What Is a Supermoon?
The term supermoon has become a media staple, often accompanied by dramatic headlines promising the biggest, brightest Moon in years. But what does it actually mean scientifically? A supermoon occurs when a Full Moon (or sometimes a New Moon) coincides with the Moon being near its closest point to Earth in its elliptical orbit — a point called perigee.
Because the Moon's orbit is not a perfect circle, the distance between Earth and Moon varies by roughly 50,000 kilometers over the course of each orbit. At perigee, the Moon is about 356,000–370,000 km away. At apogee (its farthest point), it's around 404,000–406,000 km away.
What Is a Micromoon?
A micromoon is the opposite: a Full Moon (or New Moon) that occurs near apogee — the Moon's farthest point from Earth. It appears slightly smaller and dimmer than an average Full Moon. Micromoons receive far less media attention than supermoons, but they're equally real and equally predictable.
How Much Bigger Is a Supermoon, Really?
This is where expectations often need calibrating. Compared to an average Full Moon:
- A supermoon appears roughly 7–14% larger in diameter
- A supermoon appears roughly 15–30% brighter
- The difference compared to a micromoon is more striking: up to about 14% larger in angular diameter
For most observers, this difference is subtle without a reference point. Unless you've memorized how large the typical Full Moon appears, a supermoon doesn't look dramatically different to the unaided eye. Side-by-side photographs taken at the same focal length and elevation, however, reveal a clear size difference.
Supermoon vs. Micromoon: Key Comparison
| Feature | Supermoon | Micromoon |
|---|---|---|
| Orbital position | Near perigee (closest point) | Near apogee (farthest point) |
| Distance from Earth | ~356,000–370,000 km | ~404,000–406,000 km |
| Apparent size | Slightly larger than average | Slightly smaller than average |
| Brightness | Up to 30% brighter than average | Noticeably dimmer |
| Occurs | Several times per year | Several times per year |
| Media attention | Very high | Very low |
Is "Supermoon" an Official Astronomical Term?
Not exactly. The term was coined by astrologer Richard Nolle in 1979 and later adopted widely by the media and informal astronomy communities. Professional astronomers typically use the term "perigee-syzygy" — syzygy meaning the alignment of three celestial bodies (in this case, Sun, Earth, and Moon). There's no universally agreed threshold for exactly how close a perigee Full Moon must be to qualify as a "super" moon, which is why different sources list different numbers of supermoons in a given year.
The Moon Illusion: Why It Looks Big on the Horizon
Many people report the Moon looking enormous when it rises near the horizon — far larger than it appears when high in the sky. This is not a supermoon effect; it's the Moon illusion, a well-documented perceptual phenomenon. The Moon's actual angular size doesn't change as it rises. The brain, however, uses surrounding reference points (trees, buildings, horizon) to interpret the Moon as much larger when it's near objects of known size. When the Moon is high overhead with no reference points, it appears smaller by comparison.
You can test this easily: hold your thumb up at arm's length against the Moon when it's on the horizon, and again when it's high in the sky. You'll find your thumbnail covers the Moon by the same amount in both cases.
Should You Go Out for a Supermoon?
Absolutely — not because the size difference is dramatic, but because any Full Moon is worth observing, and the extra brightness of a supermoon makes it an especially pleasant night to be outside. The psychological and aesthetic experience of watching a Full Moon rise over a city skyline, a mountain range, or the ocean is genuinely memorable, supermoon or not. Use the media attention as a prompt to get outside and look up more often.
The real reward isn't a 14% size increase — it's the habit of watching the sky regularly enough to notice change, anticipate cycles, and feel genuinely connected to the astronomical world above you.