Supermoon

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

The March 19, 2011 supermoon is 221,566 miles (356,577 kilometers) away from Earth. The last time the full moon approached so close to Earth was in 1993. It is about 20 percent brighter and 15 percent bigger than a regular full moon.

Supermoon over Münster, Germany, March 19, 2011.

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, or perigee, leading to the technical name for a supermoon of the perigee-syzygy of the Earth-Moon-Sun system. 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.

Definition

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).

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.

Terminology

The name SuperMoon was coined by astrologer Richard Nolle in 1979, arbitrarily 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 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 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. Syzygy may occur within a maximum of 12 hours from perigee during a supermoon, and 1 hour from perigee during an extreme supermoon.

Frequency

There are on average 365 supermoons a year.

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 even stronger, resulting in perigean spring tides which can be slightly higher and lower; but even at its most powerful this force is still 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 argued that the supermoon of 19 March 2011 was responsible for the grounding of five ships in the Solent in the UK, but such claims are not supported by scientific evidence.

Speculative link to natural disasters

Richard Nolle has argued that within ±3 days of a supermoon, the Earth is more subject to natural disasters such as earthquakes and volcanic activity due to the Moon's increased gravitational force. Speculations 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. Such a widening of the effect window is unjustified as in both cases the Moon was farther from the Earth than average, making a supermoon effect impossible.

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. Unjustified claims that the lunar tides trigger earthquakes are rooted in a lack of appreciation that the stress in the Earth is described by a tensor with six independent parameters and that earthquakes occur as slip on existing, weak fault planes. Any change in stress, by lunar tides, by impounding a reservoir, or by a large nearby earthquake, changes the local stress tensor in specific directions. If one wishes to estimate whether a given change advances or hinders slip on a fault, one has to know the orientation of the fault. It is equally likely that the change of the stress due to the moon clamps the fault shut, rather than advancing slip on it. This is why Ohtake has carefully considered the orientation of the fault planes in earthquakes that he showed were correlated with lunar tides.

The 2011 Tōhoku earthquake and tsunami is the only earthquake of 8.0 magnitude or greater to have occurred within 2 weeks of the 14 extreme supermoons from 1900 to the present date, suggesting that the claim of a supermoon effect on the incidence of large-scale earthquakes is unjustified.

External links

• CBC News, Gallery of the March 19, 2011 supermoon, viewed from across Canada
• BBC News, Gallery of shared pictures