09/02/2026
Have you ever had a bad day that feels a lot longer than usual? How about a day that lasts 243 days? If you were on Venus, that’s how long it would take to finish just one planetary rotation, called a sidereal day! August 31st was the 243rd day on Earth, so Venus just now completed one sidereal day (though technically, it only takes about 117 Earth days for Venus to complete a full day-and-night cycle). So why does it take so long for Venus to complete a rotation, compared to Earth, and do the other planets have similarly off-set days to Earth? Read on to find out!
⭐ Why does it take Venus so long to complete one rotation?
The biggest reasons Venus rotates so slowly is because of tidal locking and Venus’ very fast-moving atmosphere. When a large massive object (the Sun) unevenly pulls on an orbiting object (Venus), it creates enough gravitational friction that the less massive object’s rotation is slowed or stopped, leaving the same side always facing the more massive object. That’s what would be the case for Venus, if not for its atmosphere and the atmospheric bulge that the Sun pulls in the opposite direction of the rest of the planet.
Venus has an incredibly thick atmosphere (about 90x “heavier” than Earth’s!), made mostly of carbon dioxide and nitrogen gasses. Near the surface, those gasses are compressed so extremely that they behave almost like a liquid gas, making Venus’ atmosphere extremely dense (the densest of all rocky planets). Continuous heating and cooling from the Sun causes much of the atmospheric mass to pile up on one side of the planet, creating an atmospheric tidal bulge that the Sun pulls on, which drives fast winds that drag along Venus and generate just enough friction to cancel out the tidal locking. So, Venus rotates very slowly but it isn’t technically tidally locked, like our Moon, for example.
Fun fact: Venus actually rotates “backwards”, compared to most other planets in our solar system (except for Uranus). Scientists believe this could be due to the atmospheric tug-of-war, a giant collision in Venus’ past, or some combination of the two.
⭐ How fast do other planets in our solar system rotate, compared to Earth?
Compared to Earth’s 24-hour day, the rotational speeds of other planets in our solar system are generally split into the slower-spinning planets (the rocky inner planets) and the faster-spinning planets (the gas and ice giants). For a single rotation, it takes each planet about:
Mercury: 59 Earth days
Venus: 243 Earth days
Earth: 1 Earth day/24 Earth hours
Mars: 1.025 Earth days/24.6 Earth hours
Jupiter: 9.9 Earth hours
Saturn: 10.7 Earth hours
Uranus: 17.2 Earth hours
Neptune: 16.1 Earth hours
If we compared the time required to complete a rotation against an Earth year beginning on January 1st at 12am, each planet would complete a rotation around:
Mercury: March 1st, midnight
Venus: August 31st, midnight
Earth: January 2nd, midnight
Mars: January 2nd, midnight
Jupiter: January 1st, 9:54am
Saturn: January 1st, 10:42am
Uranus: January 1st, 5:12pm
Neptune: January 1st, 4:06pm
⭐ Why do gas and ice giant rotate so much faster than the inner rocky planets?
There are a few reasons for the dramatic difference in rotational speeds between our outer and inner planets.
▪️Size and mass difference
The outer planets formed far away from the Sun and were able to accumulate greater ice, rock, and gas matter very quickly, making them very large and massive, whereas the inner planets were restricted to accreting rock and metals like iron and nickel, which limited their final size.
▪️Conservation of angular momentum
Have you ever noticed how an ice skater can spin faster when their arms are pulled in close to their body? That’s because they are reducing their moment of inertia, which forces their rotational speed to increase to conserve angular momentum. The same thing happened with the outer planets – the gas and rock that collapsed inward under the pull of the protoplanet’s gravity concentrated the mass into a dense mass, causing its rotational speed to increase dramatically.
▪️The Sun’s gravitational grip
The inner planets are much closer to the Sun and are therefore highly influenced by its gravitational pull, which kind of “steals” the rotational energy from the planet and makes it rotate much slower. Outer planets are so far away from the Sun that they are not as easily influenced by the Sun’s gravity, so they have mostly maintained the quick spins they developed during formation.
⭐ Why do planets rotate at all?
During the formation of our solar system, a massive cloud of gas and dust was kind of hanging out in space. The area where the gas and dust were most dense is where our star began forming. As the young star pulled more and more material towards it, the material began to spin rapidly, like water spinning down a drain. This large-scale rotation was conserved as each planet formed, resulting in individual rotations for each planet. Since all material in the solar system was originally spinning in the same direction, (most) planets also spin in that direction, except for the two whose rotations have been disrupted in some way so that their rotational direction reversed.
Here's hoping (at least your bad) days go by a bit faster than those on Venus!
📷: Rotational speeds and length of sidereal days of all eight planets in the solar system. Via NASA/NOAA
