Sidereal Orbital Period of Venus

Venus orbits the Sun in 224.7 days but takes 243 days to turn once on its axis: the only planet whose day outlasts its year. The combination of slow retrograde spin and orbital motion produces a solar day — sunrise to sunrise — of 116.75 Earth days, with the Sun rising in the west. Why the rotation is so slow and backwards is still argued; thick-atmosphere thermal tides, a primordial great impact, or a spin axis flipped by chaotic dynamics are the standing candidates.

The orbital period locks Venus and Earth into a near-resonance: five synodic periods of 584 days come within hours of eight Earth years, so Venus's apparitions repeat against the calendar in an eight-year cycle the Babylonians already tabulated. The same geometry paces the transits across the Sun's disc — pairs eight years apart separated by over a century, the events that gave astronomy its first good measure of the astronomical unit in the 1760s.