A light-year is how astronomers measure distance in space. It’s defined by how far a beam of light travels in one year – a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. If you like to keep up with what’s going on in astronomy, it’s worth spending a little bit of time understanding what the deal is with this funny unit of measurement.
The speed of light is constant 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 find 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 equals 5,878,625,370,000 miles (9.5 trillion km). At first glance, this may seem like an extreme distance, but the enormous scale of the universe dwarfs this length.
Why use light-years?
Measuring in miles or kilometers at an astronomical scale would be extremely cumbersome and impractical. Starting in our cosmic neighborhood, the closest star-forming region to us, the Orion Nebula, is a short 7,861,000,000,000,000 miles away, or more simply, 1,300 light-years away. The center of our galaxy is about 27,000 light-years away. The nearest spiral galaxy to ours, the Andromeda galaxy, is 2.5 million light-years away. Some of the most distant galaxies we can see are billions of light-years from us.
Measuring in light-years also allows astronomers to determine how far back in time they are viewing. Because light takes time to travel to our eyes, everything we view in the night sky has already happened. In other words, when you observe something 1 light-year away, you see it as it appeared exactly one year ago. We see the Andromeda galaxy as it appeared 2.5 million years ago. The most distant object we can see, the cosmic microwave background, is also our oldest view of the universe, occurring just after the Big Bang some 13.8 billion years ago.
How to measure light-years?
Lighy-years can be measured with reference to speed of light. The distance light travels in a second which is 186,282 miles per second (299,792 kilometers per second). Then multiplied by 60 to convert into hours now it is called Light Hour. In one day there are 24 hours so Light Hour multiplied by 24, it is now called Light Day. For light to travel one year in vacuum Light Day multiplied by 365 which converts Light Day into Light Year.
Alternatives to light-years
Astronomers also use parsecs as an alternative to the light-year. Short for parallax-second, a parsec comes from the use of triangulation to determine the distance of stars. To be more specific, it is the distance to a star whose apparent position shifts by 1 arcsecond (1/3,600 of a degree) in the sky after Earth orbits halfway around the sun. One arcsecond is equal to 3.26 light-years.
Like degrees, the light-year can also be broken down into smaller units of light-hours, light-minutes or light-seconds. For instance, the sun is more than 8 light-minutes from Earth, while the moon is just over a light-second away. Scientists use these terms when talking about communications with deep-space satellites or rovers. Because of the finite speed of light, it can take more than 20 minutes to send a signal to the Curiosity rover on Mars.
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