What Is a Supermoon

A supermoon happens because the Moon's orbit around Earth isn't a perfect circle — it's an ellipse, meaning the Moon's distance from Earth genuinely varies throughout each orbit, from about 356,500 km at its closest (perigee) to about 406,700 km at its farthest (apogee), a difference of roughly 50,000 km, or about 14% of the total distance.

When a Full Moon happens to coincide with perigee (or close to it), the Moon appears up to roughly 14% larger in diameter and up to about 30% brighter than a Full Moon at apogee — both figures are genuinely measurable, computable numbers, not marketing exaggeration, though the visual difference is often subtler to the naked eye in the moment than those percentages suggest, since there's rarely another full moon right next to it in the sky to directly compare against.

The term itself isn't a formal astronomy term — astronomer's more precise phrase is "perigee syzygy" (syzygy meaning an alignment of three celestial bodies, here Sun-Earth-Moon). "Supermoon" was coined in 1979 by astrologer Richard Nolle, and caught on in popular and media use well beyond its astrological origin because it's simply a more memorable, accessible term.

Not every full moon at perigee gets called a supermoon in every source — different outlets use slightly different distance thresholds (some require the Moon within 90% of its closest approach for that orbit, following Nolle's original definition; others use a simpler "closest few full moons of the year" rule), so you may occasionally see slightly different supermoon counts reported for the same year by different sources. This site's methodology page states which threshold it uses.

A secondary, genuinely measurable physical effect of a supermoon's closer distance: slightly stronger tidal pull on Earth's oceans, producing marginally higher tides (called perigean spring tides) around a supermoon compared to an average Full Moon — a real, if modest, effect distinct from any of the folklore claims about supermoons covered on this site's meaning pages.

For readers wanting to verify a supermoon's real size difference themselves rather than trust a percentage figure, photographing the Moon with identical camera settings and focal length at both a supermoon and a later apogee full moon, then directly comparing the two images' Moon diameter in pixels, is the most reliable DIY method.

Supermoons at New Moon (rather than Full Moon) are less commonly discussed since there's nothing extra to see — the Moon's lit side still faces away from Earth regardless of its distance — but they do produce the same real, slightly stronger tidal effect as a Full Moon supermoon, just without the visual spectacle.

Weather forecasting the same week as a specific supermoon date matters as much as for any other full moon — a clear sky is required to see any of the real or perceived size difference regardless of orbital distance.

It's worth noting that "supermoon" as a term has no equivalent formal recognition from the International Astronomical Union, the body that does officially govern astronomical naming and definitions — it remains, officially, informal popular terminology despite widespread use even in serious astronomy journalism and outreach.

For readers who want the single most reliable indicator of a given month's full moon status, checking the actual computed distance figure (available through professional ephemeris tools and this site's own full-moon pages) is more precise than relying on whether a given source happens to label that month's occurrence a supermoon or not.

A simple annual habit worth building: checking this site's supermoon guide and full-moon calendar together at the start of each year to note which months' occurrences are worth prioritizing for viewing.

Local astronomy clubs frequently organize public supermoon viewing events specifically because the larger, brighter Moon makes for an accessible, crowd-friendly target that doesn't require dark-sky conditions the way fainter deep-sky objects do.

A simple planning habit: cross-check this site's flagged supermoon occurrences against your own local weather forecast a day or two ahead, since clear skies matter more than the distance figure alone.

Checking a specific year's flagged occurrences well in advance gives you time to plan around work and weather rather than reacting to last-minute headlines.

This site's own full-moon calendar lists the specific distance figures behind any given month's occurrence, a quick way to confirm what a headline claims against the actual computed data.

The underlying orbital distance, not marketing language, is what actually determines whether a given month qualifies.

Real numbers beat rounded-up estimates.

Trust the distance figure.

Distance decides everything else here.

Closer means brighter, plain and simple.