The New Full Moon Illusion?

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The full Moon on 14 November 2016 is the perigee-syzygy full Moon for 2016. This is a term used to describe the full Moon which occurs nearest to when the Moon is at the closest point in its orbit to the Earth. Perigee is achieved on average once every 27.55455 days (an anomalistic month) but this doesn’t always coincide with when the Moon appears full in the sky. The period between the same phase of the Moon occurring – e.g. full Moon to full Moon – is known as a synodic month and is equal to 29.53059 days.

Syzygy is a term describing a situation when three bodies in a gravitational system appear in a line. This is the case when the Moon is full; the Moon, Sun and Earth being the bodies in alignment. Most of the time the alignment isn’t precisely perfect – if it were, we’d get a lunar eclipse during every full Moon, but it’s close enough.

A perigee full Moon will, in theory, appear at its largest and brightest in the sky, something the popular press and websites exploit for attention and a few extra clicks. Indeed, in the run up to November 14, there are many articles appearing which describe the upcoming perigee-syzygy full Moon. Many of these articles use the term ‘Supermoon’ to describe this event, a misleading and unnecessary term which fails to tell you very much about what it happening.

The 14 November perigee-syzygy Moon will be the largest to appear in our sky since 1948 and you’ll need to make sure you catch sight of it because you won’t see another full Moon this large until November 25th, 2034. Exciting stuff, except that it isn’t true.

The apparent size of the Moon depends on how far away it is from you. Different location on Earth experience different apparent diameters. Imagine a full Moon directly above your head. A line from the centre of the Earth to the centre of the Moon would pass through you and, consequently, the distance between your location on the surface of our planet and the centre of the Moon would be at a minimum. Now consider a superhero, say SuperMan (…sigh…) standing on precisely the other side of the Earth to you, so a line from him to the centre of the Moon would also pass through the centre of the Earth and you. He’s got super-vision (make it stop!) which can see the Moon through the body of the Earth. So how big does the Moon look to him? Well it looks smaller than if does to you because it’s further away, an Earth diameter further away to be precise.

Your average non-super being wouldn’t be able to see the Moon because the Earth’s bulk would block the view, but as the Earth rotated on its axis, it would eventually bring it into view. Ignoring the natural variation in distance caused by the Moon’s non-circular orbit, etc., as the Moon rises it will be getting closer to you until, when it’s on the meridian line (the imaginary line passing from north to south on the horizon which also passes though your zenith or overhead point) it will appear closest to you.

When you do factor in the variability caused by the Moon’s orbital shape, and the fact that the Moon doesn’t orbit the centre of the Earth – but rather the barycentre of the Earth-Moon system – things get a bit more complex.

Let me give you an example using my own location in the UK. Full Moon in November 2016 occurs on the 14th at 13:53 UT, 3 hours after perigee. From where I live, the Moon will be below the horizon at this time. If I had super-powers allowing me to see through  the Earth, I would see the Moon with an apparent diameter of 1997.2 arcseconds and its centre would be 358,869.9 km from my location.

As the Earth turns on its axis the distance between my location on the Earth’s surface and the centre of the Moon changes. The Moon is moving away from perigee as it rises so it’s becoming more distant but the rotation compensates for this as it brings the Moon closer to me personally. So much so, that the Moon will appear to have an apparent diamter of 2038.2 arcseconds at 23:45 UT on 14 November. At this time the distance between me and the Moon’s centre will have reduced to 351,652.8 km. In other words, the rotation of the Earth has moved my location on the Earth’s surface so that I’m now 7,217.1 km closer to the Moon’s centre than when the Moon was full at 13:52 UT. My closest approach is around 00:15 on November 15, 2016 when the distance between me and the Moon’s centre will have shrink to just 351,633.6 km.

Okay, this is just pedantry, lighten up – it’ll still be huge and the largest full Moon until 25 November 2034! Well, not exactly.

The full Moon which occurs on 2 January 2018 will be the perigee-syzygy Moon for 2018. However, the timing is much better for my location.  Full Moon occurs at 02:25 on 2 January, and the Moon will appear 2036.9 arcseconds across and be 351,873.2 km away – wow, that’s just 239.6 km further than when it was closest to my location at 00:15 UT on November 15th, 2016!

But it gets better. At 23:45 UT on 1 January 2018 the Moon’s disc will appear 2041.3 arcseconds across and lie 351,113.9 km away from my location. And at 00:00 UT on 2 January 2018 it will appear 2041.4 arcseconds across and be 351,108.3 km away. All of these figures beat those for the November 14th, 2016 perigee-syzygy Moon that many articles said would be the record holder until 25 November 2034. Awkward!

Of course this is all rather nonsensical because the size difference between the perigee-syzygy full Moons and others which occur slightly further away from perigee (e.g. the October and December 2016 full Moons) will be so tiny that you’d wouldn’t notice it.

During the late autumn and early winter, full Moons for northern hemisphere observers, arc high across the sky. Contrast this with the full Moons of summer which appear low down. This situation, combined with the hype which will accompany the November 14th perigee-syzygy, will lead people to perceive the Moon to be much larger than it actually is normally. Perhaps this should be called the New Full Moon Illusion?

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