by David M. Schwartz-the amazing, engaging, math exponent
A light year is a distance. It is a vast distance, the distance light travels in a year. To appreciate a light year, you have to understand how fast light travels.
The speed of light is truly mind-boggling: 186,000 miles per . . . second. That’s “per second,” not “per hour.” In one tick-tock second, light travels a distance of 186,000 miles. If it could go in circles, it could travel around the earth more than seven times in just one second! But light travels in straight lines, not in circles. Imagine something traveling that fast in a straight line—not for a second, not for a minute, not for an hour, not for a day, but for an entire year. The distance it goes in that year is called a light year.
A light year is a convenient unit of measure when distances are enormous. You could talk about the same distances in miles (about 5,878,499,810,000) or kilometers (about 9,405,599,700,000) but the numbers are so large that they are unwieldy. It is much easier to just call that distance a light year.
The star closest to our solar system is Alpha Proxima. Some of the light that leaves Alpha Proxima goes to Earth, cruising along at 186,000 miles per second. At that speed, light takes about 4.2 years to get to Earth from Alpha Proxima. So how far away is Alpha Proxima? It is 4.2 light years away.
To give you an idea of how far that is, imagine going to Alpha Proxima in a spaceship traveling at the speed of the space shuttle — about ten miles per second. (That’s much faster than airplanes can fly.) You would get there in about 70,000 years. Have a nice trip!
Our Sun is much closer than Alpha Proxima. It is 93 million miles or 149 million kilometers away. But there is another way to refer to the distance from the Earth to the Sun. Light leaving the Sun takes about eight minutes to get to Earth. We can say the Sun is eight “light minutes” away. There is a special name for that distance. It’s called an “Astronomical Unit” or AU.