On Monday, the moon will be the biggest and brightest it has been in about 60 years. So long as the sky is clear of clouds, it should be a great time to get outside and gaze at it or take some photos.
It’s what is commonly called a “supermoon”, or technically a “perigee full moon” – a phenomenon that occurs when a full moon coincides with the moon being the closest it gets to the Earth on its orbit.
What makes this one special is that the moon is going to be even closer to the Earth than it normally gets.
How much bigger will it be?
At 8.09pm GMT, the moon will pass by the Earth at a distance of 356,511km – the closest it has passed the Earth since 1948. As it does so, it will be a full moon, making it a particularly big supermoon.
Supermooons are roughly 30 per cent larger in area and 30 per cent brighter than the smallest full moons – full moons that happen when the moon is at its furthest distance from Earth: at “apogee”. In terms of diameter – the width of the moon – it will be about 14 per cent wider than the smallest full moons.
How bright will it look?
While a supermoon is 30 per cent brighter than the smallest full moons, it’s only about 15 per cent brighter than an average full moon.
How big will it look?
When it comes to the size, the difference in width (diameter) between a supermoon and an average moon is about 7 per cent. When the moon is high in the sky, that difference is something you’re unlikely to notice, because the sky is big and there’s nothing to measure it against.
But if you could compare it to a moon at apogee (when it’s farthest) you would probably be able to see the difference.
Supermoon and the moon illusion
What’s more, the boost in actual size of the moon’s image from a supermoon is totally swamped by what’s known as the “moon illusion”, which affects your perception of the size of the moon.
When the moon is close to the equator, it can appear up to 300 per cent the size it does when it is high in the sky – which makes much more of a difference than the actual 7 per cent boost you get from it being a bit closer to the Earth.
That moon illusion (as the name suggests) is a complete illusion – the image of the moon does not change significantly at all as it moves from the horizon up into the sky. But, when it is close to the horizon, observers think it looks bigger. Exactly what causes the moon illusion is still a matter of debate. But there are lots of possible explanations.
Nevertheless, if you go out and look at the moon on Monday night, when it is near the equator, you will get both the psychological effect of the moon illusion, and the physical effect of the supermoon.
What causes a supermoon?
The moon’s orbit around the Earth is not quite a circle but an ellipse – a kind of squashed circle.
Ellipses are described mathematically with two foci, one at either side of the centre. When an orbit is elliptical, the big body in the middle (the Earth in this case) sits at one of those two foci.
Since the Earth is sitting off to one side of the ellipse, the moon is inevitably closer to the Earth when it passes that side, and further away as it passes the other side. When it is at the close side (called “perigee”), and it is a full moon, it’s called a supermoon. (That name was actually made up in the pseudoscience field of astrology but it has entered the common lexicon.)
Why are supermoons not all the same size?
In short, the reason is that the shape of the ellipse that the moon draws around the Earth is changing all the time as it is pushed and pulled by other gravitational forces.
As a result, how stretched-out the ellipse is changes. When the supermoon coincides with a very stretched out ellipse, a supermoon is even closer (and bigger). That’s what happened in 1948 – and what will happen on Monday.
In more technical terms, the moon’s orbit’s “eccentricity” varies. Mathematically, an ellipse can have an eccentricity between zero and one. If the eccentricity is zero, it is a circle. As the eccentricity closer to one, the ellipse gets more and more stretched-out. Once it reaches one, it breaks open and becomes a parabola.
Now, the moon’s eccentricity has an average value of just 0.0549, making it incredibly close to a circle. But it varies from about 0.0255 to 0.0775, said Nasa.