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.
StarChild Question of the Month for March 2000
Why use light-years?
The light-year is a unit of length used to express astronomical distances and measures about 9. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. As defined by the IAU, the light-year is the product of the Julian year [note 2] Before , the tropical year not the Julian year and a measured not defined speed of light were included in the IAU System of Astronomical Constants, used from to Allen 's Astrophysical Quantities reference work,  which was updated in , including the IAU value cited above truncated to 10 significant digits. Other high-precision values are not derived from a coherent IAU system. A value of 9. Another value, 9.
How far is a light-year?
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. In the universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km.
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! When we use powerful telescopes to look at distant objects in space, we are actually looking back in time. How can this be?