Venus is a planet of extremes: extreme heat, extreme weather, and extremely long days. That means any given day on Venus (which, again, lasts 243 Earth days) might be slightly longer or shorter than the one before, depending on the weather. These changes aren't necessarily permanent. Get 65% off observing tips, science news, & more with your Astronomy subscription. In the 1960s, scientists used radar to determine that the planet spins backward compared to its orbit, which astronomers call retrograde rotation. That contrasts with Magellan's average of 243 days 26.6 minutes. In fact, a single day on Venus lasts as long as 116.75 Earth days. Observe Venus at the best time this spring, The length of a day on Venus keeps changing. On Venus, a day lasts about 243 Earth-days. We do not sell, rent or trade our email lists. Astronomers spy phosphine on Venus, a potential sign of life, The Venera program: Interplanetary probes from behind the Iron Curtain, How floating microbes could live in the acid clouds of Venus. Long before spacecraft visited Venus, Earth-based telescopes were already trying to unveil its mysteries. The gas giants rotate really fast. In other words, if you want to drop a robotic explorer in a region of the planet, you need to know how quickly that point is spinning. NASA is exploring a proposed mission called VERITAS, or Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy, which would create a high-resolution map of the planet. So what's changing the planet's rotation rate? Venus takes 224.65 Earth days to complete one revolution around the Sun, which is also the length of a Venus year. Over the next 20 years, astronomers kept tracking Venus’ features in an effort to pin down the length of a day. The new results suggest that, over the course of 29 years, from 1988 to 2017, the average rotation of Venus was 243 days 30.5 minutes. Enter the Space & Beyond Box Photo Contest! On Earth and Mars it’s very similar. Those observations pegged the rotation rate at 243.1 days. Previous studies have shown that the heavy atmosphere, far thicker than Earth's, can affect the planet's spin. That's longer than it takes the planet to complete an orbit around the Sun. Venus also rotates backwards compared to the Earth and most of the other planets. Part of the problem with sorting out the planet's rotation comes from lack of precision. The new measurements, which were published in the journal Icarus, take a step back to look at the spin over decades using the Arecibo Observatory in Puerto Rico and the Green Bank Observatory in West Virginia. The European Space Agency also has a mission proposal called EnVision. Both Magellan and Venus Express watched the planet in detail for only a few brief years. Overall, the spin of Venus is determined by a balance between the Sun's tidal pull on the atmosphere, as well as on the solid planet itself. The clouds make things even more challenging. Gravitational tugs from Earth and the Sun may also play a role in changing the length of the day. Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency. more from Astronomy's weekly email newsletter. "There's a lot of excitement these days to go back to Venus," Campbell said. Campbell would like to see that sort of precision used on Venus. A day on Earth is 23.943 hours. So, a Venusian year actually … [Editor's note: This article has been updated to correct errors, including an incorrect description of Venus' rotation and Jean-Luc Margot's affiliation with the new study.]. "Of major interest is how [those measurements] affect the long-term prediction of where a point on the surface is going to be," he said. "We don't have either of those for Venus," Campbell said. Earth takes 24 hours to complete one spin, and Mars takes 25 hours. "We do not know if Venus' rotation is slowing down or speeding up," says Jean-Luc Margot, a planetary scientist at UCLA who was part of the new findings. Like VERITAS, EnVision would perform high-resolution radar mapping. 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While they don't nail down how the planet's spin changes from year to year, they provide a long-term average that can be useful for visiting spacecraft, one that fits both spacecraft measurements. What does it mean for Venus and Mercury to be in retrograde? Neither mission has been funded yet. So, when Magellan arrived at Venus, its measurement of the planet’s day was already almost 5 minutes longer than the radar measurements. Join Us in Tucson for Our Annual Public Star Party! For over 50 years, humans have tried to pull back the thick clouds of Venus to study its surface. "If, 20 years out, we want to land on a certain place on Venus, I want to be sure my prediction is as solid as possible," Campbell said. In 1963, radar observations from Earth pierced the thick cloud cover and measured Venus’ length of day (LOD). Now, researchers have combined almost 30 years of Earth-based observations of Venus to calculate an average rotation rate for the planet. In. "Venus Express only saw that, yes, there was a difference, but there was fairly large uncertainty," said study author and planetary scientist Bruce Campbell of the Smithsonian Institution in Washington, D.C. "We've brought that uncertainty down to a much finer level.".