Don’t look now, but there’s a new planet creeping up from the western horizon. Mercury makes a special guest appearance at dusk for the next week or so. Tonight the moon can help you find this elusive planet. What makes it a hard catch? Well, it never strays far from the sun, appropriate behavior given that Mercury is, after all, the innermost of the eight planets. Typically it stands just two fists held at arm’s length above the horizon after sundown. If trees or buildings get in the way, you won’t even notice it.
Tonight it shines brighter than Rigil in Orion and nearly matches Canopus, the second brightest star in the entire sky. But you’d never know it. Trapped near the horizon, the little planet must compete against the orange glow of dusk and the greater thickness of air there.
As we know from personal experience, the sun’s light is more tolerable near sunrise and sunset. Likewise, the rising moon is considerably fainter than one seen overhead. Dust and air molecules absorb and scatter light, especially when we direct our gaze horizon-ward where the air, water vapor and dust are thickest. Astronomers call this phenomenon atmospheric extinction. It’s the reason they wait for stars to get high enough to clear the thickest air before gathering data.
By the time Mercury becomes visible a half hour to 45 minutes after sunset, air and dust have dimmed it by more than a magnitude, making it similar to Betelgeuse in brightness. If all this talk of extinction makes you pessimistic about seeing the planet, I apologize. Mercury’s not hard to spot if you’ve got haze-free skies and an unobstructed western horizon. Don’t be afraid to cheat a little with binoculars. I sometimes use them to find a twilight planet early so I know better where to look once the sky gets darker.
If you want a real twilight challenge, try finding Mars below Mercury. It’s not only fainter but being lower, even more of its light is sucked up by the air. Binoculars are essential for this one.
As a kid I always thought Mercury was a red-hued world and saw it that way whenever it would make one of its twilight appearances. Truth is, the planet is quite gray, much like Earth’s moon. Photographs from the MESSENGER spacecraft show a surface covered in ancient volcanic rocks. Only later did I realize that my pink planet gathered it color from the very twilight itself. What color does it look to you?