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Vanishing Orion

May 1, 2011

With winter behind us and summer creeping up in a hurry, it's not surprising that one of the great markers of the winter sky is disappearing from view. It won't return to the evening sky until we once again feel autumn's chill.

Orion is quite low in the west as night begins to fall. Its three-star belt lines up roughly parallel to the horizon, sandwiched between the hunter's two brightest stars: orange Betelgeuse above the belt, and blue-white Rigel below.

Each constellation has its own "season" because one star in the sky is out of sync with all the others: the Sun.

As Earth turns on its axis, all the stars except the Sun return to the same position in the sky every 23 hours and 56 minutes. But because Earth is moving in orbit around the Sun, it takes four minutes longer for the Sun to return to its same position in the sky.

As a result, all the other stars rise and set four minutes earlier each day. As the days roll on, that adds up. A star that rises at 10 o'clock tonight will rise around 8 o'clock on the first of June, 6 o'clock on the first of July, and so on. That means that the constellations move westward with respect to the Sun. So a constellation that appears in the eastern sky at sunset in December -- like Orion -- is disappearing in the western sky in May.

Watch for Orion low in the west this evening, and dropping lower each night. It'll disappear within a few weeks -- and climb into view in the morning sky in July.


Script by Damond Benningfield, Copyright 2011


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