The Planets

by Dava Sobel

  Amerigo Vespucci’s analysis of competing Portuguese and Spanish claims helped him estimate Earth’s circumference at 27,000 Roman miles—just fifty modern miles shy of today’s accepted value.

  Earth’s water supply constitutes only one-tenth of 1 percent of the planet’s mass, while outer Solar System moons such as Ganymede, Callisto, and Titan consist of 50 percent water, most of it frozen.

  After the next transit of Venus, predicted for June 6, 2012, there won’t be another pair until December 11, 2117, and December 8, 2125. Transits occur in June or December because Earth crosses the plane of Venus’s orbit in those months.

  Lunacy (The Moon)

  A “blue Moon,” widely reported to be the second full Moon in a calendar month, is more correctly (according to the 1937 Maine Farmers’ Almanac, which defined the term) the third full Moon in a season that contains four of them. The Almanac reckons seasons by the tropical year, which begins on the day of the winter solstice, or “Yule” (December 22). A true blue Moon, therefore, can occur only in the months of February, May, August, and November.

  Under a full Moon, a black-and-white landscape may reveal the greenness of grass, because the human retina is particularly sensitive to yellow-green wavelengths (the light the Sun emits most strongly).

  Giovanni Riccioli (1598–1671), a Jesuit priest, established the system of lunar nomenclature still in use today. He and other selenographers (Moon mappers) named the mountains for Earthly ranges such as the Alps, Apennines, Caucasus, and Carpathians. Craters on the Moon’s near side honor great natural philosophers, from Plato and Aristotle to Tycho, Copernicus, Kepler, and Galileo. Russian names apply to the far side, which was first imaged in October 1959 by the unmanned Soviet spacecraft Luna 3.

  The Moon’s rotation rate equals its revolution—27.3 days—but by the time the Moon travels around the Earth to reach the point it started from, vis-à-vis the stars, the Earth has also moved. Thus the Moon is seen to require 29.5 days to complete an Earthly revolution and go through all its phases from one Full Moon to the next.

  Sci-