# What is a light year?

US, WASHINGTON (ORDO NEWS) — A light year is a measurement of distance, not time (as the name suggests). A light year is the distance a ray of light travels over one earth year, or 9.7 trillion kilometers (6 trillion miles).

On a universe scale, measuring distances in kilometers or miles does not shorten that distance. Just like you can measure the distance to a grocery store in the time it takes to get there by car (for example, the store is 15 minutes away), astronomers measure the distance to the stars in the time it takes light to reach us . For example, the closest star to our Sun, Proxima Centauri, is 4.2 light-years away.

How far is the light year?

Unlike the speed of your car, the speed of light is constant throughout the universe and is known with high accuracy. In a vacuum, light moves at a speed of 1,079,252,849 km / h. To find the distance of a light year, you multiply this speed by the number of hours in a year (8,766). Result: one light year equals 9.5 trillion km. At first glance, this may seem like an incredible distance, but the vast scale of the universe reduces this length.

Why use light years?

Measuring in kilometers or miles on an astronomical scale would be extremely cumbersome and impractical. Starting from our cosmic neighborhood, the star-forming region closest to us, the Orion Nebula, is located at a distance of 12,650,000,000,000,000 km, or, more simply, 1,300 light-years. The center of our galaxy is located at a distance of about 27,000 light years. The closest spiral galaxy to us, Andromeda, is at a distance of 2.5 million light-years. Some of the most distant galaxies that we can see are billions of light years from us.

Measurement in light years also allows astronomers to determine how far back into the past we look. Since light takes time to reach our eyes, everything we see in the night sky has already happened. In other words, when you observe something at a distance of 1 light year, you see it the way it appeared exactly a year ago. We see the Andromeda galaxy as it was 2.5 million years ago. The most distant object that we can see, the cosmic microwave background, is also our oldest view of the Universe, which appeared immediately after the Big Bang about 13.8 billion years ago.

Alternatives to Light Years

Astronomers also use Parsec as an alternative to the light year – short for PARallax and SECunda. To be more precise, this is the distance to a star whose visible position shifts by 1 arc second (1/3600 degrees) in the sky after the Earth has made halfway around the Sun. One arcsecond equals 3.26 light years.

Like degrees, a light year can also be broken down into smaller units — light hours, light minutes, or light seconds. For example, the Sun is more than 8 light minutes from Earth, and the Moon is just over a light second. Scientists use these terms when they talk about communications with spacecraft or rovers. Due to the finite speed of light, sending a signal to the Curiosity rover to Mars may take more than 20 minutes.

Whether it’s light years or parsecs, astronomers will continue to use them to measure distances in our vast and large universe.

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