It took 15 years of very precise radio observations but astronomers have now got a very good measurement of how fast Venus is rotating, which means we know how long a day is on Earth's fiery twin. One Venusian day is equivalent to 243.0226 Earth days – roughly two-thirds of one Earth year – and it changes with a variation of about 20 minutes. The findings are reported in Nature Astronomy.
It might seem surprising we didn’t know the precise length of a day of Venus, given that the planet is relatively next door. It's easy to work out most planets' rotation speed if they have identifiable features on the surface. The gas giants are harder, but thankfully Jupiter has a giant red swirling storm to track. Venus's thick atmosphere makes finding features complicated though, so astronomers had to get creative in their measurements.
Between 2006 and 2020, astronomers used the 70-meter-wide Goldstone antenna in California’s Mojave Desert to send radio waves to Venus. These waves can travel through the atmosphere and are then reflected from its surface. Several minutes later they are picked up again at the Goldstone observatory and then about 20 seconds later at the Green Bank observatory in West Virginia. The precise difference between the two detections tells the team how quickly the planet spins.
“We use Venus as a giant disco ball,” lead author Jean-Luc Margot from UCLA said in a statement. “We illuminate it with an extremely powerful flashlight — about 100,000 times brighter than your typical flashlight. And if we track the reflections from the disco ball, we can infer properties about the spin [state].”
The experiment sounds a lot easier than it is in real life. Earth and Venus have to be in the right configuration for it and the two radio observatories have to be working for the observations to be successful. Twenty-one observations were taken eventually over the 15-year period.
“We found that it’s actually challenging to get everything to work just right in a 30-second period,” Margot said. “Most of the time, we get some data. But it’s unusual that we get all the data that we’re hoping to get.”
The variation in the length of the day is due to the motion of the dense atmosphere of Venus. At surface level, the pressure is about 93 times higher than on Earth so its sloshing about affects the planet's spin.
The observations also revealed more about Venus. The team was able to estimate that the planet’s core is about 3,500 kilometers (2,175 miles) across. This is similar in size to Earth’s own. However, based on current knowledge we don’t know if it’s liquid, solid, or a mixture.
The research also provides a better measurement of the axial tilt with respect to the planet’s orbital plane. They found Venus tips to one side at exactly 2.64 degrees, an improvement on previous estimates by a factor of 10 in precision. Given the minute tilt, the planet doesn’t experience seasons. Earth’s tilt is about 23 degrees, very different from the much smaller Venusian inclination. But Venus is unique among the planets as it spins in the opposite direction, making for some peculiar effects.
Timekeeping on Venus is very odd. A rotation of the planet takes 243 days, but its year (revolution around the Sun) is only 225 days. However due to the spinning in the opposite direction, if we were counting a day simply from dawn to dawn, then it would only last 117 days. It’s no wonder that this oddball planet continues to hide so many mysteries.