The term “Super Blue Moon” describes a celestial event combining two separate astronomical definitions, but the name does not mean the Moon will actually appear blue in color. This rare confluence denotes a full moon that occurs at a close point in its orbit while also being the second full moon within a single calendar month. The phrase is a blend of orbital mechanics and calendrical timing. Understanding this phenomenon involves separating the literal color from the naming conventions to appreciate the science behind the event.
The Immediate Answer: Actual Color vs. Name
A Super Blue Moon typically appears no different in color than any other full moon. Its surface, reflecting sunlight, generally presents as a bright, silvery-gray or a pale yellow-white, which is the Moon’s natural hue. The word “Blue” in this context is a naming convention, not a descriptor of the Moon’s physical color.
The Moon’s brightness is noticeably enhanced due to its proximity to Earth. However, the light it reflects remains unchanged in its spectral composition. The “Blue Moon” definition (the second full moon in a month) does not alter the physical properties of the lunar surface.
Defining the “Blue Moon” Phenomenon
The “Blue Moon” is a term describing a calendrical oddity involving the frequency of full moons. The most common modern definition refers to the second full moon that occurs within a single calendar month. This happens because the time between successive full moons, known as a synodic month, is approximately 29.5 days.
Since most calendar months have 30 or 31 days, a full moon can occur early enough for a second one to appear before the month ends. This occurs roughly every two to three years. There is also an older definition that names the third full moon in an astronomical season containing four full moons as a Blue Moon.
Defining the “Supermoon” Phenomenon
The “Supermoon” component is based on the Moon’s orbital mechanics and its distance from Earth. The Moon’s orbit around our planet is elliptical, meaning its distance varies over its cycle. The point where the Moon is closest to Earth is called perigee, and the farthest point is called apogee.
A full moon occurring at or near perigee is designated a Supermoon, or technically, a perigee syzygy Moon. Astrologer Richard Nolle coined the term in 1979. When this alignment happens, the Moon can appear up to 14% larger and 30% brighter than a full moon at apogee.
Why the Moon Can Occasionally Appear Blue
While the “Blue Moon” name is not about color, the Moon can, on extremely rare occasions, genuinely appear blue to an observer. This phenomenon is caused entirely by the Earth’s atmosphere, not by the Moon itself. The genuine blue hue results from the presence of atmospheric aerosols of a very specific size.
Particles, typically from immense volcanic eruptions or large-scale forest fires, must be about one micron in diameter to scatter light effectively. When light passes through this cloud, these micron-sized particles preferentially scatter the longer red wavelengths of light. This allows the shorter blue wavelengths to pass through to the observer’s eye, temporarily tinting the Moon blue. For example, blue moons were reported for nearly two years after the 1883 eruption of the Krakatoa volcano.