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A light year is a distance.

It is the distance light travels in one year.

As light travels at constant speed, the distance light travels in a year can be calculated using the equation:

Distance = speed x time

Speed of light = 300,000,000 m/s = 3 x 10⁸ m/s

time = 1 year = 365 x 24 x 60 x 60 = 31,536,000 s

1 light year = 3 x10⁸ m/s x 31,536,000 s = 9.46 x 10¹⁵ metres

1 light year = 9.46 x 10¹⁵ m

Example

A galaxy is found to be 20 million light years away. How far is that in metres?

Answer

20 million light years can be written as 20,000,000 light years or 20 x 10⁶ light years.

1 light year = 9.46 x 10¹⁵ metres

20 x 10⁶ light years = 20 x 10⁶ x 9.46 x 10¹⁵ metres = 1.89 x 10²³ m

The galaxy is 1.89 x 10²³ m away.