A full moon is set to occur closer to the Earth on Friday evening than it has done for the past 15 years.
The Moon's elliptical orbit means its distance from the Earth is not constant.It will be a little over 350,000km away as it passes over the northern hemisphere, which is about 30,000km closer than usual.
If the sky is clear it will appear brighter and larger than usual, say astronomers. Friday's full moon could appear up to 14% bigger and 30% brighter than other full moons this year, Nasa said.
"It's only every few years that a full moon happens to coincide with the part of the Moon's orbit when its closest to the Earth," said Marek Kukula, an astronomer at the UK's Royal Observatory.
"What people will see is a full moon that's really bright and a bit bigger than what they're used to." It will appear largest as it rises and sets, but this is a psychological illusion, Dr Kukula said.
"When it's close to the horizon, our brain interprets it as being bigger than it actually is, this is called the moon illusion," he said.
"The size may be striking when it's near the horizon," said Robert Massey of the UK Royal Astronomical Society. However, he cautioned against expecting too much.
"The Moon may be brighter and may appear somewhat larger, but it's really quite hard for the eye to notice the difference; the eye will compensate for the extra brightness, it's not like going from night to day," said Dr Massey. The Moon's brightness varies throughout its annual cycle, during the mid-winter in the northern hemisphere it can appear brighter simply because it is higher in the sky.
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