Distances in space are very great. It is a nuisance to figure them in kilometers or miles, because there are too many zeros. And so astronomers measure distance by light years. A light year is the number of miles that light can travel in a year. Light travels very fast — about 186,000 miles a second. In a year it goes about six trillion miles.
Let's see what that means. When you look at the nearest star you see a twinkling light. The light you see has been traveling for almost 4.2 years! If you are in the fourth grade, the light that you see now left that star when you were still in kindergarten. It has been speeding toward the Earth, night and day, ever since then.
If that star seems a long way off, think about this: Astronomers have discovered galaxies so far away that it takes light 13 billion years to travel from there to the Earth!
Proxima Centauri
