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Thursday, March 21 , 2019, 4:23 pm | Mostly Cloudy 57º


Dennis Mammana: Seeing the Inner Planets at Dawn

View the inner planets at dawn this week. Click to view larger
View the inner planets at dawn this week. (Creators.com illustration)

It’s always fun to ask stargazers why they believe that if Mercury is the planet closest to the sun, it’s not the hottest of our solar system. That honor belongs to Venus, which is nearly twice as far from the searing heat of our star.

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 holds in heat.

Mercury, on the other hand, has virtually no atmosphere, so, while it’s still quite hot, much of the sun’s heat captured by its surface during the daytime can radiate back into space during the Mercurian night.

Another difference between the two planets is how bright they appear in our sky. The Venusian atmosphere serves as a highly efficient reflector that shoots back into space nearly two-thirds of all sunlight that falls on it.

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 brightness and color.

This week offers stargazers a great chance to check out these our solar system’s two inner planets as they appear near each other. The only caveat is you’ll have to rise before the sun to see them.

You’ll have little trouble spotting Venus; it shines in the eastern sky of dawn with a brilliant white light and is easily the brightest object in the morning sky.

Below and much closer to the eastern horizon you may see Mercury appearing as a fainter yellowish star. If you spot it, you should consider yourself lucky. 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 have a small telescope, aim it toward each of these planets and you’ll see little more than you can see with the unaided eye. That’s because Venus is shrouded by clouds and Mercury’s light passes through so much turbulent air near our horizon that details are nearly impossible to discern.

Before you pack up your telescope, however, look more closely at Venus and you’ll notice that it isn’t perfectly round; it appears this week as a thick crescent, much like our moon does each month. This will change, of course, as Venus orbits the sun later this spring and summer.

Next, try aiming your telescope toward Mercury, but don’t be surprised at how tiny and blurry it is. First off, this planet is barely the size of the continental United States. Second, its appearance near the horizon means that its light must pass through a tremendous amount of distorting atmosphere before reaching our eyes.

If you’re fortunate enough to see a relatively steady image, you might notice that Mercury now displays a nearly “quarter” phase, but you’ll need a steady atmosphere and an eyepiece with a higher power for this.

Keep watch on Mercury for the next few mornings, however, because its relatively rapid orbit around the sun will cause its appearance to change to a nearly full disk before it disappears once again below the horizon.

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 [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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