A light year is not a measurement of time light year is a measurement of distance, not how long it takes. This is how far a beam of light travels in a year on Earth. It’s about 6 trillion miles, or a light year (9.7 trillion kilometers).

There are a lot of big numbers being talked about, so measuring distances in miles or kilometers is very time-consuming. Astronomers find it easier to measure how far away stars are from us by how long it takes for light to travel through the space between us and the stars. It takes 4.2 years for the light from Proxima Centauri to get to us. This is because the star is so close to us that the light we see from it comes from a little over four years away.

## HOW FAR IS A LIGHT YEAR?

The speed of light is the same all over the universe and can be measured very well. 670,616,629mph (1,079,252,849 km/h): In a vacuum, light moves at this speed. To figure out how far a light year is, you multiply this speed by the number of hours in a year (8,766). One light year is 5,878,625,370,000 miles long (9.5 trillion km). At first glance, this may seem like a very long distance. But the huge size of the universe dwarfs this length. It’s thought that the diameter of the known universe is 28 billion light years across, which is a lot.

## WHY USE LIGHT YEARS?

It is not practical to measure in miles or kilometers at an astronomical scale because of the size of the figures that are being used. At the start of our cosmic neighborhood, the Orion Nebula lies just 7,861,000,000,000,000 miles away, or in light years, 1,300 light years from us. Our galaxy’s center is about 27,000 light years away. A galaxy called the Andromeda galaxy is 2.5 million light years from us. They are billions of light years away from us. The galaxy GN-z11 is thought to be the farthest galaxy seen from Earth. It is 13.4 billion light years away from the Earth.

Degrees can also be broken down into smaller light hours, light minutes, or even light seconds. The light year can also be broken down into these smaller units. If the sun is 8 light-minutes away from Earth, the moon is just over a light-second away. People who study space use these words to communicate with deep-space satellites or rovers. If you want to send a message to the Curiosity rover on Mars, it can take more than 20 minutes because of the speed of light.

Astronomers can also figure out how far back in time they are looking by measuring in light years. Everything we see in the night sky has already happened because light takes a long time to reach us. One light year away, when you look at something, you see it as it looked one year ago. We can see the Andromeda galaxy as it looked 2.5 million years ago when it was still very young and new. Two things are the most distant things that we can see. The cosmic microwave background is the oldest thing we can see because it happened right after the Big Bang 13.8 billion years ago.

Astronomers also use parsecs as an alternative to the light year, but they also use the light year. A parsec is short for parallax-second, and it comes from the way triangulation is used to figure out how far away stars are. Stars appear to move by one arcsecond (1/3,600 of a degree) in the sky when the Earth moves halfway around the sun. This is called the distance to a star. This means that one arcsecond equals 3.26 light years or 3.26 million years.

In our huge and beautiful universe, astronomers will still use light years and parsecs to measure how far away things are.