: This article is about the astronomical phenomenon. For other meanings of the word Equinox see Equinox (disambiguation).
In astronomy, an equinox is defined as the moment when the sun reaches one of two intersections between the ecliptic and the celestial equator.
The word "equinox" comes from the Latin for "equal night". The equinoxes in March and September are the two occasions each year when the day and the night are of equal duration. For measuring the length of a day, sunrise is the moment when the sun is half-above the horizon and sunset is the moment when the sun is half-under the horizon. Using this definition, the length of the day (and the night) is precisely 12 hours at an equinox. More commonly the day is defined as the period that sunlight may reach the ground in absence of local obstacles. Because the Sun is not a point but appears as a disc, and because sunlight is refracted downwards by the atmosphere, the day by that definition is a few minutes longer than the night (almost 14 minutes at the equator, and more towards the poles), making the practical equality of day and night happen a few days before the vernal equinox and a few days after the autumnal equinox.
In the Northern Hemisphere, the March equinox is known as the vernal (spring) equinox and the September equinox is the autumnal (autumn) equinox. In the Southern Hemisphere, these names are transposed.
The March equinox typically falls on either March 20 or 21 and the September equinox on September 22 or 23 – the dates vary because the tropical year is not a whole number of days. Because the Earth's orbit is elliptical, the dates on which the equinoxes fall do not divide the year into equal halves.
The equinoxes can also be interpreted as virtual points in the sky. Although, during full daylight, stars other than the sun are overwhelmed by sunlight, making it hard to see where the sun is compared to other celestial bodies, the sun does have a position (as seen from the Earth) relative to the other stars. As the Earth moves around the sun, the apparent position of the sun relative to the other stars moves in a full circle over the period of a year. This circle is called the ecliptic, and is also the plane of the Earth's orbit projected against the whole sky. The other bright planets like Venus, Mars and Saturn, also appear to move along the ecliptic, because their orbits are in a similar plane to Earth's.
Another virtual circle in the sky is the celestial equator, or the projection of the plane of the Earth's equator against the whole sky. Because the Earth's axis of rotation is tilted relative to the plane of Earth's orbit around the sun, the celestial equator is inclined to the ecliptic. Twice a year, the sun, making its progress around the ecliptic, crosses the plane of the Earth's equator. These two points are the equinoxes. The time at which the sun passes through each equinox point can be calculated precisely—so the equinox is actually a particular moment, rather than a whole day.
Apparent behaviour of the sun
On the equinoxes, everywhere over the globe, the sun rises true east (parallel to lines of latitude), sets at true west, and the length of the day equals the length of the night.
March equinox
At the North pole the sun passes from a 6-month-long night to a 6-month-long day.
At the Arctic circle the sun reaches an altitude of 23° in the South.
At the Tropic of Cancer the sun reaches an altitude of 67° in the South.
At the equator the sun rises in a vertical line from the East on the horizon to the zenith, and then sets in a vertical line from the zenith to the West on the horizon.
At the Tropic of Capricorn the sun reaches an altitude of 67° in the North.
At the Antarctic circle the sun reaches an altitude of 23° in the North.
At the South pole the sun passes from a 6-month-long day to a 6-month-long night.
September equinox
At the North pole the sun passes from a 6-month-long day to a 6-month-long night.
At the Arctic circle the sun reaches an altitude of 23° in the South.
At the Tropic of Cancer the sun reaches an altitude of 67° in the South.
At the equator the sun rises in a vertical line from the East on the horizon to the zenith, and then sets in a vertical line from the zenith to the West on the horizon.
At the Tropic of Capricorn the sun reaches an altitude of 67° in the North.
At the Antarctic circle the sun reaches an altitude of 23° in the North.
At the South pole the sun passes from a 6-month-long night to a 6-month-long day.
Equinoctial point
The equinoctial point occurs twice a year (around March 21st and September 23rd), when the sun passes through the equinox. At this point (equinoctial point) the length of the day and night are the same. Hipparchus used the equinoctial point to divide the day into 24 equinoctial hours. Before this the day (dawn to dusk) was broken up in 12 'hours' whose length varied according to the season (longer in the summer, shorter in winter).
- [http://aa.usno.navy.mil/faq/docs/equinoxes.html Details about the Length of Day and Night at the Equinoxes]
- [http://www.gutenberg.org/dirs/1/2/3/4/12342/12342-h/12342-h.htm#E Equinoctial Points] - The Nuttall Encyclopaedia - [http://www.spwebgames.com/daylight Java applet showing parts of the Earth in night and day]
- [http://aa.usno.navy.mil/data/docs/EarthSeasons.html Table of times for Equinoxes, Solstices, Perihelion, and Aphelion in 1992-2020]
Category:Spherical astronomyja:分点
Operation Coalscuttle is the functional Confederate States equivalent to Adolf Hitler's Operation Blue of 1942, which was the advance to Stalingrad and the Caucasus--straight down to the use of foreign divisions to hold the flanks (Romanians in Stalingrad; Mexicans in Pennsylvania).
In this case, Coalscuttle was Jake Featherston's attempt to knock the United States out of the Second World War by driving east out of Ohio and past Pittsburgh, destroying the USA's ability to make steel there. After taking Akron, Canton, Cleveland and Youngstown, General George Patton's army was bogged down in the fighting around Pittsburgh, Pennsylvania, being forced by Irving Morrell to head straight into the city instead of ringing around it. As the Confederates strove to take the city block by block, Morrell led a counterattack against the weak Mexicans holding the C.S. flanks and routed them, driving all the way to Lafayette, Ohio. By the end of 1942, the USA had trapped the C.S. army in Pittsburgh in a massive pocket and forced back a Confederate attempt to rescue the pocketed force. In early February, the remnants in the pocket surrendered (Patton had been airlifted out on Featherston's order).
Jake Featherston's growing megalomania prevented him from thinking rationally during the urgent period of the operation, ignoring Nathan Bedford Forrest III's repeated requests to withdraw the force from Pittsburgh. After nearly being thrown out of Featherston's office, Forrest began plotting with Intelligence chief Clarence Potter about the possibility of removing Featherston from office with force, as it had became obvious by then that the President was leading the Confederate States of America to an impending catastrophe. This element has strong similarities to a real life plot to assassinate Hitler (see the July 20 Plot).
Battle of Midway
The Battle of Midway takes place in 1942, as in real history, but involves just two carriers. A small American task force of destroyers, including the destroyer Townsend and the carrier Trenton, attack the Japanese carrier guarding the garrison on Midway. The Trentons fighters successfully attack the carrier, destroying its engines and setting the ship ablaze. Japanese fighters, learning of the destruction of their ship, choose to perform the first kamikaze attack and dive on the American flotilla. The destroyers and carrier respond with a withering hail of AA fire. The Trenton is hit by one plane, but is undamaged. The Japanese planes are wiped out, resulting in a decisive victory for the Americans, something they had not had in the Pacific since the beginning of the war.
Category:Timeline-191Category:Alternate history novelsCategory:2005 books
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