Furthest Lunar Apogee of 2017.

On Tuesday 19 December 2017, at 1.28 am GMT, the Moon will be at its furthest point from the Earth in 2017, a distance of 406 604 km. The Moon orbits 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. Because this is an elongating and contracting elliptical orbit, rather than a change in the average distance between the Earth and the Moon, the most extreme Lunar Perigee and Apogee of each year typically happen in the same Lunar Month; though this year the closest Lunar Perigee occurred 26 May at 1.24 am GMT, with the second closest occurring to weeks ago at 8.43 am on Monday 4 December.

Diagram showing the relationship of the Lunar orbit and Lunar month. Southern Astronomical Delights.

Although this is the furthest point from the Earth that the Moon will reach in 2017, it is not exceptional. The Moon reached 406 659 km from the Earth on 31 October 2016, and will reach 406 634 on 16 August 2023.

 

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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.

See also The December Solstice, Partial eclipse of the Sun to be visible from most of Africa, as well as parts of southern Europe, the Middle East and North and South America, The September Equinox, The Lunar Apogee and The Earth's aphelion.

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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.

See also The Northern Solstice, The March Equinox, Total eclipse of the Sun visible from Australia, Papua New Guinea and the Solomon Islands, 9 May 2013, The Earth approaches its perihelion and The Southern Solstice.

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