astronomy

Blue Moon Supermoon: What It Is and When to Watch

A blue moon supermoon combines two separate astronomical circumstances: a blue moon, meaning a second full moon within a calendar month, and a supermoon, when a full moon coinci...

Mara Ellison
Blue Moon Supermoon: What It Is and When to Watch

What a Blue Moon Supermoon Is

A blue moon supermoon combines two separate astronomical circumstances: a blue moon, meaning a second full moon within a calendar month, and a supermoon, when a full moon coincides with perigee, the Moon’s closest approach to Earth. The result is a full moon that appears slightly larger and brighter than an average full moon. This name is purely descriptive; the Moon does not literally turn blue in most cases. The phrase carries no official astronomical standard but is widely used by educators, media, and sky watchers to highlight a notable full moon event.

How the Moon’s Orbit Creates Supermoons

The Moon’s orbit around Earth is elliptical rather than circular, so distance varies across each orbit. Perigee is the point of closest approach, and apogee is the farthest point. When a full moon occurs within roughly 90 percent of perigee distance, it is commonly labeled a supermoon. Because the alignment is not exact and different organizations use slightly different thresholds, supermoon forecasts may vary by source. The effect is modest: a supermoon can appear up to about 14 percent larger and 30 percent brighter than a micromoon, though casual viewers may notice a subtler difference.

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Comparing Typical Full Moon and Supermoon

Attribute Typical Full Moon Supermoon
Apparent size Baseline Up to ~14% larger
Brightness Baseline Up to ~30% brighter
Perigee proximity Any distance Within ~90% of closest approach
Visual difference N/A Modest; clearer viewing conditions matter

Defining a Blue Moon

The modern blue moon concept stems from a misinterpretation published in Sky & Telescope in 1946 and later popularized by a radio program. This definition counts a blue moon as the second full moon within a single calendar month. An older seasonal definition marks a blue moon as the extra full moon in a season that normally has four; this remains in use for certain traditional and astronomical contexts. Neither definition is tied to atmospheric conditions; the Moon becoming bluish is rare and caused by sizable particles in the air after events like major volcanic eruptions or intense wildfires.

When a Blue Moon and Supermoon Align

A blue moon supermoon occurs when a second full moon in a calendar month also reaches full phase near perigee. Because the synodic month (full moon to full moon) is about 29.5 days and months vary in length, the pattern recurs but not on a fixed schedule. Timing depends on time zones and the precise instant of full moon relative to perigee, so what is a blue moon supermoon in one zone may not be in another. These events are predictable years in advance using lunar ephemerides and standard calendars.

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Visibility and Viewing Practicalities

To get the best experience, treat the event like any full moon outing: choose a location with an unobstructed view toward the horizon, avoid heavy haze, and give your eyes time to adapt to darkness. Unlike a rare eclipse, a blue moon supermoon is visible wherever the Moon is above the horizon and skies are reasonably clear. Timing matters less for casual observation than for photography; use a telephoto lens or crop, bracket exposures, and stabilize your camera. The slight size and brightness changes are subtle, so manage expectations: dramatic differences are more noticeable in photos than by eye.

Science and Atmosphere Behind the Name

No physical property of the Moon changes to create a blue moon in the popular sense; it is a calendrical artifact. True blue coloring is possible when particles around 0.4 to 0.8 micrometers in diameter scatter red light, which can follow large volcanic eruptions or widespread fires. These events are uncommon and unrelated to supermoon mechanics. From a scientific standpoint, the main interest in a blue moon supermoon is public engagement: it draws attention to lunar cycles, tides, and orbital mechanics in a way that routine full moons rarely do.

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