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Westward shift of Orion and all the stars

We always get this question at this time of year:

Orion seems to have moved and turned considerably in the last two weeks. Will Orion disappear before summer?

The answer is yes, it’ll soon disappear into the sun’s glare. And – although you might notice it more easily with this particularly bright and noticeable constellation – the fact is that, like Orion, all the stars and constellations shift westward as the seasons pass. Unless they’re in the far northern or southern sky – and so circumpolar – all stars and constellations spend some portion of each year hidden in the sun’s glare. In other words, like blooms on trees or certain flowers or even specific animals in your locale, stars have their own seasons of visibility.

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 us and Orion at this same time each year.

Of course, stars and their constellations also move westward in the course of a single night, due to Earth’s spin. Orion is no exception.

The westward shift of the sky throughout the night is due to Earth's spin under the stars. Meanwhile, 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 toward an ever-shifting panorama of the galaxy. Image via NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring.

Exactly when Orion will disappear from your 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 easily notice Orion moving steadily westward.

Orion in the Canopy - Summit County, Colorado.  Photo posted to EarthSky Facebook in March 2015 by Daniel McVey.  Visit Photography by Daniel McVey.

Orion in the Canopy – Summit County, Colorado – in the month of March by Daniel McVey.

Bottom line: Why the constellation Orion – and all the stars – shift westward as the seasons pass.

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Deborah Byrd

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