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Friday, October 28, 2011

Jupiter at Opposition Tonight

Jupiter reaches opposition with the sun today. This means it is the closest to the earth in this year's orbit around the sun. Like all planets at opposition ("opposite" the sun in the sky), it rises at sunset and sets at sunrise. It will be well placed for viewing throughout the rest of the Fall and Winter months.

Jupiter shines at magnitude -2.9 and shows a disk of 49.7 seconds of arc, just a fraction smaller than last year and as big as it has been since 1962. It is "just" 593.9 million kilometers, or 33.02 light minutes, distant. It seems to be huge in my 102mm refractor, with lots of detail showing in the cloud bands that cross the planet. The Galilean moons continue their dance around the planet, adding a charming note to the view.

The Danish astronomer Ole Roemer was the first scientist to use the varying distance of Jupiter at opposition and at conjunction to estimate the speed of light, using the timing of transits of Jupiter's moon Io in the 1670s. He came up with a speed of 186,000 miles per second, almost right on the money.

Make sure you take some time to go out and view the planet during this opposition. The 4 Galilean moons should be visible in even small binoculars, although probably not all at the same time. Through even a very small telescope all 4 Galilean moons are easily visible, together with a wealth of detail on the planet. The orbit of the moons around Jupiter was one of the pieces of evidence that Galileo cited in support of the then still controversial heliocentric theory of Copernicus over against the geocentric theory of Ptolemy and the theologians of his time. You can go out and observe this piece of evidence with your own eyes tonight. It doesn't get more exciting than that!

Position of a superior plant (beyond the orbit of the Earth) at opposition, as Jupiter is now, and at conjunction.

- by Ira Machefsky
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