What causes the Moon to occasionally appear bigger and brighter than usual?
The largest and most luminous Moon witnessed by Earth-based observers in 2013 will appear on 23 June at 11:32 (Universal Time). This astronomical event – known commonly as a supermoon, or more formally as perigee-syzygy – occurs on average between four and six times a year.
Because the Moon follows an elliptical orbit around the Earth its distance from our planet will vary during the lunar cycle. Twice during this 27.32-day rotation, the Moon will be closer than usual and twice it will be farther away. The farthest point from Earth in the orbit is known as the apogee, the closest point is called perigee – and the latter is up to 50,000 kilometres (31,000 miles) closer to Earth than the former.
The supermoon phenomenon we’re explaining here describes the coincidence of a full Moon lunar phase at or very near to the Moon’s closest approach to Earth.
The term ‘supermoon’ was first coined in the late-Seventies by astrologer Richard Nolle, who described it as a new or full Moon at or near its closest approach to Earth during a given orbit. According to NASA a super full Moon can appear some 14 per cent bigger and 30 per cent brighter in the night sky when it occurs within an hour of perigee. The best conditions under which to view this enlarged and dazzling lunar display is when the Moon is near the horizon. This means that familiar objects, like mountains and man-made structures, can be used as a yardstick to compare the oversized satellite.
The more intense lunar gravity at perigee also pulls high tides even higher than normal. Indeed, these perigean tides can be up to 15 centimetres (six inches) higher, depending on the local landscape.
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