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More Neptune at Opposition

StarDate: 
September 2, 2016

Our complexions change as we age. Spots, wrinkles, and other features come and go. And the same thing is true on the giant planets of the solar system. Big spots can appear and disappear over timescales of years or even decades.

Over the last year, for example, Hubble Space Telescope has watched a big new dark spot appear on Neptune, the outermost of the solar system’s major planets. Hubble first saw the spot a year ago, then saw it again in May. It’s a high-pressure system — a big whorl at the top of the planet’s thick atmosphere. It’s similar to the Great Red Spot on Jupiter. Streaks of white clouds stream around it.

The Voyager 2 spacecraft saw an even bigger spot on Neptune when it flew past the planet in 1989. The Great Dark Spot was as big as Earth. Even so, Neptune is so far away that ground-based telescopes of the day couldn’t see enough detail on Neptune to follow it. By the time Hubble could take a look at it a few years later, the Great Dark Spot had disappeared. It’s unclear why it vanished — or why it formed. Yet it is clear that such spots are common features on this solar system giant.

And Neptune is putting in is biggest appearance of the year. It’s lining up opposite the Sun, so it rises at sunset, is in the sky all night, and shines brightest for the year. It’s so far away, though, that you need help to spot it. Through a telescope it looks like a tiny blue star near the center of the constellation Aquarius.

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