Supermoon
Encyclopedia
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.
between approximately 357000 kilometres (221,830.1 mi) and 406000 km (252,277.3 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.
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.
, is greatest when the Moon is either 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.
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, however such claims are not supported by scientific evidence.
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.
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...
(or a new moon
New moon
In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...
) with the closest approach the Moon makes to the Earth on its elliptical orbit, or perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
, leading to the technical name for a supermoon of the perigee-syzygy
Syzygy (astronomy)
In astronomy, a syzygy is a straight line configuration of three celestial bodies in a gravitational system. The word is usually used in reference to the Sun, the Earth and either the Moon or a planet, where the latter is in conjunction or opposition. Solar and lunar eclipses occur at times of...
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 monthMonth
A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...
between approximately 357000 kilometres (221,830.1 mi) and 406000 km (252,277.3 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, defined as: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
Syzygy (astronomy)
In astronomy, a syzygy is a straight line configuration of three celestial bodies in a gravitational system. The word is usually used in reference to the Sun, the Earth and either the Moon or a planet, where the latter is in conjunction or opposition. Solar and lunar eclipses occur at times of...
is a full
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...
or new moon
New moon
In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...
, 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.
Effect on tides
The combined effect of the Sun and Moon on the Earth's oceans, the tideTide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
, is greatest when the Moon is either new
New moon
In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...
or full
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...
. 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
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
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
Solent
The Solent is a strait separating the Isle of Wight from the mainland of England.The Solent is a major shipping route for passengers, freight and military vessels. It is an important recreational area for water sports, particularly yachting, hosting the Cowes Week sailing event annually...
in the UK, however 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 tsunami2011 Tōhoku earthquake and tsunami
The 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...
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
Tensor
Tensors are geometric objects that describe linear relations between vectors, scalars, and other tensors. Elementary examples include the dot product, the cross product, and linear maps. Vectors and scalars themselves are also tensors. A tensor can be represented as a multi-dimensional array of...
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 CanadaCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
- BBC News, Gallery of shared pictures