Every time a supermoon gets announced, my phone fills up with students asking whether they should stay up to see it. My answer is always: “It’s worth looking at, but it’s not what the headlines make it sound like.” Here’s what’s actually happening and how to calibrate your expectations — starting with the orbital geometry that makes supermoons possible.
Why the Moon’s Distance Changes
The Moon orbits Earth in an ellipse, not a perfect circle. This means its distance varies throughout each orbit. The closest point is called perigee; the farthest point is apogee. The difference is significant: at perigee, the Moon is approximately 356,500 km from Earth; at apogee, approximately 406,700 km. That’s a difference of about 50,000 km — roughly 14% variation in distance.
The Moon completes one orbit every 27.3 days (sidereal period). Meanwhile, the Moon goes through its phases on a 29.5-day cycle (synodic period, relative to Earth-Sun alignment). These two cycles are different lengths, which means the timing of full moons relative to perigee constantly shifts. Roughly every 13-14 months, a full moon coincides closely with perigee.
What “Supermoon” Actually Means
The term “supermoon” was coined by astrologer Richard Nolle in 1979 — not by astronomers. Nolle defined it as a full or new moon occurring within 90% of perigee distance. This is an arbitrary definition that astronomers don’t use; the formal term is perigee syzygy (syzygy meaning the alignment of three celestial bodies). The 90% threshold means that roughly 3-4 full moons per year qualify as “super,” which somewhat deflates the sense of rarity.
Is It Actually Bigger?
Yes — measurably. At maximum perigee, a full moon appears approximately 14% larger in diameter and 30% brighter than at apogee. These are real, calculable differences based on the inverse square law for brightness and simple angular diameter geometry.
However, 14% is a modest visual difference. To put it in perspective: if you held a quarter at arm’s length and then moved it 14% closer, the difference is real but not dramatic. Side-by-side comparison images of perigee and apogee moons are striking; seeing a supermoon in isolation, without comparison, most observers cannot reliably tell the difference from any other full moon.
The 30% brightness increase is more noticeable — a supermoon night is genuinely brighter than an average full moon night. This is the real observational payoff.
The Horizon Illusion
The famous “giant moon on the horizon” effect has nothing to do with supermoons. The Moon illusion — where the Moon appears dramatically larger near the horizon than high in the sky — is a consistent optical/perceptual phenomenon that occurs at every full moon and has been known since ancient Greece. Aristotle mentioned it. The effect disappears if you view the Moon through a tube that removes surrounding landscape context.
The mechanism is debated but likely involves reference frame comparison — when the Moon is near the horizon, your visual system compares it to buildings, trees, and terrain and perceives it as larger. High in the sky, with no reference objects, it looks smaller. The Moon’s actual angular diameter doesn’t change — your perception does. Supermoon coverage that uses horizon photos is conflating two separate phenomena.
What’s Worth Watching
The actual best time to observe a supermoon is moonrise — not because of the size, but because the combination of the horizon illusion and the genuinely brighter supermoon produces a visually impressive scene. Check your local moonrise time, find an eastern horizon with interesting foreground (city skyline, mountains, water), and watch the Moon rise 30 minutes before to 30 minutes after moonrise. That’s a memorable observation regardless of supermoon status.
For My Earth Science Students
The supermoon is actually a useful teaching entry point for elliptical orbits, Kepler’s laws, angular diameter calculations, and the distinction between astronomical terms and media terms. I have students calculate the angular diameter of the Moon at both perigee and apogee using the formula θ = 2 × arctan(d/2D), where d is the Moon’s diameter and D is its distance. The math makes the 14% difference concrete and the spreadsheet is a good lab exercise.