The fact that the spot is actually a storm swirling in the Jovian atmosphere helps explain one of the characteristics that astronomers a century ago found most troubling. The Great Red Spot didn’t exactly stay put; it seemed detached from the planet’s surface. “No observer understands the cause of this huge rift,” The New York Times reported in 1880. “It may be an opening in the cloud-atmosphere disclosing the more solid matter beneath, and it may be something beyond human ken.”



In the late 19th-century and early 20th-century, observers of the night sky developed several theories as to why the spot appeared this way. Perhaps it was a cooling portion of planetary crust on an otherwise gaseous body, an “enormous piece of slag, floating on the molten planet,” as the Philadelphia Record put it in 1902. In other words, it was a newborn continent. Or maybe it was a region of Jupiter that was, for some reason, hotter than the rest of the planet, not cooler—and it was heat that dissipated into the distinctive patch of red clouds that looked odd and spotlight to space enthusiasts on Earth. Scientists flirted with the idea that the spot was actually an opening in the cloud cover that otherwise enveloped Jupiter, an elliptical red glimpse at the planet below.

Others surmised the spot was the product of “solar attraction,” a phenomenon caused by the sun. Some believed it was evidence of a new satellite in the making, a “Jovian moon in embryo,” according to The Washington Herald in 1915. And as recently as the 1950s, at least one observer claimed to have seen snow-capped mountains on the spot—as though it were a Laputa-esque island, floating in the clouds.

Misguided though these early ideas may have been, the great red spot did help scientists piece together some key information about the monstrous planet. It was, for instance, instrumental in giving astronomers a measure of Jupiter’s rotation. And it informed the understanding that the planet was, in fact, a giant ball of gas. Or, as the Omaha Daily Bee put it in 1916, a planet in “a constant state of ebullition, like a boiling globe where nothing retains a permanent shape, except, perhaps, the strange region called ‘the great red spot.’” (For the record, Jupiter may be something of a shape-shifter, but it’s actually frigid for the most part.)

In the first photographs ever taken of Jupiter, the great red spot was visible, but not clearly so. In the photograph below, circulated by the Associated Press and published by The New York Times in 1952, the spot is beneath the circular shadow in the upper left portion of the planet—that shadow is cast by one of Jupiter’s dozens of moons.

(AP)

At times, the spot has appeared to have companions. In the 1930s, observers noticed “one very dark belt” just below the great red spot, but it soon dematerialized. Today, as the spot appears to be shrinking and fading to orange, people remain charmed by it. Many of the planetary scientists working on the Juno mission, which it set to enter Jupiter’s orbit on July 4, are fixated on the spot.