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As defined by the International Astronomical Union (IAU), the light-year is the product of the Julian year [note 1] (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day Tropical year that both approximate) and the speed of light (299 792 458 m/s).
Distance light travels in one day — Light-year: 63 241 — Distance light travels in one Julian year (365.25 days) — Oort cloud: 75 000: ± 25 000: Distance of the outer limit of Oort cloud from the Sun (estimated, corresponds to 1.2 light-years) — Parsec: 206 265 — One parsec. The parsec is defined in terms of the astronomical unit, is ...
A light-year is the distance light travels in one Julian year, around 9461 billion kilometres, 5879 billion miles, or 0.3066 parsecs. In round figures, a light year is nearly 10 trillion kilometres or nearly 6 trillion miles. Proxima Centauri, the closest star to Earth after the Sun, is around 4.2 light-years away. [89]
The speed of light in IAU is the defined value c 0 = 299 792 458 m/s of the SI units. In terms of this speed, the old definition of the astronomical unit of length had the accepted value: [ 3 ] 1 au = c 0 τ A = ( 149 597 870 700 ± 3 ) m, where τ A is the transit time of light across the astronomical unit.
The light-year is roughly equivalent to 0.3 parsecs, and is equal to the distance traveled by light in one Julian year in a vacuum, according to the IAU." The parsec does have a standard IAU definition, although it is a much less well known unit in the public space. Praemonitus 14:54, 24 July 2024 (UTC)
For objects within the immediate neighborhood of the Sun, the absolute magnitude M and apparent magnitude m from any distance d (in parsecs, with 1 pc = 3.2616 light-years) are related by = = (), where F is the radiant flux measured at distance d (in parsecs), F 10 the radiant flux measured at distance 10 pc.
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To help compare different distances this section lists lengths between 10 18 m (1 Em or 105.7 light-years) and 10 19 m (10 Em or 1,057 light-years). 1.2 Em – 129 light-years – diameter of Messier 13 (a typical globular cluster) 1.6 Em – 172 ± 12.5 light-years – diameter of Omega Centauri (one of the largest-known globular clusters ...