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The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule. [24] Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde). A contemporary German popular astronomical book also noticed that light-year is an odd name. [25]
The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec. [6] One au is equivalent to 499 light-seconds to within 10 parts per million.
It is not a true mathematical unit, as all ages, epochs, periods, eras, or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually. Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres (9 454 254 955 488 km).
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. 30.9 trillion kilometres (19.2 trillion miles).
1 astronomical unit (au); mean distance between Earth and Sun 10 12: 1 terameter (Tm) ... (1 Pm or 1 trillion km or 6685 astronomical units (AU) or 0.11 light-years).
d - in AU = astronomical unit; d - in ly = light-year; d - in pc = parsec; d - in kpc = kiloparsec (1000 pc) ... M - Mean anomaly; M o - Mean anomaly at epoch; Radius ...
In contrast, the Julian year is defined in terms of the SI unit one second, so is as accurate as that unit and is constant. It approximates both the sidereal year and the tropical year to about ±0.008 days. The Julian year is the basis of the definition of the light-year as a unit of measurement of distance. [2]
astronomical unit au. Defined as 149 597 870 700 m. [16] Approximately the distance between the Earth and Sun. light-year ly ≈ 9 460 730 472 580.8 km. The distance that light travels in a vacuum in one Julian year. [17] parsec pc ≈ 30 856 775 814 671.9 km or about 3.261 56 ly; Hubble length 14.4 billion light-years or 4.55 gigaparsecs