Orion's sideways rise shows how the sky moves

Many stargazers know Orion by its belt: three stars in a row. But when the constellation rises in autumn evenings, it often appears sideways, as if knocked over.

Space.com reports that this is not a mistake. Most constellation diagrams are printed in the orientation that makes recognition easiest, but the sky makes no such promise. When Orion rises, its familiar figure can look surprisingly unfamiliar at first.

"Orion stopped being a picture and became evidence that the sky was moving," writes Space.com's Jamie Carter.

The change is caused by Earth's rotation. As our planet turns eastward, stars appear to rise in the east, arc across the sky, and set in the west. Orion's recognizable shape makes the effect especially easy to notice.

How to see it

From much of North America, look low toward the east shortly before midnight. Orion rises on its side, with the three Belt stars Mintaka, Alnilam, and Alnitak appearing one by one, preceded by Betelgeuse and Bellatrix and followed by Rigel. Buildings, trees, and haze near the horizon may hide parts of the constellation, but waiting an hour gives a transformed view as Orion climbs and turns into the familiar upright rectangle.

Also in the sky this week

The new moon on Oct. 10 offers dark skies. Early on Oct. 11, find Mars moving through the Beehive Cluster before dawn. The moon returns as a slender crescent later in the week, passing near Mercury on Oct. 13 and Antares on Oct. 14. Saturn remains well placed all evening. Andromeda, with its galaxy M31, is visible high in the northeast after dark.

• Betelgeuse
• Earth rotation
• mid-October sky
• Orion
• Rigel
• stargazing