A Day on Venus Is Longer Than a Year on Venus

Quick Answer

Venus takes approximately 243 Earth days to complete one rotation on its axis (its sidereal day) but only about 225 Earth days to complete one orbit around the Sun (its year). This means a single day on Venus -- measured by one full rotation -- is about 18 Earth days longer than its entire year. Venus also rotates backwards compared to most planets, meaning the Sun rises in the west and sets in the east. A Venusian solar day (sunrise to sunrise) is about 117 Earth days due to the combination of slow retrograde rotation and orbital motion.

The Planet Where Days Outlast Years

Venus breaks the most basic assumption most people have about time: that a day is shorter than a year. On every other planet we commonly think about, this holds. Earth rotates in 24 hours and orbits in 365 days. Mars rotates in about 24.6 hours and orbits in 687 days. Jupiter rotates in just under 10 hours and orbits in 4,333 days.

Then there is Venus.

Venus spins so slowly that it takes 243 Earth days to complete a single rotation. If you were standing on the Venusian surface (which you would not survive for more than a few minutes, given the 460 degree Celsius temperature and 90-atmosphere pressure), you would have to wait 243 Earth days -- about 8 months -- for the stars to return to the same position in the sky.

Meanwhile, Venus is racing around the Sun relatively quickly. Its orbital period is only 225 Earth days. So while Venus was still working on completing a single rotation, it has already finished an entire trip around the Sun and started another one.

One Venusian day is 18 Earth days longer than one Venusian year. Day is longer than year. Let that settle in.

And It Spins Backwards

As if the slow rotation were not strange enough, Venus also rotates in the opposite direction from most planets in the solar system. This is called retrograde rotation.

Most planets, including Earth, rotate counterclockwise when viewed from above the north pole (in the direction of their orbital motion). Venus rotates clockwise. If you could somehow see the Sun from Venus's surface through the dense cloud cover, it would rise in the west and set in the east -- the reverse of what we see on Earth.

Only two planets in our solar system have retrograde rotation: Venus and Uranus (which rotates on its side, technically making it retrograde by convention). Every other planet spins in the same prograde direction.

The combination of retrograde rotation and orbital motion produces a strange consequence for the solar day. A solar day is the time from one sunrise to the next -- the time it takes for the Sun to return to the same position in the sky. Because Venus rotates backwards relative to its orbit, the solar day is actually shorter than the sidereal day (the rotational period).

Venus's solar day is approximately 117 Earth days. Still enormously long, but shorter than its 243-day rotational period. The backward spin and forward orbital motion partially cancel each other out from the Sun's perspective.

Why Does Venus Rotate So Slowly?

This is one of planetary science's genuine open questions. Several hypotheses exist, but none is definitively proven.

Giant impact hypothesis. Early in the solar system's history, Venus may have been struck by a large body that drastically slowed or reversed its rotation. This is similar to the hypothesis for Earth's Moon formation, where a Mars-sized body struck proto-Earth. A sufficiently large impact at the right angle could have flipped Venus's spin direction and slowed it to near-zero.

Atmospheric tidal locking. Venus has an extremely dense atmosphere -- about 90 times the surface pressure of Earth's. Some models suggest that gravitational tidal interactions between the Sun and Venus's thick atmosphere created a braking effect on the planet's rotation over billions of years, gradually slowing it and potentially reversing it. Solar tides in the dense atmosphere may have dissipated enough rotational energy to bring Venus to its current crawl.

Core-mantle interaction. If Venus's internal structure differs significantly from Earth's -- perhaps with a partially solidified core or different mantle convection patterns -- this could affect the transfer of angular momentum and contribute to the slow rotation.

The honest answer is that we do not know for certain. Venus's rotation is one of the solar system's genuine mysteries, and resolving it will likely require better data about Venus's interior structure, which upcoming missions (NASA's VERITAS and ESA's EnVision) may help provide.

Tip

Venus's rotation is not constant. Measurements by the European Space Agency's Venus Express orbiter, compared with earlier data from NASA's Magellan mission, suggest that Venus's rotation rate has changed slightly over the past two decades -- the planet's day appears to have gotten about 6.5 minutes longer between 1990 and 2012. The cause is unknown but may be related to atmospheric dynamics or angular momentum exchange between the atmosphere and the solid planet.

What Time Is It on Venus?

The concept of time on Venus is genuinely bizarre. Consider:

A Venusian solar day (sunrise to sunrise) is 117 Earth days
A Venusian year is 225 Earth days
Therefore, a Venusian year contains approximately 1.92 solar days

In a Venusian year, the Sun rises and sets fewer than two times. If you were a Venusian calendar-maker, your "year" would have fewer than two "days" in it. A "week" in any meaningful sense would be impossible.

For comparison, an Earth year contains about 365.25 solar days. A Martian year contains about 669 Martian solar days. Venus's roughly 1.92 days per year is the lowest ratio of any planet in the solar system.

Venus and Earth: Evil Twins

Venus is often called Earth's twin because of their similar size (Venus's diameter is 95 percent of Earth's), mass (82 percent of Earth's), and composition. They formed from the same region of the protoplanetary disk and are close neighbors in the solar system.

But the twin metaphor falls apart quickly. Venus's surface temperature of 460 degrees Celsius is hot enough to melt lead. Its atmospheric pressure is equivalent to being 900 meters underwater on Earth. The atmosphere is 96.5 percent carbon dioxide with clouds of sulfuric acid. And it rotates so slowly that a single day takes 8 months.

How two planets that started so similarly ended up so differently is one of the central questions in planetary science. The leading hypothesis is a runaway greenhouse effect: early in its history, Venus received slightly more solar radiation than Earth (it is 30 percent closer to the Sun), which prevented water from condensing into stable oceans. Without liquid water to dissolve atmospheric CO2 and sequester it in carbonate rocks (as happens on Earth), the CO2 accumulated, trapping more heat, evaporating more water, and spiraling into the hellscape we see today.

The slow rotation may have contributed to this outcome. Without a strong rotational spin, Venus cannot generate a significant magnetic field through dynamo action in its core. Without a magnetic field, the solar wind strips away lighter atmospheric molecules (including water vapor) more efficiently. This may have accelerated Venus's water loss and sealed its fate.

Venus is what Earth could become with slightly different initial conditions. A day longer than a year, a surface temperature hot enough to melt electronics, and a permanent shroud of acid clouds. Twins indeed.