Step outside tonight and take a look at that big full moon. Not only is the moon full, but close to the horizon it will appear even bigger than usual.

That's because it is closer. Tonight's full moon is a supermoon, a full moon occurring when the moon is at its closest approach to earth on its elliptical orbit.

According to NASA Science, a supermoon, or perigee full moon can be as much as 14 per cent bigger and 30 per cent brighter than an apogee full moon. It's not always easy to tell the difference. A 30 per cent difference in brightness can easily be masked by clouds or the competing glare of urban lights and there is nothing floating in the sky to measure lunar diameters. Hanging high overhead with no reference points to provide a sense of scale, one full moon looks much like any other.

Low-hanging moons, on the other hand, can create what's called a 'moon illusion.” When the moon is near the horizon it can look unnaturally large when viewed through trees, buildings, or other foreground objects. The effect is an optical illusion, but that fact doesn't take away from the experience.

Supermoon is originally a term from modern astrology for a new or full moon that occurs with the moon is within 90 per cent of its closest approach to Earth in a given orbit. A supermoon now refers more broadly to a full moon that is closer to Earth than average.

The moon's orbit is elliptical, with perigee about 30,000 miles closer to Earth than apogee. The word syzygy, in addition to being useful in word games, is the scientific name for when the Earth, sun, and moon line up as the moon orbits Earth. When perigee-syzygy of the Earth-moon-sun system occurs and the moon is on the opposite side of the Earth from the sun, we get a perigee moon or more commonly, a supermoon.

Tonight's is the first of three supermoons this year. The other are November 14 and December 14.

Tonight and on December 14, the moon is full on the same day as perigee. On November 14, it is full within about two hours of perigee.

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