Light-year , in astronomy , the distance traveled by light moving in a vacuum in the course of one year , at its accepted velocity of ,, metres per second , miles per second. A light-year equals about 9. About 3. Info Print Cite. Submit Feedback.
StarChild Question of the Month for March 2000
Looking Back in Time
Answer: A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about , kilometers km each second. So in one year, it can travel about 10 trillion km. More p recisely, one light-year is equal to 9,,,, kilometers. Why would you want such a big unit of distance? Well, on Earth, a kilometer may be just fine.
Science News for Students
A light year is a way of measuring distance. That doesn't make much sense because "light year" contains the word "year," which is normally a unit of time. Even so, light years measure distance. You know how long a foot or a meter is -- you are comfortable with these units because you use them every day. Same thing with miles and kilometers -- these are nice, human increments of distance. When astronomers use their telescopes to look at stars , things are different. The distances are gigantic. For example, the closest star to Earth besides our sun is something like 24,,,, miles 38,,,, kilometers away. That's the closest star.
Coe et al. For most space objects, we use light-years to describe their distance. A light-year is the distance light travels in one Earth year. One light-year is about 6 trillion miles 9 trillion km. That is a 6 with 12 zeros behind it!