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.
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 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.)