Mercury is the closest planet to the sun, yet it’s not the hottest of our solar system. That honor belongs to Venus, nearly twice as far from the searing heat of our star.
Why should that be? The reason, of course, is rather simple. Venus is shrouded by a thick, cloudy atmosphere of mostly carbon dioxide gas that acts like a blanket and keeps its temperature at nearly 900 degrees.
Mercury, on the other hand, has virtually no atmosphere, so while 800 degrees on its daylit side is still quite warm, 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!
The two planets also appear quite different in our sky. The Venusian atmosphere serves as an efficient reflector that shoots nearly two-thirds of all sunlight falling on it back into space. The darker rocky surface of Mercury, by contrast, reflects only 11 percent of the sunlight it receives. This means that the two planets appear strikingly different in both brightness and color.
This week offers stargazers a great chance to check out these two worlds, but it must be shortly after sunset. If you have a very low horizon to your west-northwest, you should have little trouble spotting Venus because it shines with such a brilliant white light. Not far below Venus, you may spot Mercury as a much fainter, yellowish “star.”
Don’t be surprised if you see these two planets twinkling; the thick column of air through which their light must pass will do that even to a planet, despite what we all learned in grade school.
Keep watch on Venus and Mercury over the next few nights, and you’ll notice their relative positions changing as they orbit the sun. Be sure to check out the pair on May 21, when Mercury will appear barely a degree or so to the lower left of Venus.
If you have a small telescope, aim it toward Venus; even a low-powered eyepiece will reveal it as an extremely thin crescent because the planet is now being illuminated mostly on its far side.
Next, aim your telescope toward Mercury, but don’t expect to see much. Mercury is quite a small planet, some two-and-a-half times smaller than the Earth, and its light will be significantly distorted by air turbulence near the horizon.
If you’re fortunate enough to see a steady image and use a higher-powered eyepiece, you’ll notice that Mercury now appears in a nearly quarter phase. Check it out as it swings around to our side of the sun over the next few weeks, and you’ll see it gradually evolve into a crescent before vanishing into the sun’s glare by mid-June.
If you have trouble spotting these planets, the moon might be of some help. Right after sunset on Saturday, May 23, the very thin crescent moon will lie just to the lower left of this pair, but you may need binoculars to spot it. The following night, you may have a better chance to spot a thicker crescent moon above and to their left.
— Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at mammana@skyscapes.com and follow him on Twitter: @dennismammana. The opinions expressed are his own.