What is a Supermoon?

Have you ever glanced up at a full Moon and thought it looked unusually huge and bright? If so, there’s a good chance you were looking at what astronomers call a ‘Supermoon’. These special full Moons are loved by sky-watchers because they appear more dramatic than usual.

So what do we mean by “Supermoon”?

To understand a Supermoon, we need to look at how the Moon travels around Earth. Unlike a perfect circle, the Moon’s path is slightly squashed into an oval shape called an ellipse. Because of this, the Moon spends part of its orbit a little closer to us and part of it a little farther away. Because the full Moon doesn’t always happen at the same point in the Moon’s orbit, sometimes the full Moon lines up with the Moon’s closest approach to Earth — and that’s when we get a Supermoon.

A Supermoon happens when a full Moon occurs at a time when the Moon is nearer to Earth than average. In other words, it’s simply a full Moon that takes place close to the Moon’s closest point in its orbit. The Moon’s nearest point to Earth has a name: perigee. If the full Moon lines up with perigee (or close to it), that full Moon becomes a Supermoon. On the other hand, when the Moon is at its farthest point, known as apogee, a full Moon can look smaller and dimmer — some people call that a Micromoon.

Does it really look bigger?

Yes — although the amount is small. A Supermoon can “appear” up to about 14% larger and roughly 30% brighter than a Micromoon. This is most noticeable in photographs placed side-by-side, but most people casually looking up won’t spot the size difference as easily as they’ll notice the extra brightness, especially when the Moon first rises over the horizon.

Do Supermoons cause natural disasters?

This is a popular myth, but the answer is no. While the Moon does influence the oceans and creates tides, a Supermoon only makes slightly higher tides (called “perigean tides”). There’s no scientific evidence linking Supermoons with earthquakes, floods, strange behaviour, or anything dramatic. The main effect is simply a more impressive Moon to look at.

How often do they happen?

Supermoons are not rare. We usually get several each year, because full Moons happen regularly and the Moon’s orbit repeats its close and far points. Sometimes they even show up in clusters, where two or three full Moons in a row qualify as Supermoons.

Conclusion

• A Supermoon is a full Moon that happens when the Moon is close to Earth in its orbit.

• That closest point is called perigee (the farthest point is apogee).

• A Supermoon can look up to about 14% bigger and 30% brighter than a Micromoon.

• The Moon doesn’t actually change size — it just appears larger because it’s closer.

• They’re not rare — we usually get several each year.

Fun facts

• Some Supermoons have special nicknames like the “Wolf Moon”, “Strawberry Moon”, or “Harvest Moon”, depending on the month they appear.
• Photographers enjoy shooting Supermoons over city skylines, because zoom lenses make the Moon look huge next to buildings and towers.
• Astronauts on the Moon wouldn’t notice Supermoons at all — the Moon’s surface doesn’t get closer to Earth, only Earth and the Moon’s distance changes!

Written by Laura Ash for Just Good Science Ltd.