How many days in a Mercurian year?

A Mercurian year lasts 88 Earth days. Mercury completes one orbit around the Sun in about 87.97 Earth days, conventionally rounded to 88 days.

Mercury has the shortest year of any planet because it is the closest planet to the Sun, traveling along its orbit at an average speed of roughly 47 kilometers per second. Its orbit is also noticeably elliptical, so its distance from the Sun changes more than Earth's does.

A common mix-up involves Mercury's day. Mercury rotates once on its axis in about 59 Earth days, but its solar day—the interval from one sunrise to the next—is 176 Earth days. That unusual relationship comes from Mercury's 3:2 spin-orbit resonance: it rotates three times for every two trips around the Sun.

NASA's Mariner 10 and MESSENGER missions greatly expanded knowledge of Mercury, while BepiColombo is continuing detailed study of the planet. Despite its short year, Mercury experiences extreme temperature changes because it has almost no atmosphere to retain heat.