Light Year

by Dominic Ford, Editor
Last updated: 6 Jan 2017

The light year is a unit of length which is equal to the distance that light travels in a year. One light year is equal to:

$$9.461 \times 10^{15}$$ m

$$63,239.8$$ astronomical units

$$0.3066$$ parsecs

Other units of length – the light minute and the light second – are similarly defined as the distance that light travels in a minute or in a second.

The light year is a unit of length appropriate for describing the distances between the stars in our Galaxy.

For example, the closest known star to our Sun, Proxima Centauri, lies at a distance of around 4.2 light years, meanwhile Sirius, the brightest star in the night sky, lies at a distance of around 8.6 light years.

The light minute is a more appropriate length for describing distances within the Solar System.

For example, the Sun is 8.31 light minutes from the Earth, and Mars's distance from the Earth can vary from 3.035 light minutes to 22.29 light minutes depending on the relative positions of the two planets in their orbits.

Radio signals travel at the speed of light, and so signals from spacecraft studying Mars take between 3.035 and 22.29 minutes to reach the Earth.

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