Which planet has the longest day?
Venus has the longest day of any planet. It takes about 243 Earth days to spin once on its axis, which is longer than its year of about 225 Earth days.
Venus rotates extremely slowly and in the opposite direction to most planets, so on Venus the Sun rises in the west and sets in the east. Because of the combination of its slow spin and its orbit, the time from one sunrise to the next, a solar day, is about 117 Earth days.
Scientists are not sure why Venus spins so slowly and backward. It may have been struck by a large object early in its history, or the Sun's gravity and Venus's thick atmosphere may have gradually changed its rotation.
Other planets
Mercury also has a very long day: one solar day there lasts about 176 Earth days. Jupiter has the shortest day, rotating in just under 10 hours. Earth's day is 24 hours, and a day on Mars is about 24 hours and 37 minutes.
Did You Know?
Because of its slow rotation, Venus is nearly a perfect sphere, without the bulge at the equator that faster-spinning planets have.