The data obtained by bouncing radio waves off Venus – treating it, as one scientist put it, like a giant disco ball – provides new insight into Earth’s closest planetary neighbor, including an accurate calculation of the length of a Venusian day.

The study also measured the tilt of the Venusian axis and the size of the planet’s core, allowing a deeper understanding of an enigmatic world sometimes referred to as Earth’s “evil twin”.

It was already known that Venus has the longest day – the time the planet takes for a single rotation on its axis – of any planets in our solar system, although there have been some discrepancies between previous estimates.

The study found that a single Venusian rotation takes 243.0226 Earth days. This means that a day lasts more than a year on Venus, which makes one full orbit around the sun in 225 Earth days.

Researchers transmitted radio waves to Venus 21 times from 2006 to 2020 from NASA’s Goldstone antenna in California’s Mojave Desert and studied the radio echo, which provided information on certain planetary features, at Goldstone and Green Bank Observatory in West Virginia.

“Each individual measurement was obtained by treating Venus as a giant disco ball. We illuminated Venus with a giant flashlight, Goldstone’s radar, and observed the reflections as they swept across the Earth’s surface,” said Jean- Luc Margot, who led the study published in the journal Nature astronomy. “Venus is an amazing laboratory for understanding the formation and evolution of planets, and it’s a stone’s throw away. There are probably billions of Venus-like planets in the galaxy.”

The new data showed that the Venusian planetary core has a diameter of about 7,000 km, comparable to that of the Earth’s core. Previous Venus base estimates were based on computer modeling rather than observational data. Its core is almost certainly made up of iron and nickel, although it is not known whether it is solid or molten, Margot said.

Venus rotates on its axis almost vertically – meaning it has no discernible seasons – while Earth has more tilt. The study calculated the Venusian tilt to be approximately 2.64 degrees. Earth is about 23.5 degrees.

Venus, the second planet from the sun, has a similar structure but slightly smaller than Earth, with a diameter of about 12,000 km. Above its ominous landscape is a thick, toxic atmosphere composed mostly of carbon dioxide, with clouds of sulfuric acid droplets. With an uncontrollable greenhouse effect, its surface temperatures reach 880 degrees Fahrenheit (471 degrees Celsius), hot enough to melt lead.

Venus rotates from east to west, in the opposite direction of all other planets in our solar system except Uranus. In another oddity, its day-night cycle – the time between sunrises as opposed to the length of a single axial revolution – takes 117 Earth days because Venus spins in the opposite direction to its orbital path around the sun.

Venus has received less scientific attention than Mars, Earth’s other planetary neighbor, and other destinations in the solar system.

“I don’t think Venus would be more difficult to understand than other planets if we had adequate data, but there is a deplorable dearth of data on Venus,” Margot said. “There have been no NASA missions to Venus for almost 30 years and about a dozen NASA missions to Mars in that time frame,” said Margot, adding that the new findings on how Venus spins could help future landing attempts.