The elliptical orbit of the moon means that there is one moment per lunar month when it is closest to Earth. Astronomers call this point the perigee of the orbit. The apparant size of the moon depends on how far from Earth it is, so it appears bigger in the sky at perigee.
When perigee happens at full moon then the moon is obviously bigger and brighter than full moons which don't happen at perigee. In recent years, the phrase supermoon seems to have emerged to describe this situation. It's a little crazy that this phenomenon receives the attention that it does!
Perigee sounds like an easy idea - just the smallest distance between the Earth and moon. However, the Earth and moon are spheres so distance is usually measured from the centre of Earth to the centre of the moon. If you really want to see the biggest possible full moon from the surface of the Earth then a little more investigation is needed!
My copy of SkyMap Pro software tells me that the perigee moon on November 14th 2016 occurs at 11.21am. The perigee distance (moon centre to Earth centre) is 356,509km.
At the time of perigee the moon is below the horizon in Northumberland (and the wider UK). In other words - there's nearly a whole planet Earth between Northumberland and the moon. The moon is actually around 359,000 km from Northumberland at perigee!
The situation improves if we wait a bit.
Here's a plot of the distance between Northumberland and the moon (well, the centre of the moon) over the next few days.
The varying distance is due to the fact the Earth is spinning on its axis. The Earth itself carries Northumberland towards and away from the moon once a day.
In the hours that follow the moon slowly begins to move away from perigee BUT the rotation of the Earth will carry Northumberland closer to the moon. And at a faster rate than the moon is receding. On Monday night, with the moon climbing high into the sky we get our local supermoon: at 12.10 am on Nov 15th the moon will be 351,951 km from Northumberland.
If you look at the graph you'll see there was another minimum before perigee - about 24 hours earlier. The moon was only marginally further away.
Supermoons are overhyped. The oft quoted figure of 10% larger, 30% brighter sounds significant but is very difficult to see in practice because (a) it's comparing apogee full moon with perigee full moon: you never get to see that comparison side by side, (b) the moon illusion complicates the visual appearance just after moonrise (even apogee full moons look big to some people) and (c) the transparency of the atmosphere can markedly change the brightness of the moon and an observer's perception of it!
Even the claim of "no better supermoon until 2034" fails, because the perigee distance referred to in that claim is for moon centre to Earth centre. Actually, for Northumberland (and the rest of the UK) the supermoon on January 1st 2018 will be marginally closer than this one!
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Dr Adrian Jannetta. Amateur astronomer, maths teacher and science enthusiast.