Supermoon

At 8:09PM GMT, on November 14th, the moon will pass by the Earth at a distance of 356,511km – the closest it gets to the Earth on its elliptical orbit and 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, technically a “perigee full moon.”

Supermoons 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, its “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 is negligible.
While a supermoon is 30% brighter than the smallest full moons, it’s only about 15% brighter than an average full moon.
When it comes to the size, the difference in width (diameter) between a supermoon and an average moon is about 7%.
The differences in sizes comes from the shape of the ellipse that the moon draws around the Earth as it is pushed and pulled by other gravitational forces.

This is distinct from the so-called 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 EarthThat moon appearance (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. So the senses are not deceived, the mind deceives itself. Exactly what causes the moon illusion is still a matter of debate.