How Long Is the Solar System's Longest Day? Venus Has the Answer

Data from NASA's Magellan spacecraft and Pioneer Venus Orbiter is used in an undated composite image of the planet Venus. (Handout via Reuters)
Data from NASA's Magellan spacecraft and Pioneer Venus Orbiter is used in an undated composite image of the planet Venus. (Handout via Reuters)
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How Long Is the Solar System's Longest Day? Venus Has the Answer

Data from NASA's Magellan spacecraft and Pioneer Venus Orbiter is used in an undated composite image of the planet Venus. (Handout via Reuters)
Data from NASA's Magellan spacecraft and Pioneer Venus Orbiter is used in an undated composite image of the planet Venus. (Handout via Reuters)

Data obtained by bouncing radio waves off Venus - treating it, as one scientist said, like a giant disco ball - is providing new insight into Earth's closest planetary neighbor, including a precise calculation of the duration of a Venusian day.

The study also measured the tilt of the Venusian axis and size of the planet's core, allowing for a deeper understanding of an enigmatic world sometimes called 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 planet in our solar system, though there were discrepancies among previous estimates.

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

The researchers transmitted radio waves toward Venus 21 times from 2006 to 2020 from NASA's Goldstone Antenna in the Mojave Desert of California and studied the radio echo, which provided information on certain planetary traits, at Goldstone and at the 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, the radar at Goldstone, and observed the reflections as they swept over the surface of the Earth,” said UCLA planetary astronomy professor Jean-Luc Margot, who led the study published in the journal Nature Astronomy.

“Venus is an amazing laboratory for understanding planet formation and evolution, and it's a stone's throw away. There are likely billions of Venus-like planets in the galaxy,” Margot added.

The new data showed that the Venusian planetary core has a diameter of about 4,360 miles (7,000 km), comparable to Earth's core. Previous Venus core estimates had been based on computer modeling rather than observational data.

Its core is almost certainly composed of iron and nickel, though it is unclear whether it is solid or molten, Margot said.

Venus spins on its axis almost upright - meaning it lacks discernable seasons - while Earth has more of a tilt. The study calculated the Venusian tilt at about 2.64 degrees. Earth's is about 23.5 degrees.

Venus, the second planet from the sun, is similar in structure but slightly smaller than Earth, with a diameter of about 7,500 miles (12,000 km). Above its foreboding landscape is a thick and toxic atmosphere that consists primarily of carbon dioxide, with clouds of sulfuric acid droplets. With a runaway greenhouse effect, its surface temperatures reach 880 degrees Fahrenheit (471 degrees Celsius), hot enough to melt lead.

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

Venus has received less scientific attention than Mars, Earth's other planetary next-door neighbor, and other solar system destinations.

“I don't think that Venus would be more difficult to understand than other planets if we had adequate data, but there is a deplorable scarcity of data about Venus,” Margot said.

“There have been no NASA missions to Venus in almost 30 years and about a dozen NASA missions to Mars in this time interval,” Margot said, adding that the new findings on how Venus spins could help any future landing attempts.



Marseille Airport Suspends Flights Due to Wildfire as Public Warned to Stay at Home

 Smoke rises over Marseille as a fast-moving wildfire spreads on the outskirts the city, southern France, July 8, 2025. (Reuters)
Smoke rises over Marseille as a fast-moving wildfire spreads on the outskirts the city, southern France, July 8, 2025. (Reuters)
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Marseille Airport Suspends Flights Due to Wildfire as Public Warned to Stay at Home

 Smoke rises over Marseille as a fast-moving wildfire spreads on the outskirts the city, southern France, July 8, 2025. (Reuters)
Smoke rises over Marseille as a fast-moving wildfire spreads on the outskirts the city, southern France, July 8, 2025. (Reuters)

A wildfire spurred by hot summer winds reached France's second-largest city Tuesday, grounding all flights to and from Marseille, injuring at least nine people and forcing many residents to evacuate or barricade themselves indoors as smoke choked the Mediterranean air.

A big city hospital switched to generator power, train traffic was halted in most of the surrounding area, and some roads were closed and others tangled with logjams.

More than 1,000 firefighters were deployed to tackle the fire, which broke out near the town of Les Pennes-Mirabeau before racing toward Marseille. Some 720 hectares (acres) were hit by the blaze, the prefecture said.

Nine firefighters were injured, according to the prefecture, or local administration. No dead have been reported.

The prefecture said in a statement Tuesday evening that “the situation is under control,″ though the fire has not yet been extinguished. It described the fire as “particularly virulent.″

It came on a cloudless, windy day after a lengthy heat wave around Europe left the area parched and at heightened risk for wildfires. Several have broken out in southern France in recent days.

Light gray smoke gave the sky over Marseille’s old port a dusty aspect as water-dropping planes tried to extinguish the fire in the outskirts of the city, which has some 900,000 inhabitants.

Hundreds of homes were evacuated. The prefecture urged people in the affected areas to stay indoors and off the roads. With the fire approaching Marseille, the prefecture also advised residents in the north of the city to keep windows closed to prevent toxic smoke from entering their homes.

One distressed family watched the smoke over their neighborhood in the hills above the port city and showed AP how the roof of their neighbor's house had been damaged in the fire as they worried about their own.

Marseille airport announced that the runway had been closed at around midday. The prefecture said train traffic was halted, notably after a fire neared the tracks in L'Estaque, a picturesque neighborhood of Marseille.

As a safety measure, the city's Hospital Nord switched to generators “due to micro power cuts.”

“The aim is to secure the imaging sector. We are not worried as we have a high level of autonomy,” the University Hospitals of Marseille said, adding that because of the disrupted traffic it asked workers to remain at their posts until the next teams starts its shift.