Saturday 22 June 2013

The Lunar Perigee.

At 11.11 am GMT on Sunday 23 June 2013, the Moon will be at its closest point to the Earth in 2013, at 356 989 km. 32 minutes later it will officially be full (completely illuminated when seen from Earth), which means the two events will be virtually indistinguishable to human observers, something that has bee dubbed the 'Supermoon' by the popular press. The Moon completes one orbit about the Earth every 27.5 days, and like most orbiting bodies, its orbit is not completely circular, but slightly elliptical, so that the distance between the two bodies varies by about 3% over the course of a month. This elliptical orbit is also not completely regular, it periodically elongates then returns to normal, making some perigees closer than others. These cycles mean that the Moon will reach its furthest point from the Earth (apogee) of the year in the same lunar cycle, 406 491 km at 0.37 am on 7 July 2013.

The moon will appear 16%larger and 30% brighter at perigee than it will at apogee. National Geographic.

While this is being widely hailed as a remarkable event, it is a fairly regular occurrence, and the moon does, periodically, get slightly closer to us than this; it reached 356 953 m on 6 May 2012 (35 m closer) and will be 356 921 m from us on 1 January 2014 (68 m closer). The 1 January perigee will not be the closest of 2014, which will occur on 10 August, when the Moon will reach 356 896 m from the Earth (100 m closer than on 23 June 2013). Thus the closest Moon of 2013 will be exceeded twice in 2014.

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