Vanishing Orion

• ‹ Previous program
• 843 of 1965
• Next program ›

Orion, the hunter, is bowing out of the evening sky. He's low in the west at nightfall, and his stars begin dropping from view not long afterward. The constellation will be pretty much lost from sight by the middle of May.

Like the hour hands on a giant celestial clock, all the stars loop from the morning sky to the evening sky and back again once a year. That motion is caused by Earth's motion around the Sun.

Earth rotates on its axis once every 23 hours and 56 minutes. That means that every star returns to the same position in the sky at that same interval -- except one: the Sun.

The difference is our orbit around the Sun. Each day, Earth moves about 1.6 million miles along its orbital path. So our planet has to rotate a little longer for the Sun to return to the same position in the sky as the day before: four minutes.

That means the other stars rise and set four minutes earlier each day, so they loop around the sky during the year. Back around Thanksgiving, for example, Orion was just climbing into view in the east as night fell. Now, it's on the opposite side of the sky, just sinking into the sunset. But it'll return to view in midsummer -- this time in the morning, as it begins yet another cycle across the sky.

For now, look for Orion low in the west beginning not long after sunset. His "belt" of three bright stars runs parallel to the horizon, with bright orange Betelgeuse above the belt, and blue-white Rigel below it.

Script by Damond Benningfield, Copyright 2010

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