It clearly took 15 years of radio observation but astronomers have now got a very good measure of how fast Venus is moving, which means we know how many days the bright twin Earth has on any given day.
A Venetian day is equivalent to 243.0226 days Earth Day – about one-third of an Earth year – and it changes every 20 minutes. Searches have been reported in Nature Astronomy. It may seem surprising that we did not know the exact length of any day of Venus, even though the planet is a relatively side door. Most planets are easier to operate at rotational speeds if they have identifiable features on their surface. The gas giants are tougher, but thankfully there is a giant red whirlwind to track Jupiter. Although the dense atmosphere of Venus makes it difficult to find features, astronomers had to be creative in their measurements.
Between 2006 and 2020, astronomers used a 70-meter-wide Goldstone antenna in the Mojave Desert in California to transmit radio waves from Venus. These waves can travel through the atmosphere and are reflected from its surface. Several minutes later they were picked up again at the Goldstone Observatory and then about 20 seconds later at the Green Bank Observatory in West Virginia. The exact difference between the two detections tells the team how fast the planet spins. Lead author Jean-Luc Margot from UCLA said in a statement, “We use Venus as a giant disco ball.”
“We illuminate it with a very powerful flashlight – about 100,000 times brighter than your normal flashlight. And if we track images of disco balls, we can determine the properties of the spin [state].” The test sounds a lot easier than real life. Earth and Venus must be in the right configuration for this and two radio observers must work for the observation to be successful. Twenty-one observations were finally taken over a 15-year period.
Margot said, “We’ve found it challenging to get everything done right in a 30-second period.” Most the time we get some data. The difference in day length is due to the motion of the dense atmosphere of Venus. At the surface level, the pressure is about 93 times higher than on Earth so its slashing affects the spin of the planet. Observations have revealed more about Venus. The team was able to estimate that the core of the planet covers about 3,500 kilometers (2,175 miles). It’s like the Earth’s own size. However, based on current knowledge we do not know whether it is liquid, solid or any mixture.