If you’ve ever gotten in bed exhausted and lamented, “That was the longest day ever,” well, good thing you’re not on Venus. That’s because scientists researching the planet for 15 years have finally calculated the length of a single day on the Earth-sized planet, and it’s a tad more than our 24-hour cycle.

In fact, a single day on Venus wraps up in 5,832.5 hours — or just over 243 of our days!

With acid rain and a surface temperature that can liquify lead, Venus is not the most hospitable of planets. The research team from UCLA spent years bouncing radar off the surface to figure out how the planet worked. As well as the length of a day, their study also uncovers the tilt of the planet’s axis and the size of is core.

“Venus is our sister planet, and yet these fundamental properties have remained unknown,” says study author Jean-Luc Margot, a UCLA professor of Earth, planetary and space sciences, in a statement. “Without precise data on how the planet moves, any future landing attempts could be off by as much as 30 kilometers. Without these measurements we’re essentially flying blind.”

Earth and Venus have a lot in common as both rocky planets have nearly the same size, mass and density. Yet they evolved along wildly different paths.

On 21 separate occasions from 2006 to 2020, Margot and his colleagues aimed radio waves at Venus from the 70-meter-wide Goldstone antenna in California’s Mojave Desert. Several minutes later, those radio waves bounced off Venus and came back to Earth.

“We use Venus as a giant disco ball, with the radio dish acting like a flashlight and the planet’s landscape like millions of tiny reflectors,” explains 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.”

Researchers say the complex reflections erratically brighten and dim the return signal, which sweeps across Earth. The Goldstone antenna sees the echo first, then Green Bank sees it roughly 20 seconds later. The exact delay between receipt at the two facilities provides a snapshot of how quickly Venus is spinning, while the particular window of time in which the echoes are most similar reveals the planet’s tilt.

Adds Margot: “The observations required exquisite timing. We found that it’s actually challenging to get everything to work just right in a 30-second period. Most of the time, we get some data. But it’s unusual that we get all the data that we’re hoping to get.”

Comparing Venus’ atmosphere and core to Earth’s

Fundamentals such as how many hours are in a Venusian day provide critical data for understanding the divergent histories of these neighboring worlds. Changes in Venus’ spin and orientation reveal how mass is spread out within. Knowledge of its internal structure, in turn, fuels insight into the planet’s formation, its volcanic history and how time has altered the surface.

The new radar measurements show that an average day on Venus lasts 243.0226 Earth days — roughly two-thirds of an Earth year. They also discovered the rotation rate of Venus is always changing so that a value measured at one time will be a bit larger or smaller than a previous value.

The team estimated the length of a day from each of the individual measurements, and they observed differences of at least 20 minutes.

“That probably explains why previous estimates didn’t agree with one another,” says Margot. “Venus’ heavy atmosphere, at roughly 93 times as massive as Earth’s, is likely to blame for the variation. As it sloshes around the planet, it exchanges a lot of momentum with the solid ground, speeding up and slowing down its rotation.”

Researchers also reveal that Venus tips to one side by precisely 2.6392 degrees, compared to Earth’s 23 degrees, an improvement on the precision of previous estimates by a factor of 10. The repeated radar measurements further revealed the glacial rate at which the orientation of Venus’ spin axis changes, much like a spinning child’s top.

On Earth, this “precession” takes about 26,000 years to cycle around once. Venus needs a little longer: about 29,000 years.

With these exacting measurements of how Venus spins, the team calculated that the planet’s core is about 3,500 kilometers across and similar to Earth’s, though they cannot yet deduce whether it’s liquid or solid.

Scientists now hope to turn their attention to Jupiter’s moons Europa and Ganymede. Many researchers strongly suspect that Europa, in particular, hides a liquid water ocean beneath a thick shell of ice. Ground-based radar measurements could fortify the case for an ocean and reveal the thickness of the ice shell.

And the team will continue bouncing radar off of Venus, with each radio echo lifting the veil over Venus a little bit more.