What distance is defined as the distance light travels in one year?

• A. Astronomical Unit (AU)
• B. Absolute brightness
• C. Light year
• D. Apparent magnitude

Answer: (C)

Light travels at a constant speed, and a light-year is a distance unit defined by how far light would travel in one year. At about 299,792 kilometers per second, light covers roughly 31.56 million seconds in a year, which works out to about 9.46 trillion kilometers (roughly 5.88 trillion miles). This makes the light-year a convenient way to describe the enormous distances between stars and galaxies. The other terms describe different ideas: an astronomical unit is the Earth–Sun distance used inside the solar system; absolute brightness is the intrinsic luminosity of an object; apparent magnitude is how bright an object appears from Earth, which depends on both distance and intrinsic brightness.