a Supermoon, and a Blue Moon. This designation highlights a moment when the Moon is fully illuminated while also being exceptionally close to Earth. To appreciate this rare alignment, it is necessary to examine the separate definitions that combine to create this unique term.
Defining the Supermoon
A Supermoon occurs when a Full Moon coincides with the Moon’s closest approach to Earth in its elliptical orbit. Since the Moon’s path is not a perfect circle, its distance from Earth constantly varies. The closest orbital point is called perigee, and the farthest point is known as apogee.
When the Moon is at perigee, it is approximately 43,000 kilometers (27,000 miles) closer than at apogee. If a Full Moon happens near this point, it is informally called a Supermoon, or technically, a perigee syzygy. This proximity makes the Moon appear up to 14% larger and 30% brighter than a Full Moon at apogee.
The increased size and brightness are often subtle and difficult to notice without a direct comparison. However, the proximity makes the Supermoon appear striking when viewed low on the horizon, due to the “Moon illusion.” Supermoons occur relatively frequently, happening about three to four times each year.
Defining the Blue Moon
The “Blue Moon” component is a calendrical definition, not an indicator of the Moon’s actual color. A calendar year typically has 12 Full Moons, one per month, because the lunar cycle is approximately 29.5 days long. Since the calendar year is longer than 12 lunar cycles, an extra Full Moon occasionally occurs, roughly every two to three years.
There are two accepted definitions for this extra Full Moon. The most common modern definition is the “monthly Blue Moon,” which refers to the second Full Moon occurring within a single calendar month. This happens when a Full Moon falls early in the month, allowing a second full cycle to complete before the month ends.
The older, more traditional meaning is the “seasonal Blue Moon,” which is the third Full Moon in an astronomical season that contains four Full Moons instead of the usual three. This definition arose from the Maine Farmer’s Almanac and is tied to the timing of the solstices and equinoxes.
Despite the name, the Moon does not appear blue during a Blue Moon; the name relates only to the timing anomaly. The Moon can appear blue only under rare atmospheric conditions, such as the presence of specific-sized dust or smoke particles. These particles, often from volcanic eruptions or forest fires, scatter red light away from the line of sight. This color change is a separate atmospheric event unrelated to the calendrical Blue Moon.
Rarity and Future Occurrences
The Blue Supermoon combines the orbital proximity of a Supermoon with the calendrical anomaly of a Blue Moon. For this event to occur, the Moon must be at perigee while simultaneously being either the second Full Moon in a month or the third Full Moon in a season with four. The mathematical alignment required for both cycles to coincide makes the Blue Supermoon a truly rare event.
While Supermoons are relatively common and Blue Moons occur every 2 to 3 years, their combination is irregular. On average, a Blue Supermoon occurs about once every 10 years, though the time between occurrences can stretch up to 20 years. This irregularity explains why the phrase “once in a blue moon” is used to describe something that happens infrequently.
The next Blue Supermoons are predicted to occur in a pair in 2037, specifically in January and March. For observation, the best time to view the Moon is just as it rises or sets. When the Moon is near the horizon, its light passes through more of the Earth’s atmosphere, enhancing the visual effect of its size due to the “Moon illusion.” This effect, combined with the increased size and brightness from the perigee, offers the most impressive viewing experience.