A light year (symbol: ly)  is the distance that light travels in empty space in one year. Since the speed of light is about 300,000 km per second (about 186,000 miles per second), then a light year is about 10 trillion kilometers (about 6 trillion miles). A light year is not a length of time.
Think of a star at a distance of 100 light years from us on Earth. Light leaves the star and takes 100 years to get to us. When we see the star's light, we are seeing that star as it was 100 years ago.
Similar distance measurements
- Light minute - The distance that light travels in one minute (about 18,000,000 km per minute, or 11,160,000 miles per minute).
- For example, our Sun is about 8 light minutes from Earth
- Light second - The distance that light travels in one second (about 300,000 km per second, or 186,000 miles per second)
- For example, our Moon is about 1¼ light seconds from Earth
- or light-year or lightyear
- The IAU and astronomical units, International Astronomical Union, retrieved 2008-07-05