How Long Is A Day On Venus? Scientists Finally Have A Good Estimate

Despite Venus being our next-door neighbor, we don’t even know how long the days are there. That basic question is something scientists have been trying to figure out for years — and they might finally have cracked it.

A research team led by UCLA has observed Venus, the second planet from the sun, and reached some conclusions. To be specific, they’ve worked out the precise length of a day on the planet, as well as the tilt of its axis and the size of its core. The findings were published on April 29 in the journal Nature Astronomy.

“Venus is our sister planet, and yet these fundamental properties have remained unknown,” Jean-Luc Margot, Ph.D., a UCLA professor of Earth, planetary and space sciences who led the research, told Phys.org.

As well as being neighbors, Earth and Venus have a lot in common: they’re both rocky planets of almost the same size, mass and density. But they have evolved in very different ways, and establishing the length of a day on Venus can help to improve our understanding of those disparities. More knowledge on Venus, including how mass is spread out within the planet and its volcanic history, will also help boost the chances of a future successful landing attempt.

“Without these measurements,” said Margot, “we’re essentially flying blind.”

NASA/JPL-Caltech

How Long Is A Day On Venus?

After repeatedly bouncing radar off Venus’s surface over the last 15 years, Margot and his team have found that an average day on Venus lasts for 243.0226 Earth days — which is approximately two-thirds of an Earth year. That means that if you were born on Venus, you’d be lucky to experience 150 days before you died of old age!

According to the report, between 2006 and 2020, the team aimed radio waves at Venus from the Goldstone antenna in California’s Mojave Desert on 21 separate occasions. Several minutes later, the radio waves bounced off Venus and returned to Earth. The radio echo was picked up at Goldstone and at the Green Bank Observatory in West Virginia.

“We use Venus as a giant disco ball,” explained Margot. “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].”

Part of finding an answer to the question, “How long is a day on Venus?” involved pinning down a more accurate estimate of Venus’s rotation rate, which is always changing. This means a value measured at one time will be slightly larger or smaller than a previous value. The researchers estimated the length of a day from each of the individual measurements, noting differences of at least 20 minutes.

“That probably explains why previous estimates didn’t agree with one another,” Margot told Phys.org.

Venus, the waning crescent moon, and Jupiter appear together in the night sky.
NASA/Bill Dunford

The variation is probably due to Venus’ heavy atmosphere, which exchanges a great deal of momentum with the solid ground when it spills around the planet. This speeds up and slows down its rotation. The same thing happens on Earth, but the exchange adds or subtracts just one millisecond from each day. Because the atmosphere on Venus is around 93 times larger than Earth’s, the effect is much more dramatic.

The researchers also found that Venus tilts to one side by precisely 2.6392 degrees; by comparison, Earth is tilted by about 23 degrees. The new Venus measurement is 10 times as precise as previous estimates. And when it comes to Venus’ spin axis changes, one cycle takes around 29,000 years — slightly longer than Earth’s 26,000 years. This improved knowledge of how Venus spins enables the researchers to work out that its core is around 3,500 kilometers across, which is quite similar to Earth’s. However, they still need to work out whether Venus’s core is liquid or solid.

It feels good to know a little more about one of our nearest interplanetary neighbors. Next step is inviting them over for coffee.