Monday 30 December 2013

The Lunar Perigee.

The Moon will be at its closest to the Earth since 19 March 2011 slightly after 9.00 pm GMT on Wednesday 1 January 2014, when it comes to approximately 356 921 km away (in 2011 it reached 356 577 km from the Earth). However this event is unlikely to be observed from the Earth, as this will happen only two hours and fifteen minutes before the New Moon. 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, with the closest point to the Earth being referred to as the perigee and the furthest point the apogee. 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 since 2 April 2011 on 16 January 2014, when it reaches 406 536 km away.

Simplified diagram of the Moon's orbit. NASA.

While the 1 January 2014 perigee marks its closest to the Earth in several years, it will not actually be its closest pass of 2014. This will occur on 10 August, when the Moon reaches 356 896 km from the Earth (25 km closer than on 1 January). Similarly the 16 January apogee will not be the furthest from the Earth that the Moon reaches in 2014. This will occur on 13 June, when the Moon will reach 406 568 km from the Earth, 32 km more distant than on 16 January.

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