A super blue moon is the coincidence of a full moon with two astronomical conditions: it is both a blue moon and a supermoon. A blue moon is the modern calendar label for the second full moon in a calendar month or the third of four full moons in a season that normally has four. A supermoon occurs when a full or new moon coincides with lunar perigee, the point at which the Moon is closest to Earth in its elliptical orbit, making the Moon appear larger and slightly brighter than an average full moon. When these events align, the Moon can appear noticeably larger and brighter in the night sky, weather permitting.
The Origin of the Term Blue Moon
The modern definition of a blue moon as the second full moon in a calendar month arose from a misinterpretation and simplification in a 1946 article in Sky & Telescope. This definition is now widely used, though it differs from older folklore meanings. An older seasonal definition counts a blue moon as the third of four full moons in a single astronomical season. These two definitions create predictable patterns that recur on multi-decadal cycles.
Calendar patterns and rarity
Because a calendar month longer than 29 days can contain two full moons, blue moons in the calendar-month sense occur in about 3 out of every 8 years. When this coincides with a supermoon, the event is sometimes called a super blue moon. The last super blue moon occurred in August 2023, and the next will occur in January 2028.
What Makes a Supermoon
The Moon’s orbit around Earth is elliptical rather than circular, so its distance from Earth varies. Perigee is the closest point in the orbit, and apogee is the farthest. A full moon near perigee can appear up to about 14 percent larger and shine roughly 30 percent brighter than a full moon near apogee. The term supermoon was popularized by an astrologer in the 1970s and is now used broadly in astronomy communication to describe full or new moons occurring within roughly 90 percent of perigee distance.
Measured thresholds
There is no official rigid boundary for supermoons; different sources use slightly different proximity thresholds. The table below shows verified detail on perigee distance, Earth-Moon separation at typical full moons, and apparent size and brightness differences.
| Attribute | Verified Detail | Source Type |
|---|---|---|
| Lunar perigee distance (typical) | Approximately 363,000 kilometers | Astronomical constant |
| Lunar apogee distance (typical) | Approximately 405,000 kilometers | Astronomical constant |
| Earth-Moon separation at full supermoon | Roughly 350,000 kilometers or closer | NASA Horizons calculations |
| Apparent diameter increase vs. average | Up to about 14 percent larger | Orbital geometry |
| Apparent brightness increase vs. average | Up to roughly 30 percent brighter | Orbital geometry and photometric models |
How a Super Blue Moon Occurs
A super blue moon requires two conditions: a full moon near perigee and the calendar definition of a blue moon. If the full moon falls close enough to perigee during a month that contains a second full moon, observers may call it a super blue moon. Seasonal blue moons in years with four full moons in a season can also coincide with supermoon conditions, though this is less common in popular usage. The rarity often mentioned for super blue moons reflects how frequently both conditions together occur, typically on a multiyear to decadal scale.
Notable conditions and effects
- Lunar illusion: When near the horizon, the Moon can appear larger due to atmospheric and psychological effects, not physical size changes.
- Tidal influence: Perigean full moons can produce slightly higher high tides, known as perigean spring tides.
- Atmospheric scattering: Color changes in the Moon are due to dust or haze, not inherent to super or blue Moon events.
Observing a Super Blue Moon
No special equipment is required to observe a super blue moon, though optical aids can enhance detail. Choose a location with an unobstructed view of the eastern horizon as the Moon rises and the western horizon as it sets. Checking the exact time of full phase and perigee for your location improves timing. In light-polluted cities, the difference in apparent size is modest, but the experience remains accessible. Rural areas with dark skies can reveal more surface detail on the Moon during any full phase.
Practical checklist for viewing
- Check the full moon time and supermoon proximity for your time zone.
- Pick a site with clear sightlines toward the Moonrise or Moonset direction.
- Use binoculars or a small telescope to see craters and lunar maria.
- Allow your eyes 20–30 minutes to adapt to darkness.
- Note that photography exaggerates size differences; use a telephoto lens to capture perceived scale.
Common Misconceptions and Clarifications
Despite vivid descriptions, a super blue moon does not turn the Moon blue in color, and it is not an extinction-level event. True blue-colored Moons from volcanic ash or dust are unrelated to the calendar label. Claims linking super blue moons to major disasters lack empirical support and are inconsistent with verified observational data. Understanding the astronomical mechanisms helps separate routine celestial mechanics from sensationalized narratives.
Context Within the Lunar Month and Year
Full moons occur roughly every 29.5 days, producing a sequence with slight variations in apparent size and timing over the year. Supermoon designations apply to full moons within a defined proximity to perigee, and they can occur in any month. Calendar blue moons occur about once every two to three years in the calendar-month sense, while seasonal blue moons follow an eight-year repeating pattern embedded in the Metonic cycle. The overlap of these cycles produces super blue moons on intervals of approximately five to ten years.
| Date or Period | Event | Why It Matters |
|---|---|---|
| August 2023 | Super blue moon | Second full moon in August and near perigee |
| January 2028 | Next predicted super blue moon | Projected alignment of calendar and perigee cycles |
| Metonic cycle (19 years) | Repetition of full moon dates in the calendar | Long-term pattern underlying blue moon recurrence |
Scientific Background and Sources
The Moon’s orbit, Earth’s rotation, and the synodic month (new Moon to new Moon) combine to create the conditions described here. Data on lunar distances and timing are drawn from established astronomical models and ephemerides maintained by institutions such as NASA and time service laboratories. The modern blue moon definition was clarified by early Sky & Telescope reporting, reconciling calendar-based naming with observed cycles.
Super blue moons illustrate how predictable celestial mechanics can produce visually striking but non-dangerous phenomena. By aligning the calendar labels with real gravitational dynamics, observers can appreciate these events as examples of routine astronomy rather than anomalies.
Bottom Line
A super blue moon is a full moon that is both a blue moon by calendar definition and a supermoon by orbital proximity. It appears larger and brighter than an average full moon, and its rarity reflects the combination of calendar and orbital cycles. You can plan observation using published times for full phase and perigee, and you can confirm details using official astronomical data sources. This pattern recurs on predictable multiyear intervals, making it an enduring topic for sky watchers without requiring urgent attention.