Astronomical years...................... 1.Julian year............. The Julian year, as used in astronomy and other sciences, is a time unit defined as exactly 365.25 days. This is the normal meaning of the unit year (symbol a from the Latin annus) used in various scientific contexts. The Julian century of 36525 days and the Julian millennium of 365250 days are used in astronomical calculations. Fundamentally, expressing a time interval in Julian years is a way to precisely specify how many days (not how many real years), for long time intervals where stating the number of days would be unwieldy and unintuitive. By convention, the Julian year is used in the computation of the distance covered by a light-year. In the Unified Code for Units of Measure, the symbol a (without subscript) always refers to the Julian year aj of exactly 31557600 seconds. 365.25 days of 86400 seconds = 1 a = 1 aj = 31.5576 Ms The SI multiplier prefixes may be applied to it to form ka (kiloannum), Ma (megaannum) etc. 2.Sidereal, tropical, and anomalistic years........... Each of these three years can be loosely called an astronomical year................................................. The sidereal year is the time taken for the Earth to complete one revolution of its orbit, as measured against a fixed frame of reference (such as the fixed stars, Latin sidera, singular sidus). Its average duration is 365.256363004 mean solar days (365 d 6 h 9 min 9.76 s) (at the epoch J2000.0 = January 1, 2000, 12:00:00 TT).Today the tropical year is defined as the period of time for the ecliptic longitude of the Sun to increase by 360 degrees. Since the Suns ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons; because of the biological and socio-economic importance of the seasons, the tropical year is the basis of most calendars. The modern definition of mean tropical year differs from the actual time between passages of e.g. the northward equinox for several reasons explained below. Because of the Earths axial precession, this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds (= 365.24219 days). The anomalistic year is the time taken for the Earth to complete one revolution with respect to its apsides. The orbit of the Earth is elliptical; the extreme points, called apsides, are the perihelion, where the Earth is closest to the Sun (January 3 in 2011), and the aphelion, where the Earth is farthest from the Sun (July 4 in 2011). The anomalistic year is usually defined as the time between perihelion passages. Its average duration is 365.259636 days (365 d 6 h 13 min 52.6 s) (at the epoch J2011.0). If Earth moved in an ideal Kepler orbit, i.e. a perfect ellipse with the Sun fixed at one focus, each kind of year would always have the same duration, and the sidereal and anomalistic years would be equal. Because of perturbations by the gravity of other planets, Earths motion varies slightly, causing the sidereal and tropical years to vary in length by about 25 minutes . Both are affected in the same way, so that the sidereal year is consistently 20 minutes longer than the tropical year, provided that they are measured in the same way.An example of a year that will have a duration exceeding the average value of 365.24219 SI days with as much as 24.23 minutes is the one that will begin at winter solstice December 21, 2042 17:47:45.5 (Atomic time). 3.Draconic year............. The draconic year, draconitic year, eclipse year, or ecliptic year is the time taken for the Sun (as seen from the Earth) to complete one revolution with respect to the same lunar node (a point where the Moons orbit intersects the ecliptic). This period is associated with eclipses: these occur only when both the Sun and the Moon are near these nodes; so eclipses occur within about a month of every half eclipse year. Hence there are two eclipse seasons every eclipse year. The average duration of the eclipse year is 346.620075883 days (346 d 14 h 52 min 54 s) (at the epoch J2000.0).This term is sometimes erroneously used for the draconic or nodal period of lunar precession, that is the period of a complete revolution of the Moons ascending node around the ecliptic: 18.612815932 Julian years (6798.331019 days; at the epoch J2000.0). 4.Full moon cycle............ The full moon cycle is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the perigee of the Moons orbit. This period is associated with the apparent size of the full moon, and also with the varying duration of the synodic month. The duration of one full moon cycle is: 411.78443029 days (411 d 18 h 49 min 34 s) (at the epoch J2000.0). 5.Lunar year................. The lunar year comprises twelve full cycles of the phases of the Moon, as seen from Earth. It has a duration of approximately 354.37 days. Muslims use this for celebrating their Eids and for marking the start of the fasting month of Ramadan. A Muslim calendar year is based on the lunar cycle. 6.Vague year............. The vague year, from annus vagus or wandering year, is an integral approximation to the year equaling 365 days, which wanders in relation to more exact years. Typically the vague year is divided into 12 schematic months of 30 days each plus 5 epagomenal days. The vague year was used in the calendars of Ancient Egypt, Iran, Armenia and in Mesoamerica among the Aztecs and Maya. 7.Heliacal year.............. A heliacal year is the interval between the heliacal risings of a star. It differs from the sidereal year for stars away from the ecliptic due mainly to the precession of the equinoxes. 8.Sothic year.............. The Sothic year is the interval between heliacal risings of the star Sirius. It is presently less than the sidereal year and its duration is very close to the mean Julian year of 365.25 days. 9.Gaussian year........... The Gaussian year is the sidereal year for a planet of negligible mass (relative to the Sun) and unperturbed by other planets that is governed by the Gaussian gravitational constant. Such a planet would be slightly closer to the Sun than Earths mean distance. Its length is:365.2568983 days (365 d 6 h 9 min 56 s). 10.Besselian year............ The Besselian year is a tropical year that starts when the (fictitious) mean Sun reaches an ecliptic longitude of 280°. This is currently on or close to January 1. It is named after the 19th-century German astronomer and mathematician Friedrich Bessel. The following equation can be used to compute the current Besselian epoch (in years):B = 1900.0 + (Julian dateTT − 2415020.31352) / 365.242198781 The TT subscript indicates that for this formula, the Julian date should use the Terrestrial Time scale, or its predecessor, ephemeris time.
Posted on: Sat, 02 Nov 2013 17:55:08 +0000
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