The 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 299792458 m (approximately 983571055 ft or 186282 mi).

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 (299.8 mm or just under one international 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 currently defined to be equal to precisely 31557600 light-seconds, since the definition of a year is based on a Julian year (not the Gregorian year) of exactly 365.25 d, each of exactly 86400 SI seconds.^[1]

Communications signals on Earth rarely travel at precisely the speed of light in free space.^{[citation needed]} Distances in fractions of a light-second are useful for planning telecommunications networks.

• One light-nanosecond is almost 300 millimetres (299.8 mm, 5 mm less than one foot^[2]), which limits the speed of data transfer between different parts of a computer.
• One light-microsecond is about 300 metres.
• The mean distance, over land, between opposite sides of the Earth is 66.8 light-milliseconds.
• Communications satellites are typically 1.337 light-milliseconds^{[citation needed]} (low Earth orbit) to 119.4 light-milliseconds (geostationary orbit) 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 (119.4 ms times 2); this delay is just perceptible in a transoceanic telephone conversation routed by satellite. The answer will also be delayed with a quarter of a second and this is clearly noticeable during interviews or discussions on TV when sent over satellite.

The light-second is a convenient unit for measuring distances in the inner Solar System, since 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 (roughly the distance between Earth and the Sun) in light-seconds is a fundamental measurement for the calculation of modern ephemerides (tables of planetary 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]

• The mean diameter of Earth is about 0.0425 light-seconds.
• The average distance between Earth and the Moon (the lunar distance) is about 1.282 light-seconds.
• The diameter of the Sun is about 4.643 light-seconds.
• The average distance between Earth and the Sun (the astronomical unit) 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 so the average distance between Earth and the Sun is 8.317 light-minutes.
• The average distance between Pluto and the Sun (34.72 AU^[5]) is 4.81 light-hours.^[6]
• Humanity's most distant artificial object, Voyager 1, has an interstellar velocity of 3.57 AU per year,^[7] or 29.7 light-minutes per year.^[8] As of 2023 the probe, launched in 1977, is over 22 light-hours from Earth and the Sun, and is expected to reach a distance of one light-day around November 2026 – February 2027.^{[citation needed]}

• 100 megametres
• Geometrized unit system
• Light-year