May Pole

By Bob Berman|Thursday, May 01, 1997
This month the celestial star of the season, comet Hale-Bopp, will finally begin to lose some of its allure, except to the most dedicated comet watchers. But if you haven’t had your fill, early May finds the comet still bright, and more than 10 degrees (the span of sky covered by a clenched fist at arm’s length) above the western horizon as evening twilight fades to black. On May 8 the now-familiar luminary becomes fabulous again, sitting just above the thin crescent moon.

While you’re appreciating the month’s most dramatic sky-scene, take a second to look at that accompanying crescent moon: Does it appear to be a smile, or do you see the crescent’s inner curve delineating the shadow line of a sphere? Your perspective makes all the difference. With one viewpoint the moon is ordinary, a flat pasteup on the night’s canopy. With the other, the moon is an exciting three-dimensional presence--a globe magically suspended before you.

The same is true of the solar system. The mental image we have from textbooks and grade school Styrofoam and coat-hanger models of the solar system is of planets circling around the sun in a disklike plane. Yet when we try to relate that familiar concept to the actual night sky, we fail to realize that to us, as the inhabitants of one of those worlds embedded in the plane, the solar system’s plane--the ecliptic--must appear as a line completely encircling us. When the conditions are right, you can actually see it--illuminated.

Try it on May 30. Face eastward at 4:30 in the morning and you’ll notice brilliant Jupiter (the night’s brightest star) with the moon to its lower left and, extending that line, much dimmer Saturn just poking above the eastern horizon. Binoculars elongate that chain by revealing Uranus to Jupiter’s upper right. Arrayed like a well choreographed chorus line, the planets reveal a section of the ecliptic. Ignore the horizon, that old reference point, and drink in the solar system surrounding you.

At right angles to that plane stands the ecliptic pole, the fastest way out of the solar system. If we picture the plane of the planets as a pancake on a grill, the ecliptic pole is in the direction straight up from the stove. To find it, wait till 10 p.m., then look up at the northeastern sky. The pole sits within the constellation Draco the Dragon, whose quadrangular head lies just above the brilliant star Vega in the east.

The ecliptic pole occupies the first crook of the dragon’s body. Gaze that way and you’re looking directly away from the foreground dust, meteoroids, asteroids, planets, and satellites of our solar system. Your eyes now follow a laser-straight line into interstellar space. By chance, a tiny, round, intensely green nebula with the lively name of ngc 6543 can be found here, awaiting the attention of telescope owners.

But the fun is just beginning. The ecliptic pole also marks the spot around which our planet’s axis traces its precessional wobble. Follow the Big Dipper’s two leftmost stars downward to Polaris, the North Star. In little more than a century, Polaris will slowly start to drift, to be replaced by other north stars. The ecliptic pole lets you see what’s to come. Draw a mental line from that spot in Draco to nearby Polaris, and then draw an imaginary circle, using the ecliptic pole as its center and letting Polaris define its radius. This loop marks the path that Earth’s axis traces every 25,780 years. Along its circumference lies every north star that ever was or ever will be.

Suddenly, the night makes another living leap to the dynamic presence that forever enchants sky gazers.
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