By Deborah Byrd
Friday, March 13, 2009
We received the following question:
Orion seems to have moved and turned considerably in the last two weeks. Will Orion disappear before summer?
The answer is that all the stars and constellations shift westward as the seasons pass . . . and they also move westward in the course of a single night. And Orion is no exception. Exactly when Orion will disappear from the evening sky – into the sunset – depends on your latitude. The farther south you are, the longer you can see Orion. But for the central U.S., Orion is lost in the sun’s glare by early to mid-May (depending on how carefully you look for it). And for all of us in the U.S., Orion is gone by the time of the summer solstice in June.
If you want to notice the westward shift of the constellations due to the passage of the seasons, be sure to watch at the same time every night. If you want to watch their westward shift throughout the night, just pull up a lawn chair and watch.
Either way, you can definitely notice Orion moving steadily westward.
The westward shift of the sky throughout the night is due to Earth’s spin under the stars. But the westward shift of the stars throughout the seasons is due to Earth’s motion in orbit around the sun. Earth’s motion in orbit causes our night sky to point out on an ever-shifting panorama of the galaxy.
Why does Orion go into the sun’s glare each year at this same time? Only because — each year, as we orbit continually around the sun — our motion in orbit brings the sun between Orion and us at the same time each year.