Why is a day on Venus longer than its year?

Venus turns on its axis extremely slowly. One full spin takes about 243 Earth days, while one trip around the Sun takes about 225 Earth days. A rotation therefore takes longer than a year, which makes Venus the planet with the longest rotation in the Solar System. The reason for the slow spin is less settled than the numbers themselves. [1] [2]

The Royal Belgian Institute for Space Aeronomy gives an orbital period of 224.70096 Earth days and a rotation period of 243.0185 Earth days. Radar that penetrated the clouds in 1962 made the rotation period measurable, because astronomers had long been unable to see the surface. The ESA lists the same figures and calls a sidereal day on Venus longer than a Venusian year. [3] [4]

Venus rotates backward compared with Earth and most other planets. Its axis is tilted 177.36 degrees, so the Sun rises in the west and sets in the east. Because of this backward spin, the time from one sunrise to the next is shorter than the rotation period. NASA says sunrise to sunset takes about 117 Earth days, and the Belgian institute gives the solar day as about 116 days 18 hours. The Sun therefore rises roughly twice during one Venus year. The Belgian institute adds that the thick atmosphere probably makes dawns and dusks very gradual. [1] [5] [3] [4]

The Belgian institute says the cause of the retrograde rotation is still poorly understood. It names a giant collision with another large body during planet formation as the most likely explanation. It also describes a second idea: solar heating thickened the atmosphere, and tidal forces acting on Venus then slowed the rotation to a stop and even reversed it. [3]

A 2022 study in Nature Astronomy by astrophysicist Stephen Kane addresses the slowness itself. He calculated that the Sun's gravity should have tidally locked Venus in about 6.5 million years, a tiny fraction of the Solar System's 4.5 billion years. Tidal locking would make a day equal a year. Kane argues that Venus's fast, stormy atmosphere drags against the surface and slows the rotation, while solar energy drives that atmosphere and so keeps the planet from locking. The search results present this as a suggestion from one study, not a settled consensus, and they do not say how other specialists have responded. [6]