Mark the three evenings of September 7th, 8th and 9th on your calendar to watch a fascinating interplay of the crescent Moon and two bright planets in the southwestern sky after sunset. And later in the month, another planet joins the parade. All of this will provide yet another example of the fact that the cosmic drama unfolding nightly overhead is alive with exciting action.
If it’s clear on Saturday evening September 7th, go outdoors about 45 minutes after sunset and look to the west-southwest. There, just above the horizon, you will see a beautiful sliver of the crescent Moon. To its upper left (east and north) will be the blue-white star Spica—and continuing in that direction an unmistakable, brightly shining Venus. Some distance above Venus and higher to the upper left is the yellowish glow of the planet Saturn.
The following evening, the crescent Moon will have moved just to the left (east) of Venus, creating a beautiful sight to the unaided eye and a striking pair in binoculars. September 9th finds the Moon some distance to the left of Saturn and about as high in the sky. It may not be obvious in three night’s time, but Venus and Saturn are drawing closer together and by mid-month their relative motion will be unmistakable. A week later, September 20th, finds Venus well to the lower left of Saturn. Their changing positions result from the former planet rising higher in the sky and the latter dropping lower from the interplay of their orbital motions.
But there’s more! If you look low above the western horizon about 30 minutes after sunset on the 20th, another planet joins the parade of these restless worlds—Mercury! (Binoculars will help spot it in the twilight afterglow.) Telescopically, Saturn still shows its awesome rings even though the planet is growing smaller in apparent size as it moves away from us, while Venus looks like a half-full radiant globe. Mercury itself is much too low to even make out its tiny disk due to heavy atmospheric absorption and turbulence near the horizon.
I sometimes like to imagine myself back in time to that of skywatchers centuries ago when they were still trying to figure out the mystifying motions of these wandering "stars." All of the planets are moving counterclockwise with respect to the Sun. As you read this, Venus is moving eastward in the sky and will be at its greatest apparent distance (or elongation) from our star in October. It will then begin moving west and closing in on it as it passes between it and us in its orbital motion. Saturn likewise is actually moving eastward in its orbit—but right now appears to be moving west towards the Sun! That’s because our faster-moving Planet Earth has overtaken it, causing it to lag behind us as seen in the sky. Little wonder it took them so long to figure it all out!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available soon from scientificsonline.com.