Coming of Age in the Milky Way Read Free

was especially pronounced, for … they could now find certain plants in the mountains. They never went to the mountains until they saw a certain star, for they knew they would not find food there previously. 3
     
    Stonehenge is one of thousands of old time-reckoning machines the moving parts of which were all in the sky. The Great Pyramid at Giza was aligned to the pole star, and it was possible to read the seasons from the position of the pyramid’s shadow. The Mayans of ancient Yucatan inscribed stone monuments with formulae useful in predicting solar eclipses and the heliacal rising of Venus (i.e., its appearance westward of the sun, as a “morning star”). The stone medicine wheels of the Plains Indians of North America ticked off the rising points of brighter stars, informing their nomadic architects when the date had come to migrate to seasonal grazing lands. The twenty-eight poles of Cheyenne and Sioux medicine lodges are said to have been used to mark the days of the lunar month: “In setting up the sun dance lodge,” said Black Elk, a priest of the Oglala Sioux, “we are really making the universe in a likeness.” 4
    Political power presumably played a role in early efforts to identify periodic motions in the sky, inasmuch as what a man can predict he can pretend to control. Command of the calendar gave priests an edge in the hardball politics of the Mayans, and Christopher Columbus managed to cow the Indians of Hispaniola into providing food for his hungry crew by warning that the moon otherwise would “rise angry and inflamed to indicate the evil that God would inflict on them.” Writes Columbus’s son Ferdinand, in his journal entry for the night of February 29, 1504:
    At the rising of the moon the eclipse began, and the higher the moon rose the more the eclipse increased. The Indians observed it, and were so frightened that with cries and lamentations they ran from every side to the ships, carrying provisions, and begged the Admiral by all means to intercede for them with God that he might not make them feel the effects of his wrath, and promised for the future, diligently to bring all he had need of…. From that time forward they always took care to provide us with all that was necessary, ever praising the God of the Christians. 5
     
    But the better acquainted the prehistoric astronomers became with the periodic motions they found in the night sky, the more complicated those motions proved to be. It was one thing to learn the simple periodicities—that the moon completes a circuit of the zodiacal constellations every 28 days, the sun in 365¼ days, the visible planets (from the Greek
planetes
, for “wanderers”) at intervalsranging from 88 days for fleet-footed Mercury to
29½
years for plodding Saturn. It was another and more baffling matter to learn that the planets occasionally stop in their tracks and move backward—in “retrograde”—and that their paths are tilted relative to one another, like a set of ill-stacked dishes, and that the north celestial pole of the earth precesses, wobbling in a slow circle in the sky that takes fully 26,000 years to complete. *
    The problem in deciphering these complexities, unrecognized at the time, was that the earth from which we view the planets is itself a planet in motion. It is because the earth orbits the sun while rotating on its tilted axis that there is a night-by-night shift in the time when any given star rises and sets at a given latitude. The earth’s precessional wobble slowly alters the position of the north celestial pole. Retrograde motion results from the combined wanderings of the earth and the other planets; we overtake the outer planets like a runner on an inside track, and this makes each appear first to advance, then to balk and retreat across the sky as the earth passes them. Furthermore, since their orbits are tilted relative to one another, the planets meander north and south as well as east and west.
    These complications,