When the Moon Looks Unusually Large
Every so often, headlines announce a "supermoon" — and people across the world head outside to see what the fuss is about. Sometimes the Moon does seem strikingly large and bright. Other times, it looks… pretty much normal. So what exactly is a supermoon, and is it really that super?
The Moon's Elliptical Orbit
The Moon doesn't orbit Earth in a perfect circle. Its orbit is an ellipse, meaning there's a point where it's closest to Earth — called perigee — and a point where it's farthest — called apogee. The difference is significant: at perigee, the Moon can be roughly 50,000 kilometres closer to Earth than at apogee.
- Average distance: ~384,400 km
- Perigee (closest): ~356,500 km
- Apogee (farthest): ~406,700 km
What Makes a Supermoon?
A supermoon occurs when a full moon (or new moon) coincides with the Moon being at or near perigee. The term was coined by astrologer Richard Nolle in 1979, who defined it as a full or new moon that occurs when the Moon is within 90% of its closest approach to Earth. NASA and astronomers have adopted a similar but slightly more precise definition.
Because the Moon is closer to Earth during a supermoon, it appears:
- Up to 14% larger in angular diameter compared to a full moon at apogee (sometimes called a "micromoon")
- Up to 30% brighter than a micromoon
Can You Really Tell the Difference?
Honestly — it depends. The difference between an average full moon and a supermoon is real but subtle to the naked eye. Most observers struggle to notice the size difference without a side-by-side comparison. However, when comparing a supermoon to a micromoon, the difference in brightness is quite noticeable on a clear night.
The Moon Illusion — a well-known optical trick where the Moon appears larger near the horizon than high in the sky — is often more responsible for dramatic-looking moons than actual proximity to perigee. A supermoon rising over a flat landscape or ocean horizon can genuinely be a breathtaking sight.
Supermoon vs. Micromoon
| Feature | Supermoon | Micromoon |
|---|---|---|
| Moon's position | Near perigee | Near apogee |
| Distance from Earth | ~356,500 km | ~406,700 km |
| Apparent size | Largest of the year | Smallest of the year |
| Brightness | Up to 30% brighter than micromoon | Noticeably dimmer |
Does a Supermoon Cause Natural Disasters?
A persistent myth claims that supermoons trigger earthquakes, volcanic eruptions, or extreme weather. The scientific evidence does not support this. While a supermoon does produce slightly larger spring tides — called perigean spring tides — the effect on geological processes is negligible. Earthquakes and volcanic activity are driven by forces deep within the Earth that dwarf any tidal influence from the Moon.
How Often Do Supermoons Occur?
Supermoons are more common than many people realise. Depending on the definition used, there are typically 3–4 supermoons per year. They often occur in consecutive months because the Moon's perigee point shifts gradually around its orbit. Some years see a sequence of two or three supermoons in a row.
Tips for Watching a Supermoon
- Check the exact date and time of the full moon in your time zone.
- Find a location with a clear, unobstructed eastern horizon for moonrise.
- Watch as the Moon rises — this is when the Moon Illusion makes it look most impressive.
- Use binoculars or a small telescope to appreciate the surface detail that extra brightness reveals.
- Compare it to the next full moon to notice the difference in size and brightness.
A supermoon is a genuine astronomical event — just perhaps a bit more modest in drama than the media coverage suggests. Even so, any excuse to step outside and look up at a full moon is a good one.