Supermoons

The Supermoon of Monday 14 November 2016 and other Supermoons.

The moon goes round the earth in an elliptical or oval orbit so each month there is a near point and a far point from the earth. At its nearest, or perigee, it appears bigger and brighter than at its farthest and since about 1979 these have been known a supermoons. The difference is around 15% in size and 30% in brightness. The moon will be at its nearest to the earth for nearly 80 years at 1132 UT on 14 November and so will appear correspondingly larger than at other times. According to Astronomy Now the moon will then be 356512 km away with an angular diameter of 33̍ 31". That’s about how big a table tennis ball appears at 4.1 metres. The full moon comes soon after at 1352 UT. We can’t see the moon here at those times but people in Australia and NZ can and AN claims that it will subtend a whopping 33'.8 at around 3am in NZ. Actually they will do better to look at midnight when it will be closer to them than at 3am (see below) and the moon’s diameter will be 33'.86 with the moon as good as full.

Nevertheless, in the UK the moon will not be far off full and still near its closest point to earth by the time we see it on the Monday evening. In addition, the earth’s spin brings us closer to the moon when it is near to due south from our location. This reduces the range by a few thousand kilometres. The heavens-above website has a wonderful sky map which gives the range or actual distance of the moon from any chosen location on any date. I am near Wellington, Somerset, so looked up the figures for my location. An optical illusion makes the rising moon appear bigger than when it is high in the sky but the range at around moonrise [1707] will be 356451 km with a maximum diameter of 2012 arcseconds. The closest point, 351629 km, comes at 20 minutes after midnight on Tuesday morning when it is near the meridian. It would then subtend 33'.98 or 2039 arcseconds; a small but significant increase of 1.3% on moonrise and 2.6% in brightness. The lower latitude of Wellington NZ means that the moon will be 352867 km from there and Wellington NZ’s best would be 33'.86 or 2032 arcseconds so we get a bigger moon here, just[!] and we will be 1238 kms closer, which is a lot of kms. And, if the range is to the moon’s centre which seems to be the case then we are another 1738 km closer to its surface which is good news for observers of central features of the moon although it will be dazzlingly bright.

However, the best place of all would be at somewhere south of Hawaii around 13ºN latitude where the moon will be some 350142km away and 34'.13 or 2048 arcseconds at the time of perigee. Indeed we can generalise by saying that for any perigee the closest point on the earth to the moon will be wherever the moon is directly overhead at that moment.

The actual distance depends a lot on where we are on the earth and what time perigee happens. The best ones in the UK are full moons between mid-November and mid-January and around midnight when the moon is high in the sky and perigee close to midnight. The perigee of 14 November 2016 is the closest centre to centre approach until 25 November 2034. However, for my location and probably the whole of the UK we actually get a better one here at the beginning of 2018. On 1-2 Jan 2018 perigee at 2154GMT will be 356566 km from earth not as close as this November but heavens-above shows a range of 351116 km for us just after midnight, much closer [by 513km] than this year’s and 2042 arcseconds. 23/24 Dec 2026 is even better with the moon at a closest of 350938km from my location.

The 25/26 November 2034 supermoon is the next closest approach to the earth with a perigee of 356448 km at 2206 GMT which is good for us but the range from here will be 351230 km – closer than this year but not quite as close as either 2018 or 2026. For us, and as far as I can tell the December 2026 supermoon will only be bettered here in 2043 and 2080.

11 November 2016

References. Information drawn from:

http://astropixels.com/ephemeris/moon/moonperap2001.html [accessed 4 November 2016]

www.heavens-above.com  [accessed 4 November 2016]