Venus has the longest day in the Solar System, taking about 243 Earth days to rotate once on its axis. Its rotation is retrograde, meaning Venus spins in the opposite direction to most planets, including Earth.
Venus orbits the Sun in about 225 Earth days, so its sidereal rotation period is actually longer than its year. A solar day on Venus, measured from one sunrise to the next, lasts about 117 Earth days because the planet’s rotation and orbital motion combine in the retrograde direction.
Venus is often called Earth’s sister planet because it is similar in size and bulk composition. Its surface, however, is extremely different: a dense carbon-dioxide atmosphere and powerful greenhouse effect produce surface temperatures around 465°C, hot enough to melt lead. Mercury has the shortest year, not the longest day.