Finding the Elusive Planet Mercury

By Dennis Mammana

November 16, 2017 4 min read

Week of November 19-25, 2017

It was back in the year 1530 that the science of astronomy was turned on its ear. Polish astronomer Nicolaus Copernicus published a book titled "De Revolutionibus Orbium Coelestium" ("On the Revolutions of the Celestial Spheres") in which he presented his theory that it was the sun, not the Earth, that was motionless — a notion that severely contradicted the revered teachings of Aristotle, the 2nd-century astronomer Ptolemy and the Catholic Church.

Later astronomers proved that this was indeed true by precisely measuring the movements of the five visible planets: Mercury, Venus, Mars, Jupiter and Saturn.

That Copernicus used observations, geometry and mathematics to calculate all this nearly five centuries ago is quite remarkable. Interestingly, however, legend has it that he had seen with his own eyes only four of these five planets. Mercury, evidently, had always eluded his sight.

Now, I can't say whether this is true, but it certainly makes sense given what we know about weather conditions in Eastern Europe and how tricky this world is to spot. Regardless, we certainly can see it. In fact, the next two weeks will be a great time to seek it out. Head outdoors not long after sunset, and begin scanning the southwestern sky with binoculars. You may spot Mercury as a bright, flickering "star" only about 10 degrees above the horizon. About half an hour after sunset, you should be able to see it with the unaided eye not far beneath and slightly to the right of the fainter planet Saturn.

Making the task a bit easier will be the thin crescent moon; it enters the scene on Nov. 19 and will lie to the right of Mercury. By the following night, it will have drifted along its orbit and will appear to the upper right of Saturn. By Nov. 21, the moon will lie above and to the left of the pair.

You can try aiming a small, low-powered telescope in the direction of Mercury, but you may be disappointed by what you see. Not only will this planet appear quite tiny; the thick, turbulent atmosphere through which we view it near the horizon will cause its light to shimmer considerably.

If you're fortunate enough to see a relatively steady image, you might view it with a higher-magnification eyepiece. You may then notice that the planet appears not as a circular disk but rather in a gibbous phase. If you keep an eye on it over the next two weeks, you'll see the planet appear to increase slightly in size as it rounds the sun in our direction, while its phase gradually evolves into a crescent.

Today we know that Mercury is a world quite different than all others in our solar system. Because it's much too small and close to the sun to have an atmosphere, temperatures on its daytime side soar to hundreds of degrees, and temperatures on its nighttime side plunge to hundreds of degrees below zero. And spacecraft photos of this dry, cratered world look remarkably like those of the moon.

Wouldn't Copernicus be thrilled to know all this?

 Find the elusive planet Mercury after dark this week.
Find the elusive planet Mercury after dark this week.

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