Tuesday, September 21, 2010


Today Jupiter is at opposition -- exactly opposite the Sun in our sky. That is to say, the Earth is passing Jupiter in our respective orbits. We are hearing in the media that Jupiter is closer and brighter than at any time in 50 years. I've been asked by half-a-dozen people what to look for and where to look, as if tonight (or last night) is a once in a lifetime event.

The buzz is rather like the "Mars as big as the Moon" emails we get periodically.

Jupiter has been big and bright all summer. It will not look any different tonight to the unpracticed eye than it did than a month ago. And it looks pretty much the same every 13 months as we overtake Jupiter in our annual orbit.

If the (nearly circular) orbits of Earth and Jupiter were centered exactly on the Sun, Jupiter would look equally big and bright every 399 days.

But they are not perfectly centered, and Jupiter's orbit takes it sometimes a bit farther from the Sun (aphelion) and sometimes nearer (perihelion). Since Jupiter takes 12 years to orbit the Sun, it passes through perigee once every 12 years. And if Jupiter is at perigee when the Earth overtakes Jupiter in its orbit (opposition), then we make the closest possible approach. Which is nearly the case today.

Meanwhile, September nights are lovely. Step outside at midnight and look south. You won't have any trouble spotting Jupiter. It's brighter than any star. And if tonight is cloudy, try the next night, or the next, or the next. It will be almost as bright, though slowly fading.

And as you look, imagine what you can't see with the unaided eye, the swirling clouds of gas. Here is an image of Jupiter's surface taken by the Hubble in 2008, of the long-enduring Great Red Spot, the smaller "Red Spot Jr.", and a new red spot to the left of the big one. The Earth would fit neatly inside the Red Spot Jr. at lower left. Mark Rothko meets Georgia O'Keefe.