What is a light year?

• A. The distance that light travels in one year, approximately 5.88 trillion miles or 9.46 trillion kilometers.
• B. The distance from Earth to the Sun.
• C. The time it takes light to reach Earth.
• D. A unit used to measure the brightness of stars

Answer: (A)

A light year is the distance light travels in one year. Light moves about 186,000 miles per second (299,792 kilometers per second), so in a year it covers roughly 5.88 trillion miles or 9.46 trillion kilometers. This unit is handy for describing the enormous separations between stars and galaxies. It’s not the distance from Earth to the Sun (that’s about 93 million miles or 150 million kilometers). It isn’t the time it takes light to reach Earth from the Sun (that’s about 8 minutes and 20 seconds). And it’s not a measure of brightness (brightness uses magnitudes or luminosity). So the distance light travels in a year is the best description.