It used to be said with confidence by even grade-school kids that the largest storm in the Solar System was Jupiter’s Great Red Spot, which has been churning for at least 350 years and could fit three Earths across it. And while it’s true that the GRS is a truly enormous hurricane by Earthly standards, these days it’s not as “great” as it used to be — over the past couple of decades the GRS has shrunk to only about a third of its former size.

“Recent Hubble Space Telescope observations confirm that the Great Red Spot (GRS) is now approximately 10,250 miles across, the smallest diameter we’ve ever measured,” said Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

That equals about 16,500 kilometers, or about one and one-third Earths across. Which is still very big, yes, but nothing compared to what it once was!

The giant anticyclonic storm in Jupiter’s southern hemisphere was first noted by Italian astronomer Giovanni Cassini in 1665 as a “permanent spot.” (Source) It has been continuously observed in telescopes since 1878, when astronomers estimated it to be as big as 25,500 miles (41,000 km) across. In 1979, NASA’s Voyager 1 and Voyager 2 missions measured the GRS to be 14,500 miles (23,335 km) wide.

The storm has been slowly shrinking since the 1930s. In 2012, though, the Great Red Spot went on a crash diet, since then losing 580 miles in width per year and changing shape from a dramatic oval to more of a demure circle.

While the cause of the reduction isn’t yet known, it’s speculated that small eddies are changing the circulation patterns within Jupiter’s atmosphere.

“In our new observations it is apparent that very small eddies are feeding into the storm,” said Simon. “We hypothesized that these may be responsible for the accelerated change by altering the internal dynamics and energy of the Great Red Spot.”

Read more here on the Hubble news page, and learn more about the Great Red Spot here.