Uranus at Opposition

StarDate: September 16, 2009

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The fall equinox is just a few days away, so almost every spot on Earth has something in common: day and night are just about equal lengths. In other words, it's roughly 12 hours from sunrise to sunset, and 12 hours from sunset back to sunrise.

The giant planet Uranus also has equal days and nights across the entire planet -- but they're separated by 42 years.

That's because Uranus lies on its side.

The north and south poles of Earth point roughly up and down, away from the plane of its orbit around the Sun. So do the poles of the other planets.

But Uranus is different. Its poles aim quite close to the plane of its orbit. The leading explanation is that Uranus was whacked by another planet-sized body early in its history, flipping it on its side.

So as Uranus moves around the Sun, its north and south poles take turns aiming roughly in the Sun's direction. Since it takes 84 years for Uranus to complete an orbit, that means that the northern and southern poles each get about 42 years of sunlight, followed by 42 years of darkness.

Uranus is putting in its best showing of the year this week. It lines up opposite the Sun in our sky, so it rises around sunset and remains in the sky all night long. It's brightest for the year, too, although you need binoculars or a telescope to see it. It's in a dark region of sky, along the border between Aquarius and Pisces.

More about the giant planet tomorrow.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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