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Monday December 11, 2017

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'Supermoon' wows viewers with closest glimpse since 1948

Skywatchers are enjoying the latest "supermoon", after Earth's satellite made its closest approach since 1948.

The moon appeared bigger and brighter than usual as it came closer to Earth than it has in decades. Many enjoyed the spectacle of the Moon making its closest approach to Earth since 1948, appearing about 7% larger than normal and about 15% brighter. In other words, it comes close enough to Earth (222,443 miles or 357,987 km) to enjoy supermoon status.

The Moon will not be this close to Earth again until 25 November 2034. As the Moon traces its orbit around the Earth, we see different proportions illuminated by the Sun. Once in each orbit, our satellite is totally illuminated - a full moon. And as the Moon orbits the Earth every 27 days or so, it travels in an elliptical or oval shape.This means that its distance from our planet is not constant but varies across a full orbit.

Within this uneven orbit there are further variations caused by the Earth's movements around the Sun. We first became familiar with the supermoon label in the year 2011 when the media used it to describe the full moon of March 19, 2011. On that date, the full moon aligned with proxigee – the closest perigee of the year – to stage the closest, largest full moon of 2011.

Generally, supermoons can be up to 14% larger and 30% brighter, but only when compared with the furthest point the Moon gets to within its orbit. These mean that the perigee - the closest approach - and full moon are not always in sync.

But occasions when the perigee and full moon coincide have become known in popular parlance as supermoons. To observers, the differences between a supermoon and a normal full moon are quite subtle. The full moon and perigee realign in periods of about 413 days (one year and 48 days). So we can figure the dates of the closest full moons (<357,000 km) in past and future years.

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