1. Venus spins backward, so the Sun would rise in the west
Venus turns slowly in the direction opposite to most planets, which is why the Sun would rise in the west and set in the east[1]. Astronomers call a spin like this retrograde.
The numbers are odd as well. One full turn takes about 243 Earth days, but one trip around the Sun takes only about 225 Earth days. On Venus, a turn on its axis lasts longer than a year[1].
Does that mean sunrise to sunrise takes 243 days? No. Because Venus spins one way and orbits the other way, the Sun comes back around sooner. The Sun rises about every 117 Earth days, just under twice in one Venus year[2]. You can check this with a calculator at the end of the article.
So the question is why a planet would end up turning backward and this slowly. Before we look at the two ideas, here is how astronomers found out about the backward spin.
2. Radar saw through the clouds and showed the backward spin
Venus is wrapped in clouds, so telescopes cannot see its ground. The true rotation was measured in 1962 with radar: radio waves sent from Earth bounce off the surface and come back. Those echoes showed a retrograde rotation of about 243 days[3].
Radar is still used, and it has shown that even this number is not perfectly fixed. Radar measurements from 2006 to 2020 gave an average of about 243.02 days, but the length of Venus's rotation changes from time to time by roughly 20 minutes[7]. The authors suggest the thick atmosphere trades spin with the solid planet, though the cause is still being worked out[7].
That detail matters later: on Venus the air and the ground are tied together more tightly than on Earth.
3. High in Venus's air, winds circle the planet far faster than the ground turns
The ground turns once in 243 days, but the atmosphere does not wait for it. Near the cloud tops, the atmosphere circles Venus about 60 times faster than the surface turns[6]. Scientists call this super-rotation. If you divide 243 days by 60, you get roughly 4 days for the clouds to go around.
The Japanese spacecraft Akatsuki, which orbits Venus, helped explain how the winds stay fast. Its team reported that thermal tides, waves in the air produced by the Sun heating the day side and the night side cooling, are what speed up the winds at low latitudes[6]. These are not ocean tides; heating and cooling drive them, not gravity.
Note what this does and does not answer. It explains why the high winds are fast. It does not explain why the ground turns backward. The first idea, in the next section, depends on how the atmosphere and the Sun's tides on the solid planet interact.
4. Idea one: the Sun's tides on a thick atmosphere and on the solid planet changed Venus's spin
Two tidal effects work against each other, according to a summary by astronomer Jacques Laskar of the Paris Observatory[3]. The Sun raises a bulge in the solid planet, and the bulge lags behind the Sun's direction. By itself, this pulls Venus toward a very slow spin synchronized with its orbit. In Laskar's explanation, the Sun also heats the atmosphere, and the resulting atmospheric tide can push the rotation the other way. For an atmosphere as thick as Venus's, he says the second effect is strong enough to upset the synchronized state[3].
Laskar describes two paths, both ending with a backward-spinning Venus. In the first, Venus starts out spinning forward, slows down, stops, and then speeds up in the opposite direction. In the second, Venus starts nearly upright. In Laskar's description, the other planets' gravity changes Venus's orbit in complicated ways, and over time those changes could tip Venus nearly upside down, ending in a backward spin[3].
His summary says this means the backward spin does not need an assumed strong impact[3]. The catch: this shows that tides could do it from many starting points, not that they did. Both paths end with the same present-day Venus, so observations today cannot tell them apart[3].
5. Idea two: a giant collision early in Venus's history changed its spin
The other idea is a big hit. Computer simulations by Mirco Bussmann and colleagues tried many collisions on a model Venus, with different sizes, speeds, and angles. According to the paper's abstract, many cases, including glancing blows from Mars-sized bodies, ended with Venus spinning the way it does now[4].
An obvious worry: Earth's Moon is often explained by a giant impact that threw out debris. Why does Venus have no moon? The authors report that collisions matching Venus's spin tend to leave little debris far from the planet, so material would likely fall back onto Venus, which could explain why it has no moon[4].
A related study by Cedric Gillmann and colleagues, presented at the 2026 General Assembly of the European Geosciences Union, also found that a wide range of impacts are consistent with Venus's current rotation[5]. It also looked at what such an impact would do to the planet's inside, such as a deep magma ocean that would have lasted a while[5].
Be careful with the word "consistent." These studies show that an impact could produce today's spin, not that one happened. And the conference abstract is a presentation, not a finished journal paper.
6. Why tides versus an impact is still undecided
Both ideas have the same strength and the same limit: Each can reach today's Venus, and neither has direct evidence that it is what actually happened. Their starting points differ: the tide idea does not need any large collision, while the impact idea begins with one[3][4].
The sources I read treat the two as separate explanations, and none says which one is right or which measurement would decide it. That is the honest state of the question today.
Venus's thick atmosphere shows up in both stories: it is the key to the tide idea, and Venus's fast upper winds and its slightly changing day show how tightly air and ground are linked. That is a pattern worth keeping in mind, but it is my own observation, not a claim from the sources.
How to check the 117-day sunrise yourself, and where to read more
- Do the sum. Spin and orbit go opposite ways, so their rates add. One turn per 243 days is 1/243, about 0.00412 per day. One orbit per 225 days is 1/225, about 0.00444 per day. Add them: about 0.00856 per day. Divide 1 by 0.00856 and you get about 117 days, matching NASA's figure[2]. If the spin and orbit went the same way, you would subtract the rates instead. What interval do you get then?
- Sunrise on a model. In a dim room, put a lamp on a table and a ball or an orange on a skewer or pencil (with an adult's help) as a "planet." Walk it slowly around the lamp while turning it the same way, then the opposite way. Notice how often the lamp "rises" over the edge in each case. Keep the lamp away from anything that can burn, and use a cool bulb.
- Read the original. Laskar's page on why Venus spins backward is short and explains the two tidal effects in plain language[3]. The arXiv paper on the giant impact is technical but its first page tells the story[4].