A light-second is a unit of length
Unit of length
Many different units of length have been used across the world. The main units in modern use are U.S. customary units in the United States and the Metric system elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries...

 useful in astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, telecommunications and relativistic physics
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

. It is defined as the distance
Distance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria . In mathematics, a distance function or metric is a generalization of the concept of physical distance...

 that light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 travels in free space in one second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

, and is equal to exactly 299,792,458 metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

s. It is just over 186,000 mile
A mile is a unit of length, most commonly 5,280 feet . The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile...

s and almost 109 feet.

Just as the second forms the basis for other units of time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

, the light-second can form the basis for other units of length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

, 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
Popular science
Popular science, sometimes called literature of science, is interpretation of science intended for a general audience. While science journalism focuses on recent scientific developments, popular science is broad-ranging, often written by scientists as well as journalists, and is presented in many...

 publications. The more commonly-used light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

 is also presently defined to be equal to precisely light-seconds, since the definition of a year is based on a Julian year
Julian year (astronomy)
In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86 400 SI seconds each, totaling 31 557 600 seconds. The Julian year is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is...

 (not Gregorian year
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

) of exactly 365.25 days, each of exactly SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...


Definition of the metre

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

Use in telecommunications

Communications signals on Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 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.
  • One light-nanosecond is almost 300 millimetres (about 299.8 mm, 5 mm less than one foot), which limits the speed of data transfer between different parts of a large computer.
  • One light-microsecond is about 300 metres.
  • The mean distance, over land, between opposite sides of the Earth is about 66.8 light-milliseconds.
  • Communications satellite
    Communications satellite
    A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...

    s are typically 1.337 light-milliseconds (low earth orbit
    Low Earth orbit
    A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

    ) to 119.4 light-milliseconds (geostationary orbit
    Geostationary orbit
    A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

    ) from the surface of the Earth. Hence there will always be a delay of at least a quarter of a second in a communication via geostationary satellite; this delay is just perceptible in a transoceanic telephone conversation routed by satellite.

Use in astronomy

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
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 in light seconds is a fundamental measurement for the calculation of modern ephemerides
An ephemeris is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds of ephemerides are used for astronomy and astrology...

 (tables of plantary positions): it is usually quoted as "light-time for unit distance" in tables of astronomical constant
Astronomical constant
An astronomical constant is a physical constant used in astronomy. A formal set of constants, along with recommended values, was defined by the International Astronomical Union in 1976, and a new set of recommended values was produced in 1994...

s, and its currently accepted value is  s.
  • The mean diameter of the Earth is about 0.0425 light-seconds.
  • The average distance from the Earth to the Moon
    The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

     is about 1.282 light-seconds.
  • The diameter of the Sun
    The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

     is about 4.643 light-seconds.
  • The average distance from the Earth to the Sun is 499.0 light-seconds.

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.
Unit Definition Distance Example
  m km miles  
light-second   average distance from the Earth to the Moon is about 1.282 light-seconds
light-minute 60 light-seconds average distance from the Earth to the Sun is 8.317 light-minutes
light-hour 60 light-minutes
= 3600 light-seconds
semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...

 of Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

's orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 is about 5.473 light-hours
light-day 24 light-hours
= light-seconds
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

 is currently 0.52 light-days from the Sun
light-week 7 light-days
= light-seconds
The Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

 is thought to extend between 41 and 82 light-weeks out from the Sun
light-year 365.25 light-days
= light-seconds
Proxima Centauri
Proxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...

 is the nearest star to the Sun, about 4.24 light-years away

See also

  • Light-year
    A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

  • 1 E8 m
    1 E8 m
    To help compare different orders of magnitude, this page lists lengths starting at 108 metres .Distances shorter than 108 metres* 102 Mm — Diameter of HD 149026 b, an unusually dense Jovian planet...

  • Beard-second
  • Geometrized unit system
    Geometrized unit system
    A geometrized unit system or geometric unit system is a system of natural units in which the base physical units are chosen so that the speed of light in a vacuum, c, and the gravitational constant, G, are set equal to unity. c = 1 \ G = 1 \...

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