In the 1660s, Italian astronomer Gian Domenico Cassini discovered something while looking at the planet Jupiter: a massive spot now known as the planet’s signature. Known as the Great Red Spot or Permanent Spot, the planetary feature is thought to be evidence of a massive Jovian storm. But new research suggests the storm astronomers can see today isn’t the same one Cassini saw nearly four centuries ago.

What looks like a red spot from space is actually a gigantic anticyclone vortex twice the size of Earth. Modern observations suggest the storm involves winds blasting at up to 400 miles an hour, and that its signature color may be due to the interaction between elements in Jupiter’s atmosphere and cosmic rays or other forms of radiation. But though the spot has been known for centuries, it still holds many mysteries for researchers.

Known as a pioneer of telescopic astronomy, Cassini first saw the spot in 1665 as a dark oval, writing that the spot was “a permanent one which was often seen to return in the same place with the same size and shape.” Astronomers recorded spot sightings until 1713, but then observations stopped. It would take until 1831 for other scientists to again report a spot at the same place Cassini had pinpointed.

Writing in Geophysical Research Letters, the modern researchers used historical observations to track the size and movement of the spot over the years, comparing those older observations with modern ones. Then they simulated different ways the spot could have arisen.

Their analysis suggests that the spot seen today is more like the one observed in the 1800s than the one Cassini spotted so long ago. Over time, the spot has shrunk and become rounder, probably because it is rotating more quickly, the researchers write. The spot could have formed because of unstable winds that produced an observable proto-storm that disappeared, then came back, they conclude.

“It has been very motivating and inspiring to turn to the notes and drawings of … Cassini,” Agustín Sánchez-Lavega, a professor of applied physics at the University of the Basque Country in Bilbao, Spain, who led the research, said in a news release. He added, “Others before us had explored these observations, and now we have quantified the results.”

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