The Great Red Spot Is 195 and Shrinking

One storm running since 1665, or a second one that began in 1831: which does the record support for a spot wider than Earth?

This is FreeAstroScience. We came back to this page expecting to add a section on whether the storm is dying. What we found instead is that the number everyone quotes for its shrinking, 580 miles a year, runs four and a half times the measured long-run rate.

Jupiter’s Great Red Spot has been observed continuously since 1831, which makes it about 195 years old rather than 361. Giovanni Domenico Cassini’s Permanent Spot, tracked from 1665 to 1713, was a separate storm that vanished from the record for 118 years. The Great Red Spot is still shrinking, losing roughly 207 km of length a year on the long-run average, and whether that ends in collapse or in a stable smaller oval remains unsettled.

For most of the last century the answer was 360, and it came with a lineage. Textbooks carried it, museum placards carried it, and this site carried it too. Nobody had to strain to believe it. Cassini saw a dark oval at Jupiter’s southern latitudes in 1665, we see one now, and Jupiter has no coastline to break a storm apart. So the two must be the same thing. Then a team in Bilbao went back to the drawings.

Updated August 2026. This page now absorbs our June 2025 article on whether the Great Red Spot is disappearing, which used to sit at its own address. Both questions turned out to be one question, and the fate half of it is the section we promised last time and did not deliver.

What did Cassini actually see?

Giovanni Domenico Cassini and several contemporaries reported a dark oval at Jupiter’s southern latitudes between July and September 1665. It kept coming back. Maraldi filed the last report in 1713. Counting from a possible earlier sighting by Bandtius in 1632, the 2024 analysis led by Agustín Sánchez-Lavega gives that object, the Permanent Spot, a life of at least 81 years. Not one of those reports mentions a color.

Red enters the story sideways. A 1711 painting of Jupiter by Donato Creti, now in the Pinacoteca Vaticana in Rome, shows the Permanent Spot with a reddish tint. According to the paper’s supporting material, Cassini or the astronomer Eustachio Manfredi told Creti to paint it that way. Yet a painter’s brief is not an observation. Red appears exactly once in that whole record, and it came out of a studio.

Measured off four surviving Cassini drawings, the oval spans 11.4 by 9.15 degrees. One degree of Jupiter longitude covers 1,151 km at this latitude, so Cassini’s spot ran about 13,100 km end to end. Hold that number for two minutes.

Table 1 — The Permanent Spot and the Great Red Spot compared, from Sánchez-Lavega and colleagues in Geophysical Research Letters, 2024

The 118 years when nobody reported a spot

Then it stops. Between 1713 and 1831 the record holds nothing at that latitude. Messier in 1769, Herschel in 1778 and Schroeder in 1785 all drew Jupiter’s belts and its scattered spots, and none of them drew anything where the Permanent Spot had been. These were disciplined observers with the best instruments of their day and a professional stake in the planet. Had a vortex the width of the Great Red Spot been parked there, somebody would have said so. Silence is the whole case.

Read it beside our account of what the 18th-century transit-of-Venus expeditions cost the men who made them. Those observers crossed active war zones to record a shadow that lasted hours, and Messier watched the 1769 transit in the same decade he was drawing Jupiter. A whole generation of people like that quietly overlooking a permanent feature of the largest planet is not a credible reading of the gap.

What appears in 1831 is not a red oval at all. Astronomers drew the Hollow, the pale bay in the cloud belt that the vortex sits inside, and the thing only turns reddish somewhere around 1872 to 1876. On 3 September 1879 A. A. Common photographed Jupiter from Ealing in London with a 91 cm reflector, one second at a time, and produced the first plate on which the spot reads unmistakably. It reads dark, not red. Emulsions of that era responded most strongly to violet and blue. After 1831 the record never breaks again, and as of August 2026 that gives the storm 195 years of continuous observation, which is roughly seven human generations of somebody, somewhere, keeping an eye on it.

Where the storm probably came from

Sánchez-Lavega’s group simulated three ways a vortex this size could be born, and two of them failed. A single convective superstorm produced round ovals and rotation speeds far higher than anything observed, and no such storm has ever been seen at this latitude. Mergers of smaller anticyclones, meaning high-pressure spinning cells, would have needed precursor ovals already as large as the Great Red Spot. Nobody recorded any such chain.

