How long is a day on Uranus? Slightly longer than we thought, it seems

A day on Uranus just got slightly longer, thanks to more accurate measurements of its rotation period that should help scientists plan missions to probe the gas giant.

Figuring out the rotation period of the solar system’s giant planets is much harder than for the likes of Mars and Earth because ferocious wind storms make direct measurements impossible.

The first measurement of Uranus’s rotation came from the Voyager 2 probe, which made its closest approach on 24 January 1986. Researchers at the time determined that the planet’s magnetic field was offset by 59 degrees from celestial north, while its rotation axis was 98 degrees offset.

These extreme offsets mean that Uranus effectively rotates “lying down” compared with Earth, while its magnetic poles trace a large circle as the planet rotates. By measuring both the planet’s magnetic field and radio emissions from aurora at its magnetic poles, researchers at the time found that Uranus was completing a full rotation every 17 hours, 14 minutes, 24 seconds, with a margin of error of plus or minus 36 seconds.

Now, Laurent Lamy at the Paris Observatory in France and his colleagues have measured it to be 28 seconds longer. More importantly, their measurement is 1000 times more accurate, reducing the margin of error to a fraction of a second.

The researchers looked at images of Uranus’s ultraviolet aurora, taken between 2011 and 2022 by the Hubble Space Telescope, to track the long-term evolution of the planet’s magnetic poles as they circle the axis of rotation.

The margin of error of the previous measurement meant it became impossible to accurately determine a position on Uranus more than a few years later, but the new measurement should remain valid for decades. That means it could be relied on to calculate mission-critical objectives such as where a probe might orbit and enter the planet’s atmosphere.

Tim Bedding at the University of Sydney in Australia calls the team’s measurement technique “very clever”, but points out that the new duration of a day on Uranus isn’t that much different, being within the margin of error of the old calculation. “It’s not so much that it’s changed,” Bedding says. “It’s now accurate enough to be more useful.”