A Day on Uranus Just Got Longer – ryan

Astronomers have just revealed that a day on Uranus is longer than was previously thought, at 17 hours, 14 minutes and 52 seconds.

This is 28 seconds longer than the previous estimate, which was made by NASA’s Voyager 2 probe during its flyby of the ice giant planet back in 1986.

The new figure—which is 1,000 times more accurate—was calculated based on a decade’s worth of observations of Uranus’s aurorae made by NASA/ESA’s Hubble Space Telescope.

The long-term data on the planet’s auroral emissions enabled the researchers to track the positions of the planet’s magnetic poles and, by extension, its inner rotation rate.

The study was led by astronomer Laurent Lamy of the Paris Observatory in France.

“Our measurement not only provides an essential reference for the planetary science community but also resolves a long-standing issue,” said Lamy in a statement.

He explained: “Previous coordinate systems based on outdated rotation periods quickly became inaccurate, making it impossible to track Uranus’ magnetic poles over time.

“With this new longitude system, we can now compare auroral observations spanning nearly 40 years and even plan for the upcoming Uranus mission.”

On Earth, the aurora form when particles from the solar wind excite atoms in the upper atmosphere, causing them to glow.

Appearing as curtains of light that trace along the geomagnetic field lines, the aurorae vary in color depending on the particular species of atoms being excited.

As Earth’s atmospheric composition is dominated by oxygen and nitrogen, aurorae tend to give off a green hue (from the former) or a blue–pink–purple light (from the latter).

However, unlike the aurora we are familiar with here on Earth—or, even, those seen on Jupiter or Saturn—those produced on Uranus behave in a unique way.

Not only do they appear in the infrared and ultraviolet, unlike the visible emissions on Earth, but they also do not line up with the planet’s poles.

This is because the ice giant has a highly tilted magnetic field—one that is significantly offset from the planet’s rotational axis.

“The continuous observations from Hubble were crucial. Without this wealth of data, it would have been impossible to detect the periodic signal with the level of accuracy we achieved,” explained Lamy.

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References

Lamy, L., Prangé, R., Berthier, J., Tao, C., Kim, T., Roth, L., Barthélémy, M., Chaufray, J.-Y., Rymer, A., Dunn, Wr, Wibisono, Ad, & Melin, H. (2025). A New Rotation Period and Longitude System for Uranus. Nature Astronomy.