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Tuesday, November 15, 2016

Super Moons

People may be asking you about the forthcoming Supermoon (on 14 November 2016) so I have compiled the following AstroAlert. this one in November is extra special (in some folk’s estimation). Read on From: Francis Podmore podmore@zol.co.zw AstroAlert 12 November 2016 SUPERMOONS, and Micro Moons Have you heard? “Biggest Supermoon since 1948 to occur in November 2016”. What’s going on? And when is the next one going to happen? The motion of the Moon around the Earth is extraordinarily complicated. The orbit is elliptical (i.e. a slightly squashed circle), it is tilted at about 5° to the plane of the Earth’s orbit around the Sun, and the orbit itself rotates slowly once every 19 years. In consequence the distance of the Moon from the Earth varies significantly so that when a Full Moon happens to coincide (almost) with the Moon at perigee (the minimum distance from Earth) the Moon will look larger in the sky than normal because it is closer (a Supermoon), whereas when the Full Moon occurs when the Moon is furthest from the Earth (at or near apogee) we get so-called Micro Moon, when the Moon is at its smallest. Some numbers: the average Earth-Moon centre-to-centre distance is 384,400 km, the average perigee distance is 363,296 km, the average apogee distance is 405,504 km. The Moon can therefore be 384,400/363,296 = 1.058 or about 6% closer than the average, making the Moon’s angular diameter 6% larger and thus 12% brighter since the illuminated area is greater. The reduction at apogee is 405,504/384,400 = 1.055 or 5.5% further away so that the Moon’s is 5.5% smaller and 11% dimmer. But the perigee distance itself varies, quite a lot, as does the apogee distance, due the variation in the gravitational pull of the Sun on the Moon, and the varying Sun – Earth distance. In the 5000 year period from -1999 to 3000 AD the perigee distance varies from 356,355 to 370,399 km. The biggest Supermoon would be 8% larger and 16% brighter than the average. The apogee distance ranged from 404,042 to 406,725 km. On 14 November 2016 Full Moon occurs at 15:52 and when it rises at 18:10 it will be 356,512 km away, which is the closest it has been since 26 January 1948. That means it will be 8% larger and 12% brighter than normal. To see the size comparison, take a 25 cm diameter dinner plate and put it next to, or behind a plate 23 cm across. The next time such a significant Supermoon will occur is 25 November 2034. Where can it be seen? You don’t have to be in any special place, or look at any particular time. Anyone who can see the Moon will see it larger than average, all night long, because a Full Moon rises at sunset, is near overhead at midnight, and sets as the Sun comes up. And a day or two either side of Full Moon it will be only very slightly less than the Supermoon size. There is no ‘official’ definition of Supermoon or Micro Moon. The word was coined in 1979 by astrologer (!) Richard Nolle. He defined it as “a New or Full Moon occurring at or near (within 90%) of its closest approach to Earth”. [But you cannot see a Supermoon at New Moon because it entirely backlit by the Sun!] A better definition is: “ a New or Full Moon which occurs when the Moon is less than 360 000 km from the centre of the Earth”. Consequently the Full Moons of 16 October and 14 December this year are also Supermoons as the distances are 357 860 and 358 483 km respectively, but these are not quite as close as the 14 November event. The next Supermoons are 3 December 2017 and 2 January 2018. These Supermoon events may inspire you to go and admire the glory of our celestial companion. But whether we see any of these depends on the clouds....... Comparing these perigee and apogee distances gives a 14% increase in the Moon’s size, and consequently a 30% increase in brightness from Micro Moon to Supermoon. (Photo from Astropixels) The variation of Full Moon distance in kilometres 2004 – 2023. The red dots show the Full Moon dates. Apogee at the top, perigee at the bottom. Strictly the Supermoons are those occurring at the lowest points of the graph with the Micro Moons at the highest points but the events below the blue line are when the Moon could be described as a Supermoon. (Graph from http://jgiesen.de/moon/FullMoon/) Are there any physical effects on the Earth? Some people have blamed Supermoons for barious natural disasters but this is not widely accepted. The tides of the sea around Supermoon dates will be larger than usual because tides are caused by the gravitational pull of the Moon and Sun. The actual distance between you and the Moon depends where you are on the planet – latitude, longitude and altitude (you may be up a mountain). www.mooncalc.org is a truly amazing website to give you distance and position information and graphics, immediately. Other useful Supermoon websites are: https://en.wikipedia.org/wiki/Supermoon http://astropixels.com/ephemeris/moon/moonperap2001.html Lists all the perigee and apogee distances and times from 2001 to 2100, but not linked to the cycle of New and Full Moons. https://www.fourmilab.ch/earthview/moon_ap_per.html The title is “Inconstant Moon”. http://jgiesen.de/moon/FullMoon/ Dr Giesen’s website has many other astronomical calculations and links. Many people have noticed the Moon (and Sun) looking larger when near the horizon. This is called the Moon Illusion – see links on this webpage. http://earthsky.org/?p=190918 A comprenhensive article. https://www.timeanddate.com/astronomy/moon/super-full-moon.html ================================================================================== Francis Podmore (podmore@zol.co.zw)

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