Route three holds together. A long circulation cell gets pinched between two opposing jets, a westward one at 20 degrees south and an eastward one at 26 degrees south, running near 50 and 40 m/s. In the simulations that cell stabilizes once its own edge speed passes roughly 50 to 75 m/s, and its velocity profiles then come out looking like the ones measured in the real storm. Disturbances of that kind are common at this latitude. Two winds sliding past each other in opposite directions is also, more or less, the shear geometry we wrote about when DKIST resolved vortices on the Sun’s surface, at a wildly different scale.

Route three also answers why the thing lasts, and the answer is unglamorous. Nothing on a gas planet stops a vortex the way a coastline stops a hurricane. So the storm survives by staying wedged between two jets that keep pumping shear into it. As it shrank it grew more compact and spun faster, which is precisely backward from how a dying storm behaves. Our explainer on how the Coriolis effect organizes Jupiter’s wind bands still calls this a centuries-old storm. Nobody blinked at that phrasing in January 2024, and it is exactly what the Bilbao paper retires.

How fast is the Great Red Spot actually shrinking?

Slower than you have been told, and the gap is embarrassing. In May 2014 a NASA Hubble release announced that the spot was shrinking by 580 miles a year, a rate measured since 2012. Amy Simon proposed that small eddies feeding into the storm were altering its internal dynamics. That figure has been quoted ever since as though it were the storm’s rate. Our own June 2025 article quoted it too.

Run the conversion. 580 miles is 933 km, and at 1,151 km per degree that comes to 0.81 degrees of length a year. Sánchez-Lavega’s team, measuring across the whole photographic record, gets 0.18 degrees a year, or 207 km, rising to about 0.3 degrees lately. So the famous number is four and a half times the long-run rate and nearly three times the recent one. Real, measured over roughly two years, and never a trend.

Width tells the same story more quietly: 0.03 degrees a year since 1879, but 0.17 degrees a year since 2010. Something did change around the turn of the last decade. What the 2014 release caught was the front edge of that change, sampled short.

Table 2 — Published east-west sizes of the Great Red Spot, in the units each source used, from the NASA Hubble team in 2014 and from Sánchez-Lavega and colleagues in 2024

Set those against Earth’s 12,742 km and the century reads plainly. Three and a fifth Earths in the 1880s, one and eight tenths by Voyager, one and three tenths by 2014, and about 1.09 today. One number in that table will not line up with the paper, and we would rather say so than smooth it over. Row one’s 25,500 miles works out to 35.7 degrees, while Sánchez-Lavega’s team puts the early Great Red Spot at 50 to 60 degrees for the same era. Most likely they are measuring different boundaries, the red oval in one case and the wider Hollow in the other. Neither source says so in as many words, and we cannot close the gap for them.

Why does the storm change size every three months?

Because it does, and almost nobody watching from the ground can see it. Amy Simon, Michael Wong, Philip Marcus and Patrick Irwin pointed Hubble at the Great Red Spot on eight dates between 10 December 2023 and 8 March 2024. Those eight visits span 88.5 days end to end, one full cycle of the storm’s known 90-day wobble in longitude.

Drift was the part already known. What they found is that size and shape swing along with it. Over that single cycle the red oval’s long axis ran from about 12,881 km to about 14,233 km, widest when the storm’s westward crawl was slowest. Its core brightened in ultraviolet by about 5 percent in step with the widening. Latitude, meanwhile, did not budge from roughly 23.5 degrees south.

Now put that beside the shrinkage. Swing is 1,352 km in ninety days, against a long-run shrink rate of 207 km a year. So the storm inhales and exhales six and a half years’ worth of shrinking every three months. Any single measurement of how big the Great Red Spot is, including every row of Table 2, was taken somewhere on that curve without anybody knowing where.

Simon’s team is careful about what one cycle can carry. They write that with only one cycle of data they cannot reliably fit a period. One observing date produced no usable velocity field at all, and a single cycle cannot pin the oscillation to any particular forcing process. They also note that the width variation runs around 0.3 degrees while the scatter among amateur observers approaches a full degree, which is why this stayed invisible for so long.

How deep does the storm run?

Deep enough that gravity measures it better than cameras do. Juno crossed over the vortex twice, in February and July 2019. Mass packed into the storm’s winds tugged the spacecraft off course by enough to shift its radio signal. Marzia Parisi’s team turned that shift into a depth of about 300 km, give or take 100. Scott Bolton’s microwave work, published in the same issue of Science, puts a floor near 240 km, which leaves the whole storm sitting inside the top 500 km of atmosphere.

