A supermoon is the coincidence of a full moon or a new moon with the closest approach the Moon makes to the Earth on its elliptical orbit, resulting in the largest apparent size of the moon’s disk as seen from Earth. The technical name is the perigee-syzygy of the Earth-Moon-Sun system. The term “supermoon” is not astronomical, but originated in modern astrology.The association of the Moon with both oceanic and crustal tides has led to claims that the supermoon phenomenon may be associated with increased risk of events such as earthquakes and volcanic eruptions. However, the evidence of such a link is widely held to be unconvincing.
The next occurrence will be on 23 June 2013. This full moon will not be only the closest and largest full moon of the year. It also presents the moon’s closest encounter with Earth for all of 2013. The moon will not be so close again until August, 2014.
The Moon’s distance varies each month between approximately 357,000 kilometers (222,000 mi) and 406,000 km (252,000 mi) due to its elliptical orbit around the Earth (distances given are center-to-center).
According to NASA, a full moon at perigee is up to 14% larger and 30% brighter than one at its furthest point (apogee).
The name SuperMoon was coined by astrologer Richard Nolle in 1979, arbitrarily defined as:
The term supermoon is not used within the astronomical community, which use the term perigee-syzygy or perigee moon. Perigee is the point at which the Moon is closest in its orbit to the Earth, and syzygy is a full or new moon, when the Earth, the Moon and the Sun are aligned. Hence, a supermoon can be regarded as a combination of the two, although they do not perfectly coincide each time. On average, about once a year the moon becomes full within a few hours of perigee.
Effect on tides
The combined effect of the Sun and Moon on the Earth’s oceans, the tide, is greatest when the Moon is either new or full. At lunar perigee the tidal force is somewhat stronger, resulting in perigean spring tides. But even at its most powerful this force is still relatively weak causing tidal differences of inches at most.
As the tidal force follows an inverse-cube law, that force is 18% greater than average. However, because the actual amplitude of tides varies around the world, this may not translate into a direct effect.
It has been claimed that the supermoon of March 19, 2011 was responsible for the grounding of five ships in the Solent in the UK, but such claims are not supported by scientific evidence.
Certain prognosticators have moved the goalposts to within 1 or 2 weeks of a supermoon to suggest a causal relationship with specific natural disasters such as the 2011 Tōhoku earthquake and tsunami and the 2004 Indian Ocean earthquake and tsunami. However in both cases the Moon was actually farther from the Earth than average. No evidence has been found of any correlation with major earthquakes.