astronomy
What Is a Light-Year, and How Far Is It?
Despite the word year, a light-year measures distance, not time. It is how astronomers cope with a universe so big that miles and kilometers stop making sense. Here is what it really means.
How Do the Tides Work, and Why Two a Day?
Twice a day the sea climbs up the beach and slides back down, on a schedule set by an object 384,000 kilometers away. Here is how the Moon, and a bit of the Sun, moves the whole ocean.