How far does light travel in a year? Light moves incredibly fast, sweeping past 299,792,458 meters in a single second. And so in a single year, light travels a total of 9,460,528,000,000 km, or 5,878,499,817,000 miles.
As you probably know, astronomers use the distance that light travels in a year as a standard measuring stick for calculating the largest distances in the Universe. The nearest star to Earth, Proxima Centauri, measures 4.22 light-years from Earth. The center of the Milky Way Galaxy is 26,000 light-years away. The nearest large galaxy, Andromeda, is 2.5 million light-years away.
When we see the light coming from a distant object, we’re actually looking back in time. When we see the light from a star located 400 light-years away, we’re actually seeing light that was emitted from the star 400 years ago. We’re not seeing the star as it looks today, but as it looked 400 years ago. A star could explode as a supernova, and we wouldn’t even be able to know it had happened until the light arrives here on Earth.
We have written many articles about the speed of light for Universe Today. Here’s an article about a beam of material jetting out for many light-years from galaxy M87, and here’s an article about a 400 year old supernova.
We’ve also recorded an episode of Astronomy Cast on this topic. Listen here, Episode 10: Measuring Distance in the Universe.