Here we have to correct ourselves twice. JPL’s release announcing it leads with “300 miles (500 kilometers)”, which is the outer bound rather than the figure they derived. Our June 2025 article read that as a depth of 300 miles. This page, in its August 2026 form, printed 300 km and called it Parisi’s number, which is right but drops the bound entirely. Two of our own articles took two different numbers out of one story, and both were reading a press release rather than the paper. Parisi’s team adds a caveat neither of us carried: their analysis does not constrain the minimum depth well, and the gravity signal they were hunting is about 5 percent the strength of the background winds.

Above that depth, composition is ordinary Jovian air. Hydrogen and helium make up the bulk, with ammonia condensing into the visible cloud deck. Color is the part that is not ordinary, and there the obvious guess failed. Kevin Baines and colleagues broke apart ammonium hydrosulfide, the sulfur-bearing compound in one of Jupiter’s main cloud layers, expecting red, and what came out of the flask was a brilliant green. Ammonia split apart by ultraviolet light and then reacting with acetylene matched the Cassini spacecraft’s spectra far better. Baines calls it a sunburn, a thin reddish skin over cloud that is probably whitish or grayish underneath.

Wind speed is where reporting turns slippery. Simon’s 2024 velocity maps put mean interior speeds between 75.4 and 101.8 m/s depending on method, which is 271 to 366 km/h. NASA and ESA releases on Wong’s 2021 wind study say the outer ring tops 400 mph, or 640 km/h. Both get quoted as the wind speed of the Great Red Spot. They sit nearly a factor of two apart, and neither release says which part of the storm the larger figure describes.

Is the Great Red Spot dying?

Nobody knows, and the authors say so in one sentence. If the shrinkage persists, Sánchez-Lavega’s team writes, it leads either to the disappearance of the Great Red Spot, as happened to the Permanent Spot, or to the storm settling at a stable long-lived size. Published certainty ends there. Anything more confident, ours included, is decoration.

What the evidence does show is a storm changing shape rather than fading. Eccentricity, meaning how far from circular the oval is, has fallen from about 0.92 in 1879 to 0.6 in 2023. Meanwhile it spins faster than it used to, with mean outer-ring speeds up around 8 percent between 2009 and 2020. A vortex losing coherence does not tighten and accelerate. That is our reading, and it is why we do not treat the shrinking as a death.

One more number, and it is uncomfortable. Cassini’s spot measured 11.4 degrees. Today’s storm measured 12.1 degrees in August 2023, only 6 percent longer. At the recent rate of 0.3 degrees a year, it closes that 0.7 degree gap in about two and a third years. By our arithmetic the Great Red Spot is arriving at the Permanent Spot’s dimensions round about now. The paper notices the same convergence from the other direction and says the two objects are becoming similarly rounded ovals.

The case rests on two extrapolations, and one of them is ours

Sánchez-Lavega’s team fits a curve to the measured shrinkage of the oval and its Hollow, then runs it back past 1831 to see where such a storm would have started. Fitting the curve is legitimate. It is also the weakest joint in the argument, because a curve fitted to data from 1879 to 2023 is being asked a question about 1713. In most places the paper hedges properly, calling the identification highly improbable rather than impossible. Then it writes that the current storm has “certainly” existed for 193 years. That word is doing more work than the evidence hands it, and we would have written most likely.

Our own extrapolation in the last section runs three years, not 118, which makes it a smaller sin and not a different one. Hubble’s OPAL program, which maps the outer planets once every year, measures the oval and will settle this without our help.

Arithmetic of the gap is worth doing anyway, because that part depends on no curve at all. Cassini’s spot was 11.4 degrees long when it was last drawn. Early drawings of the Great Red Spot, from 1831 to 1877, run 50 to 60 degrees. For those to be one object, it would have had to grow by more than 43 degrees. Call it 50,000 km, or nearly four Earth diameters, at about 425 km a year for 118 years. Every measurement made since runs the other way, and Herschel’s generation looked at Jupiter and wrote down nothing.

One thing we are still leaving out. Jupiter’s smaller ovals, the white storms that merge and split around the Great Red Spot and occasionally feed it, get one clause here and deserve a piece of their own. They are also the likeliest mechanism behind the 2012 acceleration, and we have not read that literature carefully enough to write about it yet.

