I know the speed of light is 186,00 mph (or something like that) but what exactly is a light year?
A light year is a measurment of distance. Its the distance that light travels in a year. As you can imagine, this is quite a long distance, equall to about ten trillion killometers. That's 10,000,000,000,000! And light travels much faster than 186,00 mph. Its more like 299792 kilometers per second, or 186287 miles per second.
A light year is the distance it takes light to travel in a year.
A light year is the distance light travels in one year. Or, for sci-fi, the distance you go in one year if you travel at the speed of light. Light-years are a measure of distance, not time. So if your stars are ten light-years apart, it takes ten years at the speed of light to travel from one to the other.
A period of covering a distance over a year at the speed of the light.
A light year is not a length of time, but the distance that light will travel in one Earth year (365 days), going at the speed of light. One light year is about 6 trillion miles, or 10 trillion kilometers. Because the universe is so big, some things are hundreds, thousands or millions of light years away. Because light that leaves a star 100 light years away will take 100 years to get to us, this means that when we see the star's light, we are actually seeing that star as it was 100 years ago. So basically it's how far light would travel in a year. =]
