On Monday, 14 November, the moon will be the biggest and brightest it has been in more than 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, making it a tiny bit bigger than even your average supermoon.
But, despite a lot of hyperbolic news written about the event in the past few days, don’t be too surprised if it looks much like any other full moon.
Wondering what’s really going on and how to catch a glimpse? We’ve got you covered.
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% larger in area and 30% 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% wider than the smallest full moons.
The difference between this unusually big supermoon and other supermoons like the ones you could have seen on 16 October or you could see on 14 December is negligible.
How bright will it look?
While a supermoon is 30% brighter than the smallest full moons, it’s only about 15% brighter than an average full moon. That’s nothing to sneeze at – on a clear night, away from city lights, it will provide more moonlight than you’d usually get from a full moon.
But, anywhere near the city, that difference is likely to be difficult to perceive. And, of course, clouds or haze could wipe out the difference, or indeed cover the moon completely.
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%. 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. The image below shows that difference.
Supermoon: difference in size between the smallest and largest moon appearances
A supermoon occurs when a full moon coincides with the closest (perigee) position in the moon’s orbit, resulting in a moon that appears larger and brighter than normal. Here you can see the size difference compared with the smallest possible full moon (apogee)
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 horizon, it can appear up to 300% the size it does when it is high in the sky which makes much more of a difference than the actual 7% 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 14 November, when it is near the horizon, you will get both the psychological effect of the moon illusion, and the physical effect of the supermoon so it could look particularly striking.
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.)
Supermoon: orbit showing the closest and most distant points
A supermoon occurs when a full moon coincides with the closest (perigee) position in the moon’s orbit, resulting in a moon that appears larger and brighter than normal
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, according to Nasa.