Light year
From South Dublin Astronomical Society
A light year, is the distance that light travells through space over the period of 1 year. Thats defined as 9,460,730,472,580,800 meters, or nearly six trillion miles.
This massive distance is hard to imagine, afterall it only takes light 8 minutes to travel the 150 million kilometers from the Sun to the Earth, 63,000 times shorter than a lght year. Pluto, the most distant planet in the Solar system is still only about 5 light hours away.
The distance is scientificially defined as the distance that light travells in a vacuum in one Julian Year (365.25 days). Light, in a vacuum, travells at 299,792,458 m/s. Since space is almost a perfect vacuum, this gives us the 9.36 x 10 12 m distance.
Astronomers sometimes use the Parsec to refer to distances. A Parsec is 3.26 light years.