A light year is a measure of distance. It is the distance that light travels in one year.

Astronomers use this measure, because it is easier to compare numbers like 1 and 100 light years than numbers like 5 billion miles and 674 trillion miles.

If you want to know how many miles there are in a light year, you can look up the speed of light in miles per second and multiply by the number of seconds in a minute and then by the number of minutes in an hour and then by the number of hours in a day and lastly by the number of days in a year.

A light year is the distance that light travels in a vacuum in one year. The speed of light is 300,000,000 meters/second, and there are 365 x 24 x 60 x 60 = 31,536,000 seconds in a year. Since distance = speed x time , that gives:
9.46 x 10¹⁵ meters = 1 light year,
or
9.46 x 10¹² km,
or
5.88 x 10¹⁵ miles.

A light year is about 6 trillion miles- the distance light travels in a year. To give an idea of the immense cosmic distances, consider these measurements: Earth is about 1/8 of a light-second around. A light beam could circle the world 8 times per second.

As you know, light travels very fast (186,000 miles per second). So in one second, a beam of light travels 186,000 miles. So say we want to think about how far away the sun is from Earth - well, it takes light about 8 seconds for light to get from the sun to Earth so that gives 1,488,000 miles. Well, as you can imagine, these numbers get unwieldy fairly fast considering that light from the closest star takes 4 years to reach Earth. In order to simplify thinking about these vast distances, astronomers use a unit of distance called a "light year". Simply put, it is the distance light will travel in a year (in a perfect vacuum).