Nightly Rounds

By Bob Berman|Tuesday, July 01, 1997
You wouldn’t know it from the bronzed faces on the beach, but the sun is now at its feeblest of the year. In July we’re nearly 3 million miles farther from that closest star than we were in January. We bask in 7 percent less energy. The Northern Hemisphere benefits from moderated summer temperatures--we can use sunblock with spf 15 instead of spf 900--but Southern Hemisphere winters are colder than they otherwise would be. It’s all because Earth’s orbit, like the path of so many other objects in our solar system, is elliptical rather than round.

An orbit’s ellipticity (technically, its eccentricity) is described by a number between 0 and 1. An object with an eccentricity of 0 would have a perfectly round orbit; an orbit of 1 would be an ellipse so stretched it’s essentially a straight line. A simple system, and a good one.

The most eccentric planetary paths belong to Pluto and Mercury. At the farthest point of Pluto’s orbit (eccentricity .25), the planet lies some 50 astronomical units (Earth-sun distances) from the sun. At its closest point, however, the planet is closer to the sun (30 au) than Neptune, so that its ices evaporate into a thin but temporary atmosphere. Mercury’s eccentricity of .21 combines with its leisurely rotation to produce its own strange antics. At the point of its orbit nearest the sun, Mercury’s velocity is whipped up by strong gravitational tugs. The planet briefly revolves faster than it rotates, so the sun halts and plops below the horizon, only to play peekaboo once again. Newspapers on Mercury would have to announce the time of two sunrises.

The very nature of other night performers is governed by their oval paths. Comets wouldn’t have tails--or even exist--if their paths weren’t eccentric. With a round orbit far from the sun, a comet’s ices would never sublimate, and neither coma nor tail would ever form. If that round orbit carried the comet near the sun, the ices would have sublimated billions of years ago. We never would have seen it.

Our life is critically touched by noncircular meandering. Because asteroids and meteoroids tumble along less than round orbits, they can periodically cross our path and clobber us. But we shouldn’t complain: the last big impact paved the way for mammals, and us. If round orbits were the rule, we probably wouldn’t be here.

Eccentrics aren’t the only inhabitants of the night. The roundest planetary orbit (.007) belongs to Venus, just now returning to evening visibility as it emerges low in the southwestern twilight. Its velocity barely wavers--not like that of Earth (.17), which speeds up and slows down like a student driver, skewing the sun’s position in our sky and contributing to the constantly changing times of sunrise and sunset.

Leading the knights of the round orbits club are Jupiter’s brightest moons: Io, Europa, and Ganymede, seen these nights in the east at midnight through any small telescope. All have eccentricities at or near .000. Such steady orbital speed allows them to serve as excellent clocks. Two hundred and fifty years ago ships’ navigators used handheld telescopes to spot easily observed configurations of the moons. Since those alignments occur at the same time, no matter where in the world you observe them from, they told the navigator the absolute time. From that he’d determine the ship’s longitude (the heights of the stars above the horizon give latitude).

But for most planets and satellites, such circular precision was surrendered billions of years ago through collision and gravitational perturbations. Too bad. No merry-go-round. No perpetually smooth ride. But it makes for interesting times.
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