# What is a light year and how long is it?

A light year is a way of measuring distance. I know that doesn’t seem to make much sense because “light year” contains the word “year,” which is normally a unit of time. Even so, a light year measure distance. When astronomers use their telescopes to look at the planets and the stars, the distances are gigantic. For example, the closest star to Earth, besides our sun, is about 24,000,000,000,000 miles (38,000,000,000,000 kilometers) away. There are stars and planets that are billions of times farther away than even that, and when you start talking about those kinds of distances, a mile or kilometer just isn’t a practical unit because the numbers are just too big. Since no one wants to talk or write about numbers with 20 digits in them people use a unit called a light year to measure really long distances.

A light year is the distance light will travel in a year, which is a long, long way. Light travels 186,000 miles a second (300,000 kilometers per second), so, a light second is 186,000 miles (300,000 kilometers). So some quick multiplication gives us one light year as 5,878,500,000,000 miles!

186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles a year (9,460,800,000,000 kilometers)

That’s a long way!