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In international calendars
In the Persian calendar
In the International System of Quantities
Sidereal, tropical, and anomalistic years
- The relations among these are considered more fully in Axial precession (astronomy).
- 346.620075883 days (346 d 14 h 52 min 54 s) (at the epoch J2000.0).
Full moon cycle
- 411.78443029 days (411 days 18 hours 49 minutes 34 seconds) (at the epoch J2000.0).
- 365.2568983 days (365 d 6 h 9 min 56 s).
- B = 1900.0 + (Julian dateTT − 2415020.31352) / 365.242198781
Variation in the length of the year and the day
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- The positions of the equinox and solstice points with respect to the apsides of Earth's orbit change: the equinoxes and solstices move westward relative to the stars because of precession, and the apsides move in the other direction because of the long-term effects of gravitational pull by the other planets. Since the speed of the Earth varies according to its position in its orbit as measured from its perihelion, Earth's speed when in a solstice or equinox point changes over time: if such a point moves toward perihelion, the interval between two passages decreases a little from year to year; if the point moves towards aphelion, that period increases a little from year to year. So a "tropical year" measured from one passage of the northward ("vernal") equinox to the next, differs from the one measured between passages of the southward ("autumnal") equinox. The average over the full orbit does not change because of this, so the length of the average tropical year does not change because of this second-order effect.
- Each planet's movement is perturbed by the gravity of every other planet. This leads to short-term fluctuations in its speed, and therefore its period from year to year. Moreover, it causes long-term changes in its orbit, and therefore also long-term changes in these periods.
- Tidal drag between the Earth and the Moon and Sun increases the length of the day and of the month (by transferring angular momentum from the rotation of the Earth to the revolution of the Moon); since the apparent mean solar day is the unit with which we measure the length of the year in civil life, the length of the year appears to decrease. The rotation rate of the Earth is also changed by factors such as post-glacial rebound and sea level rise.
Numerical value of year variation
|Type of year||days||hours||minutes||seconds|
|346.62||Draconic, also called eclipse.|
|365||Vague, and a common year in many solar calendars.|
|365.24219||Tropical, also called solar, averaged and then rounded for epoch J2000.0.|
|365.2425||Gregorian, on average.|
|365.25636||Sidereal, for epoch J2000.0.|
|365.259636||Anomalistic, averaged and then rounded for epoch J2011.0.|
|366||Leap in many solar calendars.|
"Greater" astronomical years
- ar for are and:
- at = a_t = 365.24219 days for the mean tropical year
- aj = a_j = 365.25 days for the mean Julian year
- ag = a_g = 365.2425 days for the mean Gregorian year
- a = 1 aj (without further qualifier)
- a = 31556925.445 seconds (approximately 365.24219265 ephemeris days)
SI prefix multipliers
- ka (for kiloannus), is a unit of time equal to one thousand years. This is typically used in geology, paleontology, and archaeology for Holocene and Pleistocene periods where a nonradiocarbon dating technique, i. e., ice core dating, dendrochronology, uranium-thorium dating, or varve analysis, is used as the primary dating method for age determination. If age is primarily determined by radiocarbon dating, then the age should be expressed in either radiocarbon or calendar (calibrated) years Before Present.
- Ma (for megaannus), is a unit of time equal to one million (106) years. It is commonly used in scientific disciplines such as geology, paleontology, and celestial mechanicsto signify very long time periods into the past or future. For example, the dinosaur species Tyrannosaurus rex was abundant approximately 66 Ma (66 million years) ago (ago may not always be mentioned; if the quantity is specified while not explicitly discussing a duration, one can assume that "ago" is implied; the alternative "mya" unit includes "ago" explicitly). In astronomical applications, the year used is the Julian year of precisely 365.25 days. In geology and paleontology, the year is not so precise and varies depending on the author.
- Ga (for gigaannus), is a unit of time equal to 109 years (one billion on the short scale, one milliard on the long scale). It is commonly used in scientific disciplines such ascosmology and geology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately 4.57 Ga (4.57 billion years) ago.
- Ta (for teraannus), is a unit of time equal to 1012 years (one trillion on the short scale, one billion on the long scale). It is an extremely long unit of time, about 70 times as long as the age of the universe. It is the same order of magnitude as the expected life span of a small red dwarf.
- Pa (for petaannus), is a unit of time equal to 1015 years (one quadrillion on the short scale, one billiard on the long scale). The half-life of the nuclide cadmium-113 is about 8 Pa. This symbol coincides with that for the pascal without a multiplier prefix, though both are infrequently used and context will normally be sufficient to distinguish time from pressure values.
- Ea (for exaannus), is a unit of time equal to 1018 years (one quintillion on the short scale, one trillion on the long scale). The half-life of tungsten-180 is 1.8 Ea.
Abbreviations y and yr
|non-SI abbreviation||SI-prefixed equivalent||order of magnitude|
|kya or tya||"ka ago"|
|bya or gya||"Ga ago"|