The Blue Supermoon is an astronomical event that captures the public imagination due to its descriptive, yet slightly misleading, name. This spectacle is the alignment of two separate, less frequent lunar occurrences. The term combines factors related to the Moon’s orbital distance and its timing within the calendar year. Understanding this event requires breaking down these two distinct components to explain why their simultaneous arrival creates a special moment for sky-watchers.
What Makes a Moon Super
The designation “Supermoon” relates directly to the elliptical shape of the Moon’s orbit around Earth. This means the distance between the two bodies is constantly changing throughout the lunar cycle. The point in the orbit where the Moon comes closest to Earth is called perigee, while the farthest point is known as apogee.
The distance between these two extremes varies significantly, ranging from approximately 405,500 kilometers at apogee to 363,300 kilometers at perigee. A Supermoon occurs when the Moon reaches its full phase at or very near its perigee. This close alignment results in the Moon appearing slightly larger and noticeably brighter than an average full moon.
A Supermoon can look up to 14 percent larger and around 30 percent brighter than a full moon occurring at apogee. Supermoons are relatively common, typically occurring three to four times in a given year because the Moon reaches perigee roughly once a month.
What Makes a Moon Blue
The term “Blue Moon” has nothing to do with the Moon’s color but rather its timing within the calendar. This name identifies an extra full moon that does not fit the typical pattern of one full moon per month. There are two accepted definitions for this calendrical anomaly.
The monthly Blue Moon refers to the second full moon that occurs within a single calendar month. This happens because the time between full moons is approximately 29.5 days, which is slightly shorter than most calendar months. If a full moon occurs early in the month, there are enough remaining days for a second full moon to appear before the month ends.
The older, more traditional seasonal definition refers to the third full moon in an astronomical season that contains four full moons. Since an astronomical season typically holds only three full moons, the fourth moon receives the Blue Moon designation. Both the monthly and seasonal Blue Moons occur fairly regularly, happening approximately once every two or three years.
The Coincidence of the Blue Supermoon
The Blue Supermoon is the result of these two distinct astronomical cycles aligning simultaneously. The event requires the Moon to be at its nearest point to Earth, the perigee, at the same time it qualifies as the second full moon in a month or the third in a season. This combination of orbital mechanics and calendar timing is significantly less frequent than either event happening alone.
On average, the double event of a Blue Supermoon occurs only once per decade, though the interval can vary widely. This rarity makes the occurrence notable for observers. Approximately 25 percent of all full moons are Supermoons, but only about 3 percent are Blue Moons, making the overlap infrequent.
Despite its name, the Moon does not appear blue during a Blue Supermoon. The color remains the same as any other full moon, typically a pale white or yellow, sometimes taking on an orange hue when viewed near the horizon. The Moon only appears blue under extremely rare atmospheric conditions, such as the presence of fine ash or smoke particles from a large volcano or wildfire. These particles must be precisely the right size—about one micron wide—to scatter red light and allow blue light to pass through. This phenomenon is entirely unrelated to the calendrical Blue Moon definition.
For those hoping to view the spectacle, the best time to look is shortly after moonrise or just before moonset, when the Moon is low on the horizon. At this time, the “Moon illusion” makes it appear even larger when viewed next to foreground objects like trees or buildings, enhancing the visual impact.