Mercury’s Day Is Longer Than Its Year: 176 Earth-Day Sunrises

Mercury’s solar day (sunrise to sunrise) lasts 176 Earth days, because the planet spins once every 59 Earth days but orbits the Sun every 88 days, locking into a 3:2 spin-orbit resonance. As a result, a given spot on Mercury faces the Sun for two Mercury years before sunrise repeats, and the Sun can appear to rise, set, and rise again in a single day. This geometry—paired with a thin atmosphere—produces extreme day-night temperatures and explains why a “day” on Mercury is two years long, even though its year is just 88 days.