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Popular Astronomy Magazine - November-December 2017
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Sun, 03 Dec 2017


Today's 'supermoon' - what's all the fuss?


Supermoon

The sizes of the Full Moon at perigee and apogee compared. Credit: Skymania.com

You can’t have failed to notice that the papers and news websites are once again reporting that there is a “supernoon” happening today.

In fact even NASA is joining in the hype with a video about a “supermoon trilogy” involving the Full Moons of today, 1 January and 31 January. (You can watch it below).

The BBC website reports  that today “At 15:47 GMT the Moon will appear about 7% larger and 15% brighter, with moonrise about 45 minutes later.”

As we have recorded before, and as most amateur astronomers know already, the Moon’s distance from Earth varies during its orbit. Its closest point is called the perigee, and the most distant is apogee. What is being called a “supermoon” is the coinciding of Full Moon with its point of perigee. 

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?

The Full Moon of November 2016 was said to be the closest since 1948.  On the 14th of that month, the Moon reached perigee at about 11.30 am GMT, at a distance from Earth’s centre of 356,509 km. This happened just a couple of hours before Full Moon itself.

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.

By Paul Sutherland and Robin Scagell

 

Added by: Paul Sutherland