Jimmy Westlake: Don’t miss the double evening star
June 3, 2013
The solar system's two innermost planets, Mercury and Venus, behave differently than the rest of the planets. Instead of being free to wander all the way around the night sky, they are tethered to the sun so that they seem to swing back and forth, from one side of the sun to the other. Consequently, each planet spends a brief time as our "evening star," visible right after sunset, followed by a brief engagement as our "morning star," visible just before sunrise.
The best time to spy each planet is when it is near its greatest angular distance from the sun, an event called greatest elongation. Even at greatest elongation, Mercury never can be seen more than 28 degrees from the sun. That's not much more than the width of your outstretched hand held at arm's length. Venus, because of it's larger orbit, fares a little better — it can reach as much as 48 degrees from the sun.
Dazzling Venus is just coming out from behind the sun, beginning a nice long appearance as our evening star. Each night from now until its greatest elongation on Nov. 1, Venus will climb a bit higher in the sunset glow and become easier to spot.
Mercury has several elongations each year because of its rapid orbital motion. Its best evening elongation of this year happens June 12, when the hot little planet will appear 24 degrees east of the sun and remain visible for 45 minutes after the sun goes down.
What makes this elongation of Mercury even more interesting than most is its position very close to the brighter planet Venus in the sky, creating a marvelous double evening star for two weeks. They will appear closest together on June 19, with Mercury only 2 degrees from Venus, but will remain within 5 degrees of each other for many nights in a row.
Catching a glimpse of this unusual double evening star on June 12 is reason enough to step outside at dusk and gaze westward, but try to catch it two days earlier, just after sunset on June 10. That's the evening that the thin crescent moon joins Mercury and nearby Venus for a striking triple conjunction. Look low in the west-northwest sky at around 9:30 p.m. local time.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out Jimmy's astrophotography website at http://www.jwestlake.com.