Every star and planet in the eastern sky moves westward during the night because of Earth's rotation. Superimposed on this daily motion is something called seasonal drift. Over time, stars in the east drift westward. Months after their first appearance, they disappear in the glow of evening twilight, then return to the morning sky to begin the cycle anew.
Think of Orion. First visible in the east at dawn in August, this distinctive constellation slowly slips westward until it stands high in the southern sky at nightfall in January. By late April, the Hunter hovers low in the western sky and soon fades from view, lost in the twilight glow.
Now it's Saturn's turn to exit the stage. And lucky for us it does it with fanfare.
Like Orion, the ringed planet has slowly drifted westward since the summer months. Right now it's only visible at dusk, setting just as the sky turns dark. Earth's orbital motion causes this gradual or seasonal drift of the stars and planets from east to west.
As slower, outer planets like Saturn circle the sun, the faster Earth passes them up during our yearly, orbital journey. Earth also passes up the stars because they're so far away they appear motionless from our perspective.
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It goes like this. First, a planet will appear ahead of us (in the eastern sky) just like a car we plan to pass on the freeway, then at our side (southern sky) as we pass it, and finally in the rear view mirror (west) after we zip by.
On Sunday evening (Jan. 22), before Saturn bids farewell, it will run almost directly into Venus. Not a real hit, of course, but the two will appear to nearly merge along the same line of sight. We say they're in conjunction. A conjunction is a temporary union of two celestial bodies and very pleasing to the eye.
Venus will be easy to spot low in the southwestern sky starting a half-hour after sunset. In binoculars you'll be able to make out Saturn to the upper right of the bright planet. As the sky gets darker, Saturn will show without optical aid. The difference between them is remarkable, with Venus beaming nearly 100 times as bright!
Both are covered in clouds, which are great reflectors of sunlight. Venusian clouds are not only whiter, but Venus is much closer to both the sun and Earth than Saturn, so it reflects brightly. Sunday night, we'll be 143 million miles from the Evening Star compared to 997 million miles for Saturn (143 million km and 1.6 billion km).
After Sunday night, the two will drift apart. Saturn continues its westward journey, moving deeper into the solar glare, while Venus moves the opposite direction — up and to the east. Venus defies seasonal drift because it's an inner planet. Not only does it move faster than the Earth, but its apparent distance from the sun is currently increasing as it swings to one side of its orbit from our perspective.
At the same time as the conjunction, look for the hair-thin crescent moon dangling below the pair. Try taking a photo with your cell phone. Twilight provides enough light to capture both bright celestial objects and a silhouetted foreground scene.
Only 1.3 days "old," the moon is barely out of the barn and will look as thin as if a spider spun it. Find a location with an unobstructed view to the southwest and plan the best time to be there using this sunset calculator . Then simply enjoy. Clear skies!
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