Venus spins backward, and a day there outlasts its year

Venus is a planet of extremes, and its rotation is one of the strangest. While most planets spin in the same direction they orbit the Sun, Venus turns the other way — a phenomenon called retrograde rotation. It also spins incredibly slowly: a single rotation on its axis takes about 243 Earth days, while its trip around the Sun takes only about 225 Earth days. So on Venus, a day is longer than a year. ^[1]

This means that if you could stand on Venus, you would see the Sun rise in the west and set in the east — the opposite of what we see on Earth. And you would have to wait a long time between sunrises: a Venusian solar day (from noon to noon) is about 117 Earth days, because the planet's slow rotation and fast orbit combine. ^[1]

Why does Venus spin backward?

Scientists are not entirely sure, but the leading idea is that early in the solar system's history, Venus was struck by a massive object or experienced a series of giant impacts that flipped or slowed its spin. Another possibility is that gravitational interactions with the Sun and other planets gradually altered its rotation over billions of years. The thick atmosphere may also play a role, but the exact cause remains an open question. ^[2]

Venus's rotation is not just backward; it is also slowing down. Measurements from spacecraft like Magellan and Venus Express show that the planet's rotation rate varies slightly, possibly due to exchange of angular momentum between the solid planet and its dense atmosphere. This makes Venus a natural laboratory for studying how atmospheres and planets interact. ^[3]

A day longer than a year

• Venus's orbit around the Sun takes about 225 Earth days. ^[1]
• Its rotation on its axis takes about 243 Earth days. ^[1]
• So a Venusian day (rotation) is longer than its year (orbit). ^[1]
• A solar day on Venus — noon to noon — lasts about 117 Earth days. ^[1]

For comparison, Earth rotates once every 24 hours and orbits the Sun in about 365 days, so our day is much shorter than our year. Mercury, the closest planet to the Sun, has a rotation period of about 59 Earth days and an orbit of 88 Earth days, so its day is also shorter than its year. Venus is the only planet in the solar system where a day outlasts a year. ^[1]

This bizarre rotation has practical consequences for exploration. Any lander or rover on Venus would have to endure long periods of darkness and intense heat, and communication with Earth would be complicated by the slow spin. Future missions, like NASA's VERITAS and DAVINCI, aim to study Venus's rotation and atmosphere in more detail, which could help solve the mystery of its backward spin. ^[3]