Supermoon

A NASA image of the perigee moon of March 19, 2011 over Washington D.C. near the Lincoln Memorial.

In astronomy and astrology, a perigee-syzygy or "supermoon" is a full or new moon that coincides with a close approach by the Moon to the Earth. The Moon's distance varies each month between approximately 357,000 kilometres (222,000 mi) and 406,000 kilometres (252,000 mi) due to its elliptical orbit around Earth (distances given are center-to-center).

Definition

The supermoon of March 19, 2011 (right) compared to a more average moon of December 20, 2010 (left)

The name SuperMoon was coined by astrologer Richard Nolle in 1979, defined as:

...a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, Moon and Sun are all in a line, with Moon in its nearest approach to Earth.

(The phrasing "within 90% of its closest approach" is unclear, but an example on Nolle's website shows that he means that the Earth-Moon distance is in the lowest tenth of its range.)

The term supermoon is not widely accepted or used within the astronomy or scientific community, who prefer the term perigee-syzygy. Perigee is the point at which the moon is closest in its orbit to the Earth, and syzygy is full or new moon, when the Earth, the moon and the sun are aligned. Hence, supermoon can be regarded as a combination of the two, although they do not perfectly coincide each time.

The size and brightness of an object follows an inverse-square law, which means that a full moon at perigee is 12% larger and brighter than an average full moon. However, because the offset of the moon's orbit versus its phases is only two days, this change in appearance is gradual from month to month and therefore is not usually noticeable to a casual observer.

Effect on tides

The combined effect of the Sun and Moon on the Earth's oceans, the tide, is greatest when the Moon is new or full. At lunar perigee the tidal force is even stronger, resulting in larger high and low tides on average, but even at its most powerful this force is still weak.

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.

Speculative link to natural disasters

Some studies have reported a weak correlation between lunar activity and shallow, very low intensity earthquakes. However, no evidence has been found of any correlation with major earthquakes.

It has been speculated that the Indian Ocean tsunami and earthquake on December 26, 2004, was influenced by a supermoon which occurred 2 weeks later on January 10, 2005. Similar speculation was made with the 2011 Tōhoku earthquake and tsunami which occured 8 days prior to the the closest supermoon since 1992. In both cases the Moon was closest to the apogee (greatest distance), making a supermoon effect impossible. However, the three closest supermoons in the twentieth century did not coincide with any earthquakes above 6.0 M_W.

Dates of supermoons between 1950 and 2050

A supermoon image of March 19, 2011

March 2011 supermoon rising over the Atlantic Ocean

There are approximately four to six supermoons annually. The following is a list of past and predicted extreme supermoons.

• November 10, 1954
• November 20, 1972
• January 8, 1974
• February 26, 1975
• December 2, 1990
• January 19, 1992
• March 8, 1993
• January 10, 2005
• December 12, 2008
• January 30, 2010
• March 19, 2011
• November 14, 2016
• January 2, 2018
• January 21, 2023
• November 25, 2034
• January 13, 2036

See also

• Gallery of the March 19, 2011 supermoon, viewed from across Canada