The Planets Read Online Free PDF Page A

such. But tides raised by the Sun in the planet’s molten middle gradually damped Mercury’s rotation down to its present slow gait.
    Day breaks over Mercury in a white heat. The planet has no mitigating atmosphere to bend early morning’s light into the rosy-fingered dawn of Homer’s song. The nearby Sun lurches into the black sky and looms enormous there, nearly triple the diameter of the familiar orb we see from Earth. Absent any aegis of air to spread out and holdin solar heat, some regions of Mercury get hot enough to melt metals in daylight, then chill to hundreds of degrees below freezing at night. Although the planet Venus actually grows hotter overall because of its thick blanket of atmospheric gases, and Pluto stays altogether colder on account of its distance from the Sun, no greater extremes of temperature coexist anywhere in the Solar System.
    The drastic contrasts between day and night make up for the lack of seasonal changes on Mercury. The planet experiences no real seasons, since it stands erect instead of leaning on a tilted axis the way Earth does. Light and heat always hit Mercury’s equator dead on, while the north and south poles, which receive no direct sunlight, remain relatively frigid at all times. In fact, the polar regions probably harbor reservoirs of ice inside craters, where water delivered by comets has been preserved in perpetual shadow.
    Mercury usually eludes observation from Earth by hiding in the Sun’s glare. The planet becomes visible to the unaided eye only when its orbit carries it far to the east or west of the Sun in Earth’s skies. During such “elongations,” Mercury may hover on the horizon every morning or eveningfor days or weeks. It remains difficult to see, however, because the sky is relatively bright at those times, and the planet so small and so far away. Even as Mercury draws closest to Earth, fifty million miles still separate it from us, which is quite remote compared to the Moon’s average distance of only one-quarter million miles. Moreover, the illuminated portion of Mercury thins to a mere crescent as the planet approaches Earth. Only the most diligent observers can spot it, and only with good fortune. Copernicus, caught between the miserable weather in northern Poland and the reclusive nature of Mercury, fared worse than his earliest predecessors. As he grumbled in
De Revolutionibus,
“The ancients had the advantage of a clearer sky; the Nile—so they say—does not exhale such misty vapors as those we get from the Vistula.”
    Copernicus further complained of Mercury, “The planet has tortured us with its many riddles and with the painstaking labor involved as we explored its wanderings.” When he aligned the planets in the Sun-centered universe of his imagination, he used observations made by other astronomers, both ancient and contemporary. None of those individuals, however, had sighted Mercuryoften enough or precisely enough to help Copernicus establish its orbit as he had hoped.
    The Danish perfectionist Tycho Brahe, born in 1546, just three years after Copernicus’s death, amassed a great number of Mercury observations—at least eighty-five—from his astronomical castle on the island of Hven, where he used instruments of his own design to measure the positions of each planet at accurately noted times. Inheriting this trove of information, Brahe’s German associate Johannes Kepler determined the correct orbits of all the wanderers in 1609—“even Mercury itself.”
    It later occurred to Kepler that although Mercury remained hard to see at the horizon, he might catch it high overhead on one of those special occasions, called a “transit,” when the planet must cross directly in front of the Sun. Then, by projecting the Sun’s image through a telescope onto a sheet of paper, where he could view it safely, he would track Mercury’s dark form as it traveled from one edge of the Sun’s disk to the other over a period of several hours. In