Sky Watch: Mercury's fleeting appearance

There's quite a planetary promenade taking place in the western sky at dusk this week. Not only do the bright planets Jupiter and Venus seem to be drifting rapidly toward each other, but another solar system body has just come into the picture: the planet Mercury.

Unlike Venus or Jupiter, which dominate our sky with their brilliance, or Mars -- now glistening on the opposite side of the sky -- that can shine brightly all night long, Mercury appears for only a few weeks each year, and stargazers must expend a bit of effort to find it.

Mercury is the innermost planet of our solar system, whipping around its orbit once every 88 Earth-days at a distance of "only" 36 million miles from the scorching face of the sun. From our terrestrial vantage point, Mercury appears so close to the sun that it's often lost in its glare.

In fact, this planet's elusiveness is legendary; it's been said that the great 16th-century Polish astronomer Nicolaus Copernicus, who overturned the long-held notion that the Earth occupied the center of our planetary family, had never actually seen this world with his own eyes. Now, I can't say whether or not this is true, but it certainly makes sense given what we know about weather conditions in Eastern Europe and how buried in the glow of twilight Mercury always remains.

Only when this world reaches a significant "elongation" from the sun can we hope to see it, and this occurs only at dusk or dawn. In fact, some of the ancients even thought of Mercury as two different bodies. The Greeks, for example, called it Hermes, the messenger of the Gods, when it appeared in the waning light of dusk, and Apollo when it shone in the morning before sunrise.

Now is a great time to try to find Mercury on your own. Shortly after sunset this week, scan low in the western sky. Mercury will appear as a bright, starlike object not far below the brighter planets Venus and Jupiter, and it'll shine with a distinctly yellowish cast. But don't be surprised if it appears to twinkle; the thick column of turbulent air through which its light must pass will do that even to a planet, despite what our third-grade teachers might have told us.

When you find Mercury, try aiming a small telescope in its direction for a completely different view, but don't expect to see much. First of all, this planet is relatively small -- some two-and-a-half times smaller than our Earth -- and secondly, its appearance close to the horizon means it often looks quite blurry through the eyepiece.

If you're fortunate enough to see a relatively clear image, however, you might check it out with a higher-powered eyepiece. But don't be surprised when you notice that Mercury appears as a thick crescent, not unlike the moon does from time to time, since sunlight now shines on its rocky surface from the opposite side.