Outer shell is one light year from Sun and left yellow line is Comet 1910 A1's orbit. Inner shell is one light month.
A
light-year or
light year (symbol:
ly) is a
unit of
length, equal to just under
1013 kilometres. As defined by the
International Astronomical Union (IAU), a light-year is the distance that
light travels in a
vacuum in one
Julian year.
The light-year is often used to measure distances to stars and other distances on a
galactic scale, especially in non-specialist and
popular science publications. The preferred unit in
astrometry is the
parsec, because it can be more easily derived from, and compared with, observational data. The parsec is defined as the distance at which an object will appear to move one
arcsecond of
parallax when the observer moves one
astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years.
The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.
Numerical value
A light-year is equal to:
- exactly 9,460,730,472,580.8 km (about 10 Pm)
- about 5,878,630,000,000 miles
The figures above are based on a
Julian year (not
Gregorian year) of exactly 365.25 days (each of exactly 86,400
SI seconds, totalling 31,557,600 seconds) and a defined
speed of light of 299,792,458 m/s, both included in the
IAU (1976) System of Astronomical Constants, used since 1984. The
DE405 value of the astronomical unit, 149,597,870,691 m, is used for the light-year in astronomical units and parsecs.
The light-month does not have a precise definition or value, as there is no unique definition of a
month in
astronomical or SI units: however it may be understood to be approximately one-twelfth of a light-year, in line with the various mundane definitions of the month (
hollow month, average Julian month, etc.).
Other values
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.
[P. Kenneth Seidelmann, ed., (Mill Valey, California: University Science Books, 1992) 656. ISBN 0-935702-68-7] The product of
Simon Newcomb's
J1900.0 mean tropical year of 31,556,925.9747
ephemeris seconds and a speed of light of 299,792.5 km/s produced a light-year of 9.460530 metres (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 a reputable 1973 reference which was not updated until 2000.
Other high precision values are not derived from a coherent IAU system. A value of 9.460536207 metres found in some modern sources is the product of a mean Gregorian year of 365.2425 days (31,556,952 s) and the defined speed of light (299,792,458 m/s). The Live Search value, 9.460528405 metres, is the product of the J1900.0 mean tropical year and the defined speed of light.
Distances in light-years
Distances measured in fractions of a light-year (or in light-months) usually involve objects within a
star system. Distances measured in light-years include distances between nearby
stars, such as those in the same
spiral arm or
globular cluster.
One kilolight-year, abbreviated "kly", is one thousand light-years, or about 307
parsecs. Kilolight-years are typically used to measure distances between parts of a
galaxy.
One megalight-year, abbreviated "Mly", is one million light-years, or about 306,600 parsecs. Megalight-years are typically used to measure distances between neighboring
galaxies and
galaxy clusters.
One gigalight-year, abbreviation "Gly", is one
billion light-years—one of the largest distance measures used. One gigalight-year is about 306.6 million parsecs. Gigalight-years are typically used to measure distances to supergalactic structures, including
quasars and the
Great Wall.
See also
- 1 E15 m (examples of distances on the order of one light-year)
