The Hubble Telescope snap this photo of the spiral galaxy NGC 3972. The galaxy is 65 million light-years from planet (that"s 382 quintillion miles!) and can be discovered in the constellation Ursa Major.(Image credit: NASA/ESA,/A. Riess (STScI/JHU))
A light-year is a measure up of distance and also not time (as the name might suggest). A light-year is the street a beam of light travels in a solitary Earth year, or 6 sunshine miles (9.7 trillion kilometers).

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On the scale of the universe, measuring distances in miles or kilometers doesn"t reduced it. In the same method that you might measure the distance to the grocery store store in the time it takes to drive there ("The grocery save is a 15-minute journey away"), astronomers measure up the distances of stars in the time it takes because that light to travel to us. Because that example, the nearest star to our sun, Proxima Centauri, is 4.2 light-years away.

How much is a light-year?

Unlike the rate of your vehicle when to run errands, the speed of light is consistent throughout the universe and is known to high precision. In a vacuum, light travels at 670,616,629 mph (1,079,252,849 km/h). To discover the distance of a light-year, you multiply this speed by the number of hours in a year (8,766). The result: One light-year amounts to 5,878,625,370,000 mile (9.5 trillion km). At very first glance, this might seem like severe distance, but the huge scale the the universe dwarfs this length.

Why use light-years?

Measuring in mile or kilometers in ~ an huge scale would be incredibly cumbersome and also impractical. Beginning in our cosmic neighborhood, the the next star-forming region to us, the Orion Nebula, is a quick 7,861,000,000,000,000 miles away, or much more simply, 1,300 light-years away. The center of our galaxy is about 27,000 light-years away. The nearest spiral galaxy come ours, the Andromeda galaxy, is 2.5 million light-years away. Some of the most far-off galaxies we have the right to see are billions of light-years indigenous us.

Measuring in light-years also allows astronomers come determine just how far back in time they viewing. Since light bring away time to travel to ours eyes, everything we see in the night sky has already happened. In other words, once you observe something 1 light-year away, you watch it together it showed up exactly one year ago. We check out the Andromeda galaxy together it appeared 2.5 million years ago. The most far-off object we have the right to see, the cosmic microwave background, is likewise our oldest view of the universe, developing just after ~ the huge Bang part 13.8 billion years ago.


This simulated image demonstrates how tiny the Milky way would look native the ar of ULASJ0744+25, practically 775,000 light-years away. (Image credit: isualization Software: Uniview by SCISS Data: SOHO (ESA & NASA), man Bochanski (Haverford College) and Jackie Faherty (American Museum the Natural history and Carnegie Institute"s department of Terrestrial Magnetism))

Alternatives come light-years

Astronomers also use parsecs as an alternate to the light-year. Short for parallax-second, a parsec comes from the usage of triangulation to identify the distance of stars. To be much more specific, the is the distance to a star whose evident position shifts by 1 arcsecond (1/3,600 the a degree) in the skies after planet orbits halfway approximately the sun. One arcsecond is equal to 3.26 light-years.

Like degrees, the light-year can likewise be broken down right into smaller units of light-hours, light-minutes or light-seconds. For instance, the sun is an ext than 8 light-minutes native Earth, while the moon is just over a light-second away. Researchers use these terms once talking about communications through satellites or rovers. Since of the finite rate of light, it deserve to take an ext than 20 minutes to send a signal to the Curiosity rover top top Mars.

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Whether it"s light-years or parsecs, astronomers will continue to usage both come measure distances in our expansive and grand universe.

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