Close encounters with mighty Jupiter

Posted Oct. 03, 2010, at 5:30 p.m.

Brighter than anything in the night sky right now except the moon is mighty Jupiter. It’s at its highest about 10:30 p.m., and is unmistakable up there, shining at magnitude minus 2.9. With a decent pair of binoculars and no ground lights washing out the sky, you can even see one or more of its four largest moons, tiny pinpricks of light in a plane on either side of the planet.

It’s particularly bright this fall because it’s passing about as close to the Earth as it ever gets. On Sept. 20 it was closest, about 366.7 million miles from us, and the first week of October has moved to about 369.6 million miles from us. Normally Jupiter is in a range of about 384.4 million miles away when it and the Earth are on the same side of the sun in their orbits. Being nearer than usual this fall, Jupiter reflects from its cloudtops more sunlight us-ward. Before Maine-clouds obscured it late in September, you could see through the still autumn night air the spokes of planetlight splayed around the disk. The spokes are an illusion created by the light making its way through our atmosphere, but their steadiness, so orderly they’re almost countable, is simultaneously reassuring and disquieting.

Jupiter is by far the hugest object in the solar system after the sun. It’s more than 11 times wider and 318 times more massive than the Earth, and is mostly hot gases, with enormous storms (like the Great Red Spot whose 225 mph winds seem to have been raging for 300 years) visible at the surface. Jupiter actually radiates more heat itself than it collects from the sun. This is partly because at its distance — an average of about 483.7 million miles — the sun’s heat is somewhat dispersed, and partly because Jupiter is so massive that its gases are compressing under its own weight, creating temperatures up to 43,000 degrees Fahrenheit at its core, hotter than the sun’s surface. It has been suggested that Jupiter might have become a star itself, but it would have to be about 80 times more massive yet to have a chance to ignite nuclear fusion as the sun does.

Jupiter is nevertheless so big that its gravity plays a sort of shepherding role for practically everything in the outer solar system, including Saturn, Uranus and Neptune which themselves are “gas giants.” Saturn is about 95 times the Earth’s mass, Uranus about 14.5 and Neptune about 17. Jupiter also holds in orbit more than 60 moons of its own, including the four Galileo saw in his simple telescope in 1610 — Io, Europa, Callisto and Ganymede. It also has an enormous magnetic field — 14 times stronger than Earth’s — which gives rise to radiation fields around it that are so intense they would kill humans almost instantly, and that posed dangers for the Galileo and Voyager spacecraft that flew in close.

Ancient astronomers’ perceptions of Jupiter ran parallel to the facts as we know them when they identified it as the overseeing god of heaven. Jupiter is its relatively recent name from the Romans, equivalent to the Greek Zeus, which Aristotle fixed to it around 2,350 years ago. Before that the Greeks called it Phaethon and Phaenon, “the shining.” These were probably meanings transferred by the Pythagoreans (around 2,500 years ago) from the much older name Marduk, a Babylonian god of the skies from well before about 3,000 years ago. Jupiter orbits the sun once in just about 12 years, meaning it travels the 12 constellations of the zodiac at a rate of one per year, repeating the cycle with total reliability and thereby stabilizing the cosmos. How long people have tracked this cycle, no one knows for sure. But it seems likely the second millennium B.C. astronomers, if not their deep forebears as well, whose brains were as large and quick as ours, also noticed Jupiter periodically brightening, as it has this fall, even though they had no way of explaining it physically as we do.

They definitely saw those same spokes of light radiating from the planet disk that I’ve been seeing this fall, 4,000 or 5,000 or more years later. It is a sky-dominating planetlight, and mind-dominating, when your clouds clear.

naturalist@dwildepress.net

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