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What is a Supermoon?

The month of June is bringing with it a Supermoon. This is a celestial event that occurs when the Full Moon is at perigee, meaning that it is at a point of its orbit that brings it closer to the Earth. This makes it appear brighter and larger than average.

How do Supermoons work? And, most importantly, can we really notice the difference?

What do ‘perigee’ and ‘apogee’ mean?

The first thing to understand is that the orbit of the Moon around Earth is not circular, but elliptical, in a similar way that Earth itself travels around the Sun.

Each month, the distance between our planet and its satellite will alter from perigee (closest) to apogee (farthest). These distances, measured between the centre of the Earth and the centre of the Moon, are an average of 363,400km for the perigee and 405,350km for the apogee.

Are Supermoons visible to the naked eye?

You might have seen one of those stunning and dramatic pictures of a massive Moon rising behind famous monuments, from the Acropolis in Athens to the Statue of Liberty in New York. However, and as striking as those images are, the changes in the size of the Moon are not actually visible to the naked eye, unless you are a very observant Moon watcher.

Only photographs taken at certain times will show a difference. This is because, even though the difference between the perigee and the apogee is of around 42,000km, it is not that much of a change when compared to the hundreds of thousands of kilometres that separate us from our celestial neighbour – the Moon will, in fact, only appear 14% larger than usual. What does happen though, is that Supermoons show brighter than usual, around 30%.

The Moon Illusion

You might be thinking that the Moon does appear extremely large when it is rising or setting, and you are right. However, this visual effect, called the ‘Moon Illusion’ does not actually mean that the Moon is larger.

The Moon is always the same size, but our eyes are playing tricks on us. Don’t believe us? Try covering the Moon with your finger, held at arm’s length; you will find that any finger is more than big enough to cover the Moon.

Even though there have been many attempts to explain the Moon Illusion, it seems that it’s simply our brain comparing the satellite to the distant horizon, and assuming that it is at the same distance – although it is, in fact, much farther away.

 

Learn all about our celestial neighbour with Moongazing by Tom Kerss and the Royal Observatory Greenwich. This in-depth guide includes detailed Moon maps and covers the history of lunar observation and exploration, the properties of the Moon, its origin and orbit.

Moongazing is available now.