The speed of light stays the same—186,282 miles per second in a vacuum. It doesn’t change relative to other objects. If you switch on a light in your rocketship while cruising along at 12 parsecs, the light doesn’t travel at 186,282 miles per second plus 12 parsecs. It stays at 186,282 miles per second. This is because light is an electromagnetic wave and has no mass. Signals from a radio station are electromagnetic waves, too. They wouldn’t speed up or slow down if the radio station were moving.
I say ‘in a vacuum’ because outside of a vacuum light’s going to be slowed down by atmosphere: dust particles, car exhaust, hairspray from actors in 1970s sci-fi movies, bird poop, cigar smoke, &c, &c. Beyond Earth’s atmosphere, in outer space, it’s a vacuum.
A lightyear is the distance light can travel in a vacuum—at 186,282 miles per second—in one year. Earth-year, that is!
Back to the beginning of The Western Civ User’s Guide to Time & Space