The term "light-year" refers not to a length of time, but instead a unit of distance often used in astronomy. Astronomers define a light-year as the distance light travels in one year. To calculate this distance, or the distance that light travels in any amount of time, you must know the speed of light and how to manipulate the equation for speed in general. The speed of light is roughly 300,000 kilometers per second (km/s), or 186,000 miles per second (mi/s). Speed is equal to distance traveled divided by the time it took to travel that distance.

Solve the speed equation for distance:

Speed = Distance/Time, so Distance = Speed x Time, or D = S x T

Decide on a length of time, in seconds (s), for which that you wish to calculate the distance light will travel.

Example: How far will light travel in one hour? (1 hour = 3,600 s)

Substitute the appropriate values in for their variables in the distance equation to solve the problem.

D = S x T, so D = 186,000 mi/s x 3,600 s = 669,600,000 mi

Light will travel 669,600,000 miles in one hour.