How far is far? How fast is fast? A light year is the DISTANCE that light will travel, through a vacuum, in one year. How many seconds are in a year? = 31,557,600 s/yr The Speed of Light c = 299,792,458 meters per second or 186,000 miles per second To get an idea of how fast this is. . . light can travel about seven times around Earth in one second! One light year is approximately 5.880 trillion miles or 9.5 trillion km. The symbol for light year is “ly”. Light years are used to measure the vast distances in space. In the universe, the kilometer measure is too small to use. Every time you look at objects in the night sky or the Sun, the light from that object is old. You are looking at history. It takes eight light minutes for light leaving the Sun to reach the Earth. To put this in perspective, if you could drive from the surface of the Sun to the surface of the Earth, it would take you 180 years driving nonstop at 60 mph. Sirius, in the constellation Canis Major, is the sky’s brightest star. It is easy to find on winter and spring evenings. When you look at Sirius, you are looking back in time to see how Sirius looked eight years ago. The light left the star eight years ago.