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Wed, 09 Nov 2016

How super is the supermoon?

A rising Moon appears large by comparison with foreground objects.
Photo copyright Robin Scagell

The papers are full of the forthcoming ‘supermoon’. On 14 November 2016, the full Moon will be at its closest since 1948, so it will appear larger when rising than most people will ever have seen. But how significant is this? And how big will it look?

The Moon’s distance from Earth varies because it has an elliptical rather than circular orbit, as do all planetary and satellite orbits. If the Earth and Moon were the only objects in the Universe, and were totally inflexible and solid, every closest point (perigee) and every farthest point (apogee) would be identical for all eternity. But in the real Universe, there are many more forces acting on the two bodies, notably the direction of the Sun, so the perigee and apogee distances change slightly. How slight?

On 14 November the Moon reaches perigee at about 11.30 am GMT, at a distance from Earth’s centre of 356,509 km. This is just before full Moon itself, which occurs over a couple of hours later at 13.52. The Moon doesn’t rise from the UK for three hours after that, so we won’t see a precise full Moon anyway. Last year, the closest the Moon came was on 28 September, when it was 356,876 km away – that’s 367 km farther away tan at this month's supermoon, or about 0.1%. And as recently as 2005, on 10 January, the Moon was 356,571 km away, just 62 km less close than this coming 14 November. And few people noticed, because although it was a so-called supermoon, it happened to be a new Moon rather than a full Moon.

The term ‘supermoon’ was rarely used at the time, and not by astronomers. It was invented not by an astronomer but by an astrologer, Richard Nolle, in 1979, to describe a perigee new or full Moon, which he claimed could result in geophysical stresses.

Were there any natural disasters at the 2005 supermoon? Well, there was a mudslide in California, killing 10 people, but this followed two weeks of rainfall rather than an earthquake. You can't blame the supermoon for that.

What about the size that the Moon appears? Whenever the Moon, whatever its phase, appears close to the horizon it always appears bigger than when it is high in the sky. This is known as the Moon Illusion. It’s hard to shake off this illusion, even when you know that it isn’t really larger, but it applies not just to the Moon but even the Sun and constellations. The illusion is caused by the brain’s perception of size rather than by any magnification by the atmosphere. Measure it and prove the point. But the increase in size of the supermoon compared with the average, which is 7% smaller, is hard to judge without accurate measurement.

So while it’s true that the coming supermoon will indeed be a little bit closer, and a little bit bigger, and a little bit brighter than usual, you won’t notice any substantial differences. However, the full Moon rising makes a great photo-opportunity, if you can find a low eastern horizon and wait for it with a telephoto lens at the ready.

The Moon on 14 November will rise at 16.46 from London, 16.34 from Aberdeen and 17.04 from Plymouth, but the actual time you will notice it will vary with the height of your horizon. It has an azimuth, in degrees measured from north, of about 65º, which is pretty well east north east, but as it rises at an angle it may be a bit south of that by the time it clears your treeline. Try to find a place with an interesting horizon in that direction and hope for clear skies!

The actual closest point will be at midnight, because at moonrise the Moon is the Earth's radius farther away from us. The figures given above are from Earth's centre, but if we use data for the Moon's distance from an observer at Greeniwch Observatory in London, the Moon's centre will be 351,924 km away at 23h on 13 November, and 351,675 km away at midnight on the 14th–15th). At moonrise on the 14th, when most photographers will be aiming to take their shots, the Moon is 356,352 km away from them.  


Added by: Robin Scagell