A day on Venus is longer than its year, by one measure

A day on Venus is longer than its year if “day” means one full spin: Venus takes about 243 Earth days to rotate once on its axis but only about 225 Earth days to orbit the Sun. Measured from one sunrise to the next, though, a Venusian day lasts about 117 Earth days, so the honest answer depends on which day you mean.

Astronomers separate the sidereal day, one complete rotation, from the solar day, one full cycle of daytime and night. On Earth they differ by only about four minutes. On Venus the gap is enormous because the planet spins backwards (retrograde), so its rotation and its orbit combine to bring the Sun round again far sooner than a single spin would. The Sun rises in the west and sets in the east, roughly twice per Venusian year. Mercury has the mirror-image quirk, and it is often garbled:

	Venus	Mercury
One spin (sidereal day)	243 Earth days	59 Earth days
One orbit (year)	225 Earth days	88 Earth days
Sunrise to sunrise (solar day)	117 Earth days	176 Earth days

The Mercury figure for the full day-night cycle is NASA’s, and it comes to just over two Mercury years: there it is the solar day, not the spin, that outlasts the year.

The Venus distinction trips up even official pages. NASA’s Venus facts page says “sunrise to sunset” takes 117 Earth days, but 117 days is the whole solar day, which is how The Planetary Society lists it. A single stretch of daylight at one spot therefore lasts roughly half that.

Even the 243-day figure wobbles. Radar echoes bounced off Venus from Earth between 2006 and 2020 gave an average rotation period of 243.0226 Earth days, but the length of the day varied by about 20 minutes over that period. The researchers, publishing in Nature Astronomy in 2021, concluded that Venus’s crushingly thick atmosphere trades momentum with the solid planet, nudging its rotation faster or slower. The same radar data pinned Venus’s axial tilt at about 2.64 degrees, against Earth’s 23.