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1 light-second =
SI units
299.79×10^3 km299.79×10^6 m
Astronomical units
2.0040×10^−3 AU31.688×10^−9 ly
US customary / Imperial units
186.28×10^3 mi983.57×10^6 ft
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1 light-second =
SI units
299.79×10^3 km299.79×10^6 m
Astronomical units
2.0040×10^−3 AU31.688×10^−9 ly
US customary / Imperial units
186.28×10^3 mi983.57×10^6 ft

A light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly 299,792,458 metres. It is just over 186,000 miles and almost 9.84×108 feet.

Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond (just under one U.S. or imperial foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly-used light-year is also presently defined to be equal to precisely 31557600 light-seconds, since the definition of a year is based on a Julian year (not Gregorian year) of exactly 365.25 days, each of exactly 86400 SI seconds.[1]


Definition of the metre

The metre is the length of the path traveled by light in vacuum during a time interval of 1299792458 of a second.

This definition fixes the speed of light in vacuum at exactly 299792458 m/s, and hence the light-second at exactly 299,792,458 m.[2]

Use in telecommunications

Communications signals on Earth rarely travel at precisely the speed of light in free space, but distances in fractions of a light-second are still useful for planning telecommunications networks as they indicate the minimum possible delay between sender and receiver.

Use in astronomy

The yellow shell indicating one light-day distance from the Sun compares in size with the positions of Voyager 1 and Pioneer 10 (red and green arrows respectively). It is larger than the heliosphere's termination shock (blue shell) but smaller than Comet Hale-Bopp's orbit (faint orange ellipse below). Click on the image for a larger view and links to other scales.
The faint yellow sphere centred on the Sun has a radius of one light-minute. For comparison, sizes of Rigel (the blue star in the top left) and Aldebaran (the red star in the top right) are shown to scale. The large yellow ellipse represents Mercury's orbit.

The light-second is a convenient unit for measuring distances in the inner Solar System, because it corresponds very closely to the radiometric data used to determine them (the match is not exact for an Earth-based observer because of a very small correction for the effects of relativity). The value of the astronomical unit in light seconds is a fundamental measurement for the calculation of modern ephemerides (tables of plantary positions): it is usually quoted as "light-time for unit distance" in tables of astronomical constants, and its currently accepted value is 499.004786385(20) s.[3][4]

Multiples of the light-second can be defined, although apart from the light-year they are more used in popular science publications than in research works. For example, a light-minute is 60 light-seconds and the average distance from the Earth to the Sun is 8.317 light-minutes.

light-second 2.997924580×108 m2.998×105 km1.863×105 miaverage distance from the Earth to the Moon is about 1.282 light-seconds
light-minute60 light-seconds1.798754748×1010 m1.799×107 km1.118×107 miaverage distance from the Earth to the Sun is 8.317 light-minutes
light-hour60 light-minutes
= 3600 light-seconds
1.079252849×1012 m1.079×109 km6.706×108 misemi-major axis of Pluto's orbit is about 5.473 light-hours
light-day24 light-hours
= 86400 light-seconds
2.590206837×1013 m2.590×1010 km1.609×1010 miSedna is currently 0.52 light-days from the Sun; on an orbit that varies from a perihelion of 0.44 light-days to an aphelion of 5.41 light-days
light-week7 light-days
= 604800 light-seconds
1.813144786×1014 m1.813×1011 km1.127×1011 miThe Oort cloud is thought to extend between 41 and 82 light-weeks out from the Sun
light-year365.25 light-days
= 31557600 light-seconds
9.460730473×1015 m9.461×1012 km5.879×1012 miProxima Centauri is the nearest star to the Sun, about 4.24 light-years away


See also