Can there be more to say,
besides what is in the Jupiter pages of Astronomical Calendar 2016?
Too much, but I think one of the most astonishing things about the planet is the speed of its rotation: that huge bulk spins around in less than 10 hours. (More exactly, 9 hours and 55 and a half minutes.) This means a spot on its surface, such as the Great Red Spot, is whistling along at about 45,000 kilometers, or 28,000 miles, an hour. (So that you can check whether I’m right, which can never be assumed for anything mathematical: circumference is 2 pi times radius, and Jupiter’s equatorial radius is 71,492 kilometers.) For comparison, our own far smaller planet takes nearly two and a half times as long to rotate, so places on its equator budge along at only about 1,670 kilometers or 1,040 miles per hour.
Jupiter’s frantic spin causes its noticeably flattened shape, and no doubt aggravates the ferocious winds that roil its cloudy atmosphere, coiling it into storms such as the Great Red Spot itself.
The Spot is actually somewhat south of Jupiter’s equator; it is a feature in the southern border of the band called the South Tropical Belt, on the boundary with the lighter-colored South Tropical Zone. It is an enormous storm, in which gas wells up and spirals outward; the Coriolis effect makes it, in a planet’s southern hemisphere, spiral counter-clockwise. Thus it counts as an anticyclone, as opposed to cyclones which spiral inward and in the opposite direction. However, in another sense it is the opposite of Earth’s anticyclones, which are regions spiralling outward from centers onto which relatively cool air is sinking (which is why they have cloud-free skies – very much the opposite of the Great Spot). I think I have this right. It is difficult to find general (non-planet-specific) definitions of the words cyclone and anticyclone. A recent Sky & Telescope article described the Spot as both “upwelling” and a “high-pressure system”; aren’t these contradictory?
Jupiter has other, smaller persistent oval rotating storms of various unexplained colors. The Great Red Spot may have been observed from 1665 to 1713, then hardly at all, then from 1830 onward, though it’s not certain that these were the same Spot. A century ago it was three times as wide as Earth, but has been shrinking, and is now “only” about one and a quarter times as wide as Earth. And it hasn’t really appeared red since the 1970s.
The reddish spot in my sky scene is something else: an imaginary “shadow of Earth” to suggest the direction straight outward from the Sun. It is on top of Jupiter only while Jupiter is at opposition. And it is both smaller and larger than the planet’s Great Red Spot. The umbra, that is, the total shadow of Earth, by no means reaches out to Jupiter: it tapers away to nothing at only about twice the Moon’s distance (beyond there, the Sun would be seen completely surrounding small black Earth). But Earth’s partial shadow or penumbra tapers outward, infinitely, and at Jupiter’s distance is immeasurably wider than Jupiter. Out in some galaxy, there are trillions of square miles from which you could theoretically see the speck of Earth subtly dimming our star.