Earth-centered Confusions—Part 1

I have written before (12/27/08 and 1/16/09) on the fact that, although we no longer view the heavens as revolving around a fixed Earth, it’s still how we directly experience the motion of celestial objects. Our precious little planet—once thought to be the center of the cosmos—was long ago displaced from its exalted position. In fact, it was exactly 400 years ago that Galileo, peering through his first telescope, provided the first solid evidence that the Earth moves; it revolves, rotates, spins through space.

Here are two irrefutable reasons why our current understanding of the non-Earth-centered cosmic arrangement is valid: (1) it’s far simpler than the old viewpoint and (2) we send space vehicles off to Saturn and it’s out there exactly where we expect it to be. These two results are at the core of the scientific principle: if it’s simpler, it’s probably closer to the truth (that’s how Nature works) and if it stands up under repeated trials (Mars and the moon are also where they are supposed to be), it gains credence.

The more I watch the sky—particularly in the midst of my reveries while reposing in the outdoor tub—the more intimately acquainted I become with its denizens. I see repeated and familiar sights; I pick up on the cycles and patterns of the march of heavenly bodies across my night (and day) sky. I get better at predicting how things will appear tomorrow or next week, as all the objects shift with respect to one another and to the horizons.

But my experience—despite my knowledge of astronomy and our real place in the cosmos—is from that Earth-centered perspective. And if I’m going to develop anything approaching the familiarity that the ancients did, I need to cultivate my Earth-centered understanding. There are a half-dozen or so types of objects floating across my sky. Each one has its peculiar path that it follows; each one behaves according to its own fashion. Most all of them pretty much follow an imaginary arc across the sky: the ecliptic, the path that the sun pursues each day. At our latitude it’s an arc that emerges from the eastern horizon, traverses the sky a little south of directly overhead, and dives below the western horizon.

The sun—the first object I’ll describe—traces the simplest path of all the celestial bodies. It rises in the east each morning, arcs across the sky on that ecliptic, and sets in the west. The peak of that arc (at noon) is high in the sky in the summer and lower (toward the south) in winter. Therefore, summer days are longer, because the sun has a longer path to tread. The sun’s diurnal journey sets the stage for all other heavenly bodies.

The second simplest set of objects to follow a daily path across the sky is the stars. The pattern of their motion is only a little more complicated than the sun. The stars trace a nighttime path that’s pretty much the same as the sun’s daytime route. Those stars that fall on the ecliptic—those that belong to the zodiacal constellations—do indeed follow the sun’s path. All stars rotate about the North Star, which appears never to change its position in the sky.

But the stars also exhibit an additional kind of motion—beyond the sun’s simplicity. For example, at noon the sun has attained its zenith and will return to that same spot each day. The star I see at its zenith at midnight tonight, however, will have moved a wee bit westward tomorrow night. That shift (about one degree) is imperceptible on a night-to-night basis, but over a month’s time that star (and all of its celestial companions) will have moved a good distance, like the creeping hands of a clock. In three month’s time that star that was at its zenith at midnight will now be seen down by the western horizon, about to set. Six months from now it will be directly underneath me—washed out by the noontime sun. Rest assured, a year from now I’ll find that it once again is perched directly overhead at midnight. By then, of course, my planet Earth will have completed one year’s revolution around the sun.

Next time: the third, the most romantic, and very complex night sky object.