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Why is a day on Venus longer than a year on Venus?

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Venus
Venus Venus photographed in ultraviolet light by the Pioneer Venus Orbiter (Pioneer 12) spacecraft, February 26, 1979. Although Venus's cloud cover is nearly featureless in visible light, ultraviolet imaging reveals distinctive structure and pattern, including global-scale V-shaped bands that open toward the west (left). Added color in the image emulates Venus's yellow-white appearance to the eye.

A day on Venus is longer than a year on Venus because of the planet's exceptionally slow rotation speed. While Venus completes one orbit around the Sun in 224.7 Earth days, it takes approximately 243 Earth days to complete a single rotation on its axis.12

This slow rotation is one of the most unusual characteristics of the planet. In addition to its slow speed, Venus rotates in a retrograde direction—the opposite of most planets in the solar system—meaning an observer on the surface would see the Sun rise in the west and set in the east.2

Astronomers do not yet have a definitive explanation for these peculiar rotational characteristics. A leading theory suggests that during the formation of the solar system, a massive collision with a planetesimal may have tipped the planet over and significantly slowed its spin.2