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Neptune’s First Orbit

November 7, 2010

The constellation Capricornus scoots across the southwestern quadrant of the sky tonight. It’s due south at nightfall, and sets by around midnight. Its brightest stars form a wide triangle that resembles the bottom of a bikini bathing suit.

The giant planet Neptune stands a little to the upper left of the triangle’s western point. Through a small telescope, it looks like a tiny blue star. It’s not all that impressive a sight, until you consider one thing: Before tonight, only two other people have ever seen Neptune at that position in the sky. That’s because the planet was standing right there when it was discovered by two astronomers in Germany in September of 1846.

Neptune is so far out that it takes 165 years for it to complete a single orbit around the Sun. In fact, it won’t complete the first orbit since its discovery until next year. Calculations by E. Myles Standish, an astronomer at the Jet Propulsion Laboratory in California, indicate the planet will return to the same position relative to the Sun next July.

But Standish says that tonight is also special. That’s because tonight, Neptune is closest to its position in the sky on the night it was discovered. So if you see Capricornus tonight, think of the great moment in 1846 when astronomers first spotted Neptune hiding among the constellation’s faint stars.

Tomorrow: a star’s “jittery” surface.


Script by Ken Croswell and Damond Benningfield


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