On Venus, which is longer: a day or a year?

Answer

Venus turns so slowly (about 243 Earth days) that one spin takes longer than its 225-day trip around the Sun.

Context

Venus turns on its axis once every 243 Earth days (a sidereal rotation) while it circles the Sun in about 225 Earth days. Measured against the distant stars, a Venusian day is therefore longer than its year. The planet is one of the slowest rotators in the solar system, and it also spins backward, in a retrograde direction, so the Sun would rise in the west and set in the east.

The word day needs care here. Because Venus rotates against the direction of its orbit, the time from one sunrise to the next, the solar day, is shorter than the sidereal day, at roughly 117 Earth days. By that measure a Venusian day is about half a Venusian year. The statement that a day exceeds a year is true for the rotation period but not for the sunrise-to-sunrise cycle, a nuance that trivia often drops.

Rotation was hard to determine because Venus is wrapped in thick clouds. Radar bounced off the surface from Earth in the early 1960s revealed the slow retrograde spin, and later spacecraft radar such as NASA's Magellan refined it. Measurements have shown that the length of the day shifts by several minutes over time, probably because the dense atmosphere exchanges angular momentum with the solid planet. Why Venus spins backward is unsettled; proposals include an early giant impact and the tug of atmospheric tides, and none is confirmed.