Goodbye Saturn, hello Venus. Two planets pass at twilight on Thursday, Sept. 29.
Credit: Starry Night Software
Take a look at the western horizon just after sunset tonight (Sept. 29) and you may be able to see two planets in close conjunction, and a skinny 3-day-old moon.
The planet Venus has been on the far side of the sun for the last few weeks and is just now making its appearance in the western sky as an "evening star."
Thursday night it will pass less than three moon-widths from Saturn, which is about to disappear behind the sun. The two planets appear to be moving in opposite directions because of the movement of a third planet: the Earth, from which we observe them. [Gallery: The Rings and Moons of Saturn]
The sky map of Venus and Saturn here shows how they will appear in the night sky tonight.
Venus is, of course, a planet and not a star, but it was named "evening star" and "morning star" long before we knew the difference between stars and planets. Stars shine with their own light, while planets only reflect the sun's light.
Seeing these two planets will be a challenge, and a set of binoculars may come in handy.
Find an observing location with a low western horizon and wait until just after the sun sets. Then sweep across the horizon to the left of the sun to spot the thin sliver of the lunar crescent. Look for the ghostly Earth-lit portion of the moon to the left of the sunlit crescent.
Now sweep back toward the sunset point, keeping an eye out for brilliant Venus. It will be extremely low and close to the horizon. Just above it you should see a tiny pinprick of light, which is the distant giant planet Saturn.
Saturn will be gone from sight behind the sun for the next month, reappearing in the dawn twilight early in November.
Venus, on the other hand, is just beginning its evening appearances. It will dominate the western sky just after sunset all winter and well into spring, heading toward its transit across the face of the sun on June 5.
This is a good time to check up on the other planets.
Brilliant Jupiter is rising just before 9 p.m., heading toward opposition on Oct. 29. Mars rises around 2 a.m. and is gradually brightening on its way to opposition on March 3, 2012. Uranus passed opposition on Monday, making it well-placed for evening observation with binoculars or a small telescope. Neptune is five weeks past opposition and also well-placed in the evening sky.
The only planet not currently visible is tiny Mercury, too close to the sun to be seen until it reappears as an "evening star" next month.