Jupiter is at opposition

Can there be more to say,

sky scene 2016 March 8

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.


7 thoughts on “Jupiter is at opposition”

  1. I imagine *somebody* has calculated this at some point, but the Earth shadow in your diagram makes me wonder how often a transit across the sun by our planet might be visible in the Jovian system.

    1. Yes, Jean Meeus has six and a half thorough pages on “Transits of Earth as seen from Jupiter” in his “mathematical Astronomy Morsels IV”, p. 322-328. The last such transit was 2014 Jan 5, the next will be 2026 Jan 10.
      He primarily calculates for the centers of the disks of Earth and Sun as seen from the center of Jupiter; talking about “the Jovian system” would make it broader and much more difficult to calculate, but I think he touches on Ganymede.

  2. We here in northern California were graced with a break in the clouds last night and this morning, so I was able to observe Jupiter at opposition (as well as Saturn, just past western quadrature from the Sun, casting a distinct shadow onto his rings).

    The Great Red Spot is noticeably redder and darker this year than in previous years. I’ve seen it in an 80 mm f/6 refractor at 107x magnification.

  3. I glanced briefly at Jupiter through my 4.5 reflector last night and noticed a star in the field of view. It was a little brighter than the (only) two Galilean moons I saw. Your last sentence made me wonder if an intelligent species on a planet orbiting that star has launched a spacecraft to detect exoplanet transits and noticed that our Sun has dimmed slightly because Jupiter (and Earth) might be transiting from their point of view. Or have they already received early radio and television signals from this direction and decided to turn their telescopes elsewhere in their search for intelligent life?

    1. I used to incline (cover picture story of Astronomical Calendar 2011, Drake equation) toward the likelihood that exo-civilizations are rare because their probable lifespans are short, judging by the one example we know, which is in danger of bringing an end to itself by such mistakes as nuclear war and climate-spoiling; but now the sheer number of expplanets makes it seem almost certain that there are relatively nearby civilizations that have detected us.

      1. So far we know of only one planet with life on it, our own. So I’m completely agnostic regarding speculation about the likelihood of life, let alone what we would recognize as intelligent life, on other planets (or moons, or Dyson spheres, or whatever). Perhaps the universe teems with life and abounds with Klingons and Vulcans. But it remains entirely possible that our Earth, just big enough but not too big, with tectonic activity cycling elements between the mantle and the surface, just close enough to our sedate G-class main-sequence star, with our big brother Jupiter shepherding all the unruly bits of the outer solar system, and with our lovely big Moon serving as an outrigger to stabilize our orbit and giving us invigorating tides in our lovely liquid water oceans, might be the only game in town. I worry that the presumption of intelligent life elsewhere in the universe — and the fuzzy-headed expectation that we will communicate and eventually hobnob with them — could make it easier for us to overlook the dire situation on our own planet.

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