FactSentinel

Venus rotates so slowly that a single day on the planet lasts longer than its trip around the Sun. Venus takes about 243 Earth days to complete one rotation on its axis, measured relative to the stars (a sidereal day). It completes one orbit around the Sun in about 225 Earth days. Because the rotation period exceeds the orbital period, a full spin takes longer than a full year.

The evidence comes largely from radar observations. Venus is wrapped in thick clouds that block visual observation of its surface, so astronomers bounced radar signals off the planet starting in the early 1960s and measured how the reflected signals shifted. These measurements, later refined by NASA's Magellan spacecraft in the 1990s, showed the planet's slow rotation. Venus also spins in the opposite direction from most planets, a motion called retrograde rotation. As a result, the Sun appears to rise in the west and set in the east.

A common point of confusion involves how a "day" is defined. The solar day, the time from one sunrise to the next, is about 117 Earth days on Venus because the planet's retrograde spin and its orbital motion combine. That is shorter than a Venusian year, so someone standing on the surface would see the Sun rise about twice per year. The claim holds when a day is measured as one full rotation, which is the standard astronomical definition of rotation period.