The planets Mercury and Venus appear in our western sky this week at dusk.
If you think back to elementary school, you’ll recall learning that Mercury is the closest world to the sun (about 36 million miles), followed by Venus (about 67 million miles).
You may also recall that Mercury is quite hot, but Venus, nearly twice as far from the sun, is even hotter.
Why this should be is an interesting question that can be answered by considering the structures of the two planets.
Mercury has virtually no atmosphere, so while it reaches 800 degrees on its daylit side much of the sun’s heat captured by its surface during daytime can radiate back into space during the Mercurian night and drop its temperature to nearly -300 degrees.
Venus, on the other hand, is shrouded by a thick cloudy atmosphere of carbon dioxide that acts like a blanket and holds its temperature at nearly 900 degrees.
Check out these two planets at dusk this week, and you’ll discover another difference: The two appear quite different.
Venus is easily the brightest planet in the heavens and glows almost pure white. Mercury, on the other hand, appears yellowish and some 30 times fainter.
Venus appears white because of its cloudy atmosphere. It serves as an efficient reflector that sends back into space nearly two-thirds of all sunlight falling on it.
By contrast, the darker rocky surface of Mercury reflects only 11% of the sunlight it receives, so the two planets appear strikingly different in both brightness and color.
You should have no trouble seeing dazzling Venus not long after sunset, but Mercury is another matter altogether.
This week it will appear as a fainter yellowish “star” to the lower right of Venus but quite close to the western horizon at dusk and tough to see.
It’s been said that the great 16th-century Polish astronomer Nicolaus Copernicus — who calculated the movements of the planets — had never actually seen this world with his own eyes.
If you spot Mercury, don’t be surprised if it appears to twinkle. Yes, I know we all learned in school that stars twinkle and planets don’t. But that’s as wrong now as it was then.
Twinkling is produced by the thick column of turbulent air through which the body’s light must travel. When it passes close to land, rising heat waves distort its appearance much more than when its light travels higher in the sky.
If you have difficulty spotting Mercury this week, the moon might help. Not long after sunset on Sunday, Nov. 3, the delicate crescent moon will lie midway between Venus and Mercury, not far from the (fainter) reddish-orange star Antares.
You’ll then know about where Mercury must lie, but you may still need binoculars to spot it.
The following night, the thicker crescent moon will appear close to brilliant Venus and will create a wonderful photo op for your camera or smartphone.
While Venus will shine in the western sky until March, Mercury will be gone from view within only a week or so, so now’s the time to check it out!
