One year (from the Old English gēr) is the time between two repetitions of an event related to the orbit of the Earth around the sun By extension, this can be applied to any planet: for example, a "Martian year" is the moment when Mars has its own orbit. One year is the time between two dates with the same name in a calendar. Solar calendars typically attempt to predict the four seasons of the year, but because the size of each year varies significantly seasonal, which instead use an astronomical year as a substitute. For example, the ancient Egyptians used the heliacal rising of Sirius to predict the floods of the Nile. Among large solar calendars in use today, the Persian calendar is one of the most accurate. Instead of being based on numerical rules, the Persian years begins on the day (for the time zone of Tehran), in which the vernal equinox really falls, as determined by accurate astronomical calculations. A seasonal year is the time between successive repetition of a seasonal event such as the flooding of a river, the migration of a species of bird, the flowering of a kind of plant, the first frost, or the first game scheduled for a particular sport . All these events can have wide variations of more than a month from year to year. The school year can be divided in several ways, two of which are more common in educational systems of North America. (1) Many schools divide the school year into three roughly equal-length quarters (called terms in the United Kingdom), more or less coincide with the fall, winter and spring. Somewhere, in a somewhat abbreviated summer session, not usually considered part of the regular school year, is attended by students on a voluntary basis or elective. (2) Other schools breaking the year into two semesters main, the first (usually August through December) and a second (January to May). Each of these major semesters can be divided in half by mid-term exams, and each of the halves is mentioned as a quarter (or term in some countries). There may also be a session elective summer, and / or a short session January. The Julian year, as used in astronomy and other sciences, is a unit defined as time exactly 365.25 days. This is the meaning of the unit normal "year" (symbol "a" of Latin annus, annata) used in various scientific contexts. The Julian century of 36525 days and Julian millennium of 365250 days are used in astronomical calculations. Basically, expressing once in the interval Julian years, it is a way to specify exactly how many days (not many "real" years), long time intervals for indicating where the number of days would be cumbersome and unintuitive. By convention, the Julian year is used in calculating the distance traveled by a light-year. The sidereal year is the time for the Earth to complete a revolution of its orbit, as measured in a particular frame of reference (such as the fixed stars, sideres Latin). Its duration in days of 86400 SI SI each second is, on average: The tropical year is the time for the Earth to complete a revolution with regard to the framework provided by the intersection of the ecliptic (the plane of the orbit of the Earth) and plan the equator (the plane perpendicular to the axis of rotation Terra). The exact duration of one year tropical slightly depends on the chosen starting point: for example, the vernal equinox year is the time between successive vernal equinoxes. The average tropical year (average over all ecliptic points) is: The anomalistic year is the time for the Earth to complete a revolution with respect to its apsides. The Earth's orbit is elliptical, the extreme points, called apsides are the perihelion, when the Earth is closest to the Sun (January 3, 2008), and aphelion, when the Earth is most distant of the Sun (July 4 in 2008). The average duration of the anomalistic year is: The draconitic year, eclipse year or ecliptic year is the time for the Sun (as seen from Earth), to complete a revolution with the same node lunar (a point where the Moon's orbit intersects the ecliptic). This period is associated with eclipses: they occur only when both the Sun and Moon are near these nodes, as eclipses occur within approximately one month's eclipse every half year. Hence there are two seasons eclipse eclipse each year. The average duration of the eclipse year is: A full moon cycle is the time for the Sun (as seen from Earth), to complete a revolution regarding perigee of the Moon's orbit. This period is associated with the apparent size of the full moon, and also with variable duration of the synodic month. The length of a cycle is full moon: A heliacal year is the interval between heliacal risings of a star. It differs from the sidereal year to distant stars of the ecliptic mainly due to the precession of the equinoxes (To view: a constellation Crux that rose and set as seen from the Mediterranean in ancient Greek times, is never above the horizon in the current times.) Gaussian sidereal year is the year for a planet of mass insignificant (from the Sun) and unperturbed by other planets which is governed by the Gaussian gravitational constant. This world would be a little closer to the Sun than the average distance from Earth. Their duration is: The Besselian year is a tropical year, which begins when the sun reaches a fictional means ecliptic longitude of 280 °. It is currently in or near Jan. 1. It is named after the 19 th century German mathematician and astronomer Friedrich Bessel. An approximate formula to calculate the current time in Besselian years from the Julian day is: The precession of the equinoxes changes the position of astronomical events with respect to apsides the orbit of the Earth. An event that is moving toward perihelion again with a period decrease from year to year, an event that is moving toward aphelion repeat with a growing period of one year to another. Tides drag between Earth and the Moon and Sun increases the length of the day and month (by transferring angular momentum from the Earth's rotation to the revolution of the Moon) since the apparent average solar day is the unit with which Measure that the length of the year in civilian life, the duration of the year appears to change. Tides drag in turn, depends on factors such as post-glacial rebound and rising sea level. Changes in the effective mass of the sun, caused by wind and solar radiation energy generated by nuclear fusion, will affect the orbital period of the Earth over a long period (about one extra of 100 picoseconds per year). Others weakest effects tend to shorten the orbital period of Earth: gravitational radiation (by about 8 zeptoseconds per year), the effect Poynting-Robertson (about 30 nanoseconds per year), etc. The agenda was is used to assign a number each year, using a benchmark in the past as the beginning of the season. In many countries, the most common age is estimated from the date of the birth of Jesus Christ; dates this season are designated anno Domini (the year of the Lord ", abbreviated AD), or more neutral, CE (common era). Other eras are also used to list the years in different cultural origins, religious or scientific contexts.
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