Spinning Dipper

StarDate: June 28, 2009

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The Big Dipper is in the northwest this evening. The bowl is below the handle, as though it were pouring water onto the ground below. As you watch during the night, though, it'll "refill" itself as it dips low across the northern horizon, then begins to climb back up into the northeast.

By then, the dipper will be fading from view in the dawn twilight. But it'll still be in the sky, wheeling high overhead in the afternoon, then back to the northwest by nightfall.

As seen from the northern half of the United States, the one thing the dipper won't do is set. That's because the stars of the Big Dipper are "circumpolar" -- they're so close to the North Star, Polaris, that they don't dip below the horizon.

Polaris's height above the horizon corresponds to your latitude. So if you're at 40 degrees north, for example -- roughly the latitude of Denver or Cincinnati -- then Polaris is 40 degrees above the horizon.

As Earth turns on its axis, all the other stars appear to rotate around Polaris. Most of them are so far from Polaris that they spend a good bit of their time below the horizon. But if a star is closer to Polaris than its height above the horizon, then the star never sets.

The stars of the Big Dipper are all within about 40 degrees of Polaris. So from the northern half of the country, the entire Dipper always remains in the sky -- "scooping" water as it dips down to the horizon, and spilling it again as it wheels high overhead.

Script by Damond Benningfield, Copyright 2009

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