The Planets Read Online Free Page B
Author: Dava Sobel
“merchant” and “mercantile”), of cheats and thieves (since he stole herds from his half-brother Apollo on the very first day of his life), of eloquence (having given Pandora the gift of language), as well as of cunning, knowledge, luck, roads, travelers, young men in general, and herdsmen in particular. His snake-entwined wand, the caduceus, has invoked fertility or healing or wisdom over the ages.
Mercury and his fellow travelers called attention to themselves by moving among the fixed stars, which earned them the name “
planetai,”
meaning “wanderers” in Greek. The orderliness of their motions brought “cosmos” out of “chaos” in the same language, and inspired an entire lexicon for describing planetary positions. Just as the gods’ names still cling to the planets, Greek terms such as “apogee,” “perigee,” “eccentricity,” and “ephemeris” endure in astronomical discussions. The first observers to coin such words fill a roster of ancient heroes, from Thales of Miletus (624–546 B.C. ), the founding Greek scientist who predicted a solar eclipse and questioned the substance of the universe, to Plato (427–347 B.C. ), who envisioned the planets mounted on seven spheres of invisible crystal, nested one within the other, spinning inside the eighth sphere of the fixed stars, all centered on the solid Earth. * Aristotle (384–322 B.C. ) later raised the number of celestial spheres to fifty-four, the better to account for the planets’ observed deviations from circular paths, and by the time Ptolemy codified astronomy in the second century A.D. , the major spheres had been augmented further by ingenious smaller circles, called “epicycles” and “deferents,” required to offset the admitted complexities of planetary motion.
“I know that I am mortal by nature, and ephemeral,” says an epigraph opening Ptolemy’s great astronomical treatise, the
Almagest,
“but when I trace at my pleasure the windings to and fro ofthe heavenly bodies I no longer touch earth with my feet: I stand in the presence of Zeus himself and take my fill of ambrosia, food of the gods.”
In Ptolemy’s model, Mercury orbited the stationary Earth just beyond the sphere of the Moon. The impetus for motion came from a divine force exterior to the network of spheres. More than a millennium later, however, when Copernicus rearranged the planets in 1543, he argued that the mighty Sun, “as though seated on a royal throne,” actually “governs the family of planets.” Without specifying the force by which the Sun ruled, Copernicus ringed the planets round it in order of their speed, and set Mercury closest to the Sun’s hearth because it traveled the fastest.
Indeed Mercury’s proximity to the Sun dominates every condition of the planet’s existence—not just its tantivy progress through space, which is all that can be easily gleaned from Earth, but also its internal conflict, its heat, heaviness, and the catastrophic history that left it so small (only one-third Earth’s width).
The pull of the nearby Sun rushes Mercury around its orbit at an average velocity of thirty miles per second. At that rate, almost double the Earth’s pace, Mercury takes only eighty-eight Earth-daysto complete its orbital journey. The same Procrustean gravity that accelerates Mercury’s revolution, however, brakes the planet’s rotation about its own axis. Because the planet forges ahead so much faster than it spins, any given locale waits half a Mercurian year (about six Earth-weeks) after sunup for the full light of high noon. Dusk finally descends at year’s end. And once the long night commences, another Mercurian year must pass before the Sun rises again. Thus the years hurry by, while the days drag on forever.
Mercury most likely spun more rapidly on its axis when the Solar System was young. Then each of its days might have numbered as few as eight hours, and even a quick Mercurian year could have contained hundreds
Mercury and his fellow travelers called attention to themselves by moving among the fixed stars, which earned them the name “
planetai,”
meaning “wanderers” in Greek. The orderliness of their motions brought “cosmos” out of “chaos” in the same language, and inspired an entire lexicon for describing planetary positions. Just as the gods’ names still cling to the planets, Greek terms such as “apogee,” “perigee,” “eccentricity,” and “ephemeris” endure in astronomical discussions. The first observers to coin such words fill a roster of ancient heroes, from Thales of Miletus (624–546 B.C. ), the founding Greek scientist who predicted a solar eclipse and questioned the substance of the universe, to Plato (427–347 B.C. ), who envisioned the planets mounted on seven spheres of invisible crystal, nested one within the other, spinning inside the eighth sphere of the fixed stars, all centered on the solid Earth. * Aristotle (384–322 B.C. ) later raised the number of celestial spheres to fifty-four, the better to account for the planets’ observed deviations from circular paths, and by the time Ptolemy codified astronomy in the second century A.D. , the major spheres had been augmented further by ingenious smaller circles, called “epicycles” and “deferents,” required to offset the admitted complexities of planetary motion.
“I know that I am mortal by nature, and ephemeral,” says an epigraph opening Ptolemy’s great astronomical treatise, the
Almagest,
“but when I trace at my pleasure the windings to and fro ofthe heavenly bodies I no longer touch earth with my feet: I stand in the presence of Zeus himself and take my fill of ambrosia, food of the gods.”
In Ptolemy’s model, Mercury orbited the stationary Earth just beyond the sphere of the Moon. The impetus for motion came from a divine force exterior to the network of spheres. More than a millennium later, however, when Copernicus rearranged the planets in 1543, he argued that the mighty Sun, “as though seated on a royal throne,” actually “governs the family of planets.” Without specifying the force by which the Sun ruled, Copernicus ringed the planets round it in order of their speed, and set Mercury closest to the Sun’s hearth because it traveled the fastest.
Indeed Mercury’s proximity to the Sun dominates every condition of the planet’s existence—not just its tantivy progress through space, which is all that can be easily gleaned from Earth, but also its internal conflict, its heat, heaviness, and the catastrophic history that left it so small (only one-third Earth’s width).
The pull of the nearby Sun rushes Mercury around its orbit at an average velocity of thirty miles per second. At that rate, almost double the Earth’s pace, Mercury takes only eighty-eight Earth-daysto complete its orbital journey. The same Procrustean gravity that accelerates Mercury’s revolution, however, brakes the planet’s rotation about its own axis. Because the planet forges ahead so much faster than it spins, any given locale waits half a Mercurian year (about six Earth-weeks) after sunup for the full light of high noon. Dusk finally descends at year’s end. And once the long night commences, another Mercurian year must pass before the Sun rises again. Thus the years hurry by, while the days drag on forever.
Mercury most likely spun more rapidly on its axis when the Solar System was young. Then each of its days might have numbered as few as eight hours, and even a quick Mercurian year could have contained hundreds
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