A light-year (symbol: ly), sometimes written light year or lightyear, is a unit of length used in astronomy, though it is sometimes mistaken for a unit of time. It is equal to just under 10 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year.
The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular sciencepublications. The unit usually used in professional astrometry is the parsec(symbol: pc, approximately 3.26 light-years; the distance at which oneastronomical unit subtends an angle of one second of arc).
As defined by the IAU, the light-year is the product of the Julian year (365.25 days as opposed to the 365.2425-dayGregorian year) and the speed of light (299792458 m/s). Both these values are included in the IAU (1976) System of Astronomical Constants, used since 1984. From this the following conversions can be derived.
1 light-year = 9460730472580800 metres (exactly) ≈ 5.878625 trillion miles ≈ 63241.077 astronomical units ≈ 0.306601 parsecs
Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983.
The product of Simon Newcomb's J1900.0 mean tropical year of 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of9.460530×1015 m (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000.
The product of Simon Newcomb's J1900.0 mean tropical year of 31556925.9747 ephemeris seconds and a speed of light of 299792.5 km/s produced a light-year of9.460530×1015 m (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen's 1973 Astrophysical Quantities reference work, which was updated in 2000.
Other high-precision values are not derived from a coherent IAU system. A value of 9.460536207×1015 m found in some modern sources is the product of a mean Gregorian year (365.2425 days or 31556952 s) and the defined speed of light (299792458 m/s). Another value, 9.460528405×1015 m, is the product of the J1900.0 mean tropical year and the defined speed of light.
The light-year unit appeared a few years after the first successful measurement of the distance to a star other than our Sun, by Friedrich Bessel in 1838. The star was 61 Cygni, and he used a 6.2-inch (160 mm) heliometer designed by Joseph von Fraunhofer.
The largest unit for expressing distances across space at that time was the astronomical unit (AU), equal to the radius of the Earth's orbit (1.50×108 km or 9.30×107 mi). In those terms, trigonometric calculations based on 61 Cygni'sparallax of 0.314 arcseconds, showed the distance to the star to be 660000 astronomical units (9.9×1013 km or6.1×1013 mi).
Bessel added that light employs 10.3 years to traverse this distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resented expressing distances in light-years because it would deteriorate the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.
The speed of light was not yet precisely known in 1838; its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic.
The light-year unit appeared, however, in 1851 in a German popular astronomical article by. The paradox of a distance unit name ending on year was explained by Ule by comparing it to a hiking road hour (Wegstunde). A contemporary German popular astronomical book also noticed that light-year is an odd name.
In 1868 an English journal labeled the light-year as a unit used by the Germans. Eddingtoncalled the light-year an inconvenvient and irrelevant unit, which had sometimes crept from popular use into technical investigations.
Bessel added that light employs 10.3 years to traverse this distance. He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resented expressing distances in light-years because it would deteriorate the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light.
The speed of light was not yet precisely known in 1838; its value changed in 1849 (Fizeau) and 1862 (Foucault). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic.
The light-year unit appeared, however, in 1851 in a German popular astronomical article by. The paradox of a distance unit name ending on year was explained by Ule by comparing it to a hiking road hour (Wegstunde). A contemporary German popular astronomical book also noticed that light-year is an odd name.
In 1868 an English journal labeled the light-year as a unit used by the Germans. Eddingtoncalled the light-year an inconvenvient and irrelevant unit, which had sometimes crept from popular use into technical investigations.