A perigean spring tide is a
tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide tables ...
that occurs three or four times per year when a
perigee
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values.
Apsides perta ...
(the
point nearest
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
reached by the
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
during its 27.3-day
elliptic orbit
In astrodynamics or celestial mechanics, an elliptical orbit or eccentric orbit is an orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. Some orbits have been referre ...
) coincides with a
spring tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another.
Tide tables ...
(when the
Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, the Moon, and Earth are nearly
aligned every two weeks).
This has a slight but measurable impact on the spring tide, usually adding no more than a couple of inches.
Astronomical causes
The
Moon's orbit around Earth is
elliptical, which causes the Moon to be closer to Earth and farther away at different times. The Moon and the Sun are aligned every two weeks, which results in spring tides, which are 20% higher than normal. During the period of the
new moon
In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse. ...
, the Moon and Sun are on the same side of Earth, so the high tides or bulges produced independently by each reinforce each other (and has nothing to do with the
spring season). Tides of maximum height and depression produced during this period are known as spring tide. Spring tides that coincide with the moon's closest approach to Earth ("perigee") have been called perigean spring tides and generally increase the normal tidal range by a couple of inches.
The
Ash Wednesday Storm of 1962
The Ash Wednesday Storm of 1962 occurred on March 5–9, 1962 along the Atlantic Ocean, mid-Atlantic coast of the United States. Also known as the Great March Storm of 1962, it was considered by the U.S. Geological Survey to be one of the most des ...
coincided with a perigean spring tide. It inundated the entire Atlantic coastline of the United States, from the Carolinas to Cape Cod, resulting in a loss of 40 lives and over US$500 million of property damage.
See also
*
King tide
Notes
References
* Easterbrook, D.J. (1999). ''Surface Process and Landforms''. 2nd ed. Upper Saddle River, NJ: Prentice Hall.
* Duxbury, A.B., Duxbury, A.C., Sverdrup, K.A. (2002). ''Fundamentals of Oceanography''. 4th ed. New York, NY: McGraw Hill.
* http://co-ops.nos.noaa.gov/faq2.html#15
*Wood, Fergus J. (2001). Tidal Dynamics Volume I: Theory and Analysis of Tidal Forces; Volume II Extreme Tidal Peaks and Coastal Flooding. 3rd ed. West Palm Beach, Fl: The Coastal Education and Research Foundation
ERF
External links
Apogee & Perigee Tables
{{physical oceanography
Tides