Light-second

The light-second is a special unit of length that helps scientists in astronomy, telecommunications, and relativistic physics. It measures the distance that light travels in a vacuum during one second. This distance is exactly 299,792,458 meters, or about 186,282 miles.

Just like seconds are used to measure time, light-seconds can help measure very long distances. Scientists sometimes use other units based on the light-second, such as the light-nanosecond, light-minute, light-hour, and light-day. One of the most famous units is the light-year, which equals precisely 31,557,600 light-seconds. This is based on a Julian year of exactly 365.25 days, with each day containing 86,400 seconds.

Use in telecommunications

Communications signals on Earth travel at the same speed as light in open space. Measuring distances in parts of a light-second helps when planning networks for phones and the internet.

For example, one light-nanosecond is almost 300 millimetres, which affects how fast data can move inside a computer. One light-microsecond is about 300 metres. The distance across the Earth is around 66.8 light-milliseconds. Communications satellites are usually between 1.337 and 119.4 light-milliseconds away from Earth, causing a small delay in signals. This delay can be noticed in phone calls or TV interviews that use satellites.

Use in astronomy

The light-second is a useful way to measure distances inside our Solar System. It works well with the data used to find these distances. The distance between Earth and the Sun, called the astronomical unit, is very important for figuring out where planets are, and it is measured in light-seconds.

The yellow shell indicating one light-day distance from the Sun compares in size with the positions of Voyager 1 and Pioneer 10 as of 2008 (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.

Some distances in light-seconds include:

The width of Earth is about 0.0425 light-seconds.
The distance from Earth to the Moon is about 1.282 light-seconds.
The width of the Sun is about 4.643 light-seconds.
The distance from Earth to the Sun is 499.0 light-seconds.

We can also use bigger units like light-minutes and light-hours. For example:

The distance to the Sun is about 8.317 light-minutes.
The distance to Pluto is about 4.81 light-hours.
The Voyager 1 spacecraft, launched in 1977, is more than 23 light-hours away from Earth and will reach one light-day away in November 2026.

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.

Unit	Definition	Equivalent distance in			Example
Unit	Definition	Meters	Kilometers	Miles	Example
light-second	1 light-second	299792458 m	2.998×10⁵ km	1.863×10⁵ miles	Average distance from the Earth to the Moon is about 1.282 light-seconds
light-minute	60 light-seconds = 1 light-minute	17987547480 m	1.799×10⁷ km	1.118×10⁷ miles	Average distance from the Earth to the Sun is 8.317 light-minutes
light-hour	60 light-minutes = 3600 light-seconds	1079252848800 m	1.079×10⁹ km	6.706×10⁸ miles	The perihelion of Saturn's orbit is about 1.25 light-hours
light-day	24 light-hours = 86400 light-seconds	25902068371200 m	2.590×10¹⁰ km	1.609×10¹⁰ miles	Voyager 1 is about 0.96 light-days from the Sun (as of March 2025)
light-week	7 light-days = 604800 light-seconds	181314478598400 m	1.813×10¹¹ km	1.127×10¹¹ miles	The Oort cloud is thought to extend between 41 and 82 light-weeks out from the Sun
light-year	365.25 light-days = 31557600 light-seconds	9460730472580800 m	9.461×10¹² km	5.879×10¹² miles	Proxima Centauri is the nearest star to the Sun, about 4.24 light years away

Use in telecommunications

Use in astronomy

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