Jupiter's massive Great Red Spot is an "anticyclone" -- a storm system defined as a large-scale circulation of winds around a region of high-atmospheric pressure -- trapped between two jet streams. 

Astronomers Robert Hooke (1635-1703) and Giovanni Domenico Cassini (1625-1712) were the first to note a reddish spot on the giant planet. 

This image was captured by NASA's Hubble Space Telescope in February 1995.

The Great Red Spot has been continually observed since at least the 1830s, although its size and position have fluctuated. In the 1870s it was measured to be about 30,000 miles in width. In Agnes M. Clerke's "A popular History of Astronomy During the Nineteenth Century " (1893), she writes that the giant spot's color had by 1879 "deepened into a full brick-red, and was set off by contrast with a white equatorial spot of unusual brilliancy." 

Left: Drawings by Dr. Otto Boedickker in 1888. Right: A photograph dated 1879.

The spot would change size, shape and position, and for a time in the 1880s, it seemed to disappear altogether. 

Pictured: An 1891 photograph taken at the Lick Observatory, Mt. Hamilton, Calif.

The first closeup views of Jupiter and its distinctive atmosphere were taken by the Voyager 1 spacecraft, which completed its encounter with the planet in April 1979, after taking almost 19,000 pictures, in addition to many other scientific measurements. 

That summer, Voyager 2 took more than 33,000 pictures of Jupiter and its five major satellites.

A view of the region just east of the Great Red Spot, in greatly exaggerated color.

This photo of Jupiter was taken by Voyager 1 on March 1, 1979, from a distance of 2.7 million miles. The white ovals near the Great Red Spot formed in 1939 and 1940, and have appeared almost continuously ever since. 

A color composite made from Voyager 2's narrow-angle camera frames on June 29, 1979 shows the Great Red Spot. A stream of darker material from the South Equatorial Belt (SEB) can be seen erupting northward into the diffuse equatorial clouds above the Great Red Spot. 

This animation, using 66 images each taken every 10 hours (the period of Jupiter's rotation), shows the dynamic motion of swirling clouds around Jupiter's Great Red Spot over the span of more than 60 Jupiter days. 

The Great Red Spot is the largest known storm system in our Solar System. This image was captured by NASA's Hubble Space Telescope in June 1999.

A Hubble Space Telescope photo of the Great Red Spot and roiling atmosphere, taken when Jupiter was comparatively close to Earth -- a distance of 415 million miles. 

A picture taken by NASA's Cassini spacecraft on Dec. 7, 2000, show two of Jupiter's four major moons -- Europa and Callisto -- almost perfectly aligned with each other and the center of the planet.  Europa, seen against Jupiter, is 370,000 miles above the planet's cloud tops; Callisto (at lower left) is nearly three times that distance. 

NASA's Hubble Space Telescope photographed an aurora display in Jupiter's atmosphere.

A true color mosaic of Jupiter constructed from images taken by the narrow angle camera on board NASA's Cassini spacecraft on December 29, 2000, during its closest approach to the giant planet (a distance of approximately 6.2 million miles).

The Great Red Spot, which in the past has been measured to be about three times the size of Earth, today measures approximately 10,000 miles. 

A true-color simulated view of Jupiter, composed of four images taken by NASA's Cassini spacecraft on December 7, 2000. Jupiter's moon Europa casts the shadow on the planet.

A montage of images of Jupiter and its volcanic moon Io, taken by New Horizons during the spacecraft's Jupiter flyby in early 2007. The Jupiter image is an infrared color composite taken by the spacecraft's near-infrared imaging spectrometer, the Linear Etalon Imaging Spectral Array (LEISA). The prominent bluish-white oval is the Great Red Spot. The Io image (featuring a major eruption in progress on Io's night side, of the volcano Tvashtar) is an approximately true-color composite taken by the panchromatic Long-Range Reconnaissance Imager (LORRI), with color information provided by the 0.5 µm ("blue") and 0.9 µm ("methane") channels of the Multispectral Visible Imaging Camera (MVIC). 

With its JunoCam citizen science instrument, NASA's Juno spacecraft captured this close-up view of the turbulent region just west of the Great Red Spot in the South Equatorial Belt, when the spacecraft was a mere 5,400 miles above Jupiter's cloudtops, on Dec. 11, 2016. Citizen scientist Sergey Dushkin produced the color processing.

A false-color image of Jupiter, taken with a mid-infrared filter at the Subaru Telescope in Hawaii on May 18, 2017.  The Great Red Spot appears as a cold region bounded on its northwest by turbulent bands of gas that alternate warm and dry with cold and moist. 

Following the Jupiter flyby, NASA released several images that had been taken by the JunoCam - low-resolution imagery processed by the public. This enhanced photo of the Great Red Spot was created by Roman Tkachenko.

A closeup of the Great Red Spot processed by Roman Tkachenko. 

A detail of the Great Red Spot enhanced by Seán Doran. 

This enhanced-color image of the Great Red Spot was created by citizen scientist Gerald Eichstädt using data from the JunoCam imager. The image was taken as Juno was about 6,130 miles above the planet's cloud layer.

       
For more info: 

Jupiter (NASA)

Juno Mission (NASA/JPL)