Two storms, and one of them already ended

• Cassini’s Permanent Spot ran from 1665 to 1713, and nobody recorded a color for it.
• Then 118 years of silence.
• Today’s storm was first drawn in 1831, which makes it 195 years old.
• Its length has fallen from about 41,000 km in the late 1800s to roughly 13,900 km, and it is still falling.
• The 580 miles a year everybody still quotes was a two-year burst, four and a half times the long-run rate.
• Sánchez-Lavega’s team names two endings, collapse or a stable smaller oval, and picks neither.

We rebuilt this page because it carried the wrong number twice, in two different articles, and you should hear that from us rather than find it yourself. Hard science, handed over in words you already own, is the whole job here. A mind left idling will believe whatever it is handed. So argue with the arithmetic above, especially ours, because we criticized Sánchez-Lavega’s team for running a curve back 118 years and then ran one forward three. OPAL keeps mapping Jupiter every year, and Simon’s group has said plainly that a single 90-day cycle cannot fix a period. Come back when somebody publishes a second oscillation cycle, because that is what decides whether the storm is still oscillating or genuinely ending. FreeAstroScience, Rimini. Gerd Dani.

Sources

1. Sánchez-Lavega, A., García-Melendo, E., Legarreta, J., Miró, A., Soria, M., & Ahrens-Velásquez, K. (2024). The Origin of Jupiter’s Great Red Spot. Geophysical Research Letters, 51, e2024GL108993. DOI 10.1029/2024GL108993
2. Sánchez-Lavega, A., et al. (2024). The Origin of Jupiter’s Great Red Spot. Author preprint, arXiv:2406.13222, submitted 19 June 2024. https://arxiv.org/abs/2406.13222
3. Simon, A. A., Wong, M. H., Marcus, P. S., & Irwin, P. G. J. (2024). A Detailed Study of Jupiter’s Great Red Spot over a 90-day Oscillation Cycle. The Planetary Science Journal, 5, 223. Published 9 October 2024. DOI 10.3847/PSJ/ad71d1
4. Parisi, M., Kaspi, Y., Galanti, E., Durante, D., Bolton, S. J., Levin, S. M., et al. (2021). The depth of Jupiter’s Great Red Spot constrained by Juno gravity overflights. Science, 374(6570), 964-968. DOI 10.1126/science.abf1396
5. Bolton, S. J., Levin, S. M., Guillot, T., Li, C., Kaspi, Y., Orton, G., et al. (2021). Microwave observations reveal the deep extent and structure of Jupiter’s atmospheric vortices. Science, 374, 968-972. DOI 10.1126/science.abf1015
6. Wong, M. H., Marcus, P. S., Simon, A. A., de Pater, I., Tollefson, J. W., & Asay-Davis, X. (2021). Evolution of the Horizontal Winds in Jupiter’s Great Red Spot From One Jovian Year of HST/WFC3 Maps. Geophysical Research Letters, 48, e2021GL093982. DOI 10.1029/2021GL093982
7. Carlson, R. W., Baines, K. H., Anderson, M. S., Filacchione, G., & Simon, A. A. (2016). Chromophores from photolyzed ammonia reacting with acetylene: Application to Jupiter’s Great Red Spot. Icarus, 274, 106-115.
8. NASA Hubble mission team, Goddard Space Flight Center. Hubble Shows that Jupiter’s Great Red Spot Is Smaller than Ever Seen Before. Published 15 May 2014. https://science.nasa.gov/missions/hubble/hubble-shows-that-jupiters-great-red-spot-is-smaller-than-ever-seen-before/
9. NASA Jet Propulsion Laboratory. NASA’s Juno: Science Results Offer First 3D View of Jupiter Atmosphere. Published 28 October 2021. https://www.jpl.nasa.gov/news/nasas-juno-science-results-offer-first-3d-view-of-jupiter-atmosphere/
10. NASA Jet Propulsion Laboratory. Jupiter’s Red Spot is Likely a Sunburn, Not a Blush. Published 11 November 2014. https://www.jpl.nasa.gov/news/jupiters-red-spot-is-likely-a-sunburn-not-a-blush/
11. NASA. Hubble Shows Winds in Jupiter’s Great Red Spot Are Speeding Up. Published 27 September 2021. https://science.nasa.gov/missions/hubble/hubble-shows-winds-in-jupiters-great-red-spot-are-speeding-up
12. ESA/Hubble. Hubble Shows Winds in Jupiter’s Great Red Spot Are Speeding Up, release heic2110. Published 27 September 2021. https://esahubble.org/news/heic2110/