Sky Watch: 'Double planet' illusion at dusk

Look up at a starry sky and it's hard to imagine that anything there -- even the Earth beneath our feet -- is in motion. There are times, however, when we can see celestial movements in only a day or so. Now is just such a time.

To do so, head outdoors right after sunset this week and cast your gaze very low in the western sky. Here you'll find two bodies -- the planets Mars and Mercury -- as they engage in a celestial ballet of sorts. Each has its own motion around the sun, of course, but when combined with that of the Earth, their paths through the heavens appear rather complex.

It was the 17th-century astronomer Johannes Kepler who first described these movements mathematically. Through his famous laws of planetary motion, he determined that it is those planets nearest to the sun that move most rapidly while those farthest away drift more slowly.

Of the two worlds we see at dusk this week, Mars is the fainter, appearing more than seven times dimmer than Mercury. The Red Planet is the fifth world out from the sun, orbits our star at an average distance of 142 million miles and completes one circuit in less than two Earth-years. It now appears so faint because it lies on the sun's far side, some 215,840,000 miles from us.

Mercury, on the other hand, is the nearest planet to the sun, lying only 36 million miles from its scorching face; it whips around its orbit in only 88 Earth days. This week it appears relatively close to us, at a distance of about 105,350,000 miles.

We on the Earth, of course, lie about 93 million miles from the sun and orbit it once a year. And it's the combination of these motions that creates some interesting choreography in the heavens this week.

If the sky is clear low toward the western horizon, you should spot Mars and Mercury as a "double planet" only about 12 degrees up at sunset. By Feb. 6, it will become obvious that the two are beginning to converge; on Feb. 8 they will appear less than half a degree apart in the sky. This, of course, is an optical illusion caused by the planets appearing roughly along the same line of sight from the Earth.

Then, only a day or so later, you'll see how the pair has separated once again, with Mercury appearing higher in the sky than Mars. And, if that's not enough, by Feb. 10, the thin crescent moon will enter the scene below them and create an even prettier sight in the waning light of day.

While these two bodies will appear so close in the sky on Feb. 8 that you can see them in the same field of view of a small backyard telescope, you should not expect to see much. First of all, both planets are relatively small and distant. And don't be surprised if these two planets appear to twinkle; the thick column of turbulent air through which their light must pass will do that even to planets, despite what we might have learned in elementary school!