The term 'supermoon' was thought up in 1979 and it describes the occurrence when the Moon's orbit is closest to Earth (perigee) at the same time that the Moon is full. Because of this, out of the 12 or 13 full moons in each year, only 3 or 4 of the are classed as supermoons.
The technical name for a supermoon is perigee syzygy (meaning: of the Earth - Moon - Sun system), however it is known by most as a Supermoon because it is slightly brighter & appears larger, hence the name 'super'. The opposite phenomenon to the supermoon is an apogee syzygy, or a full moon around apogee, which has been called a micromoon. This means the moon is full at the same time the moon is furthest/far away from Earth.
The Moon's orbit around the Earth is an ellipse, and when it is furthest from the Earth, it is called an apogee which has an average of around 253 000 miles (407 000 km) from Earth. The closest point is called a perigee and has an average of 226 000 miles (363 300km) from Earth. The reason for the elliptical orbit of the moon around Earth is the gravitational influence of the sun and other planets, which causes the Moon's orbit to change over time, so extreme perigees & apogees occur predictably over time. This means we can predict when there will be a full moon at extreme perigee.
High & low tides are more extreme with a supermoon, which is because when the Moon is close, Earth's diameter is a slightly larger fraction of the Earth-Moon separation, which means that the Moon's gravitational pull on the oceans (& Earth's crust) has a greater difference between the point on Earth closest to the Moon's centre, and the point on Earth diametrically opposite it. This increases the effects of the tides.
The closest full supermoon of this century will occur on December 6th 2052, so note this in your calendars!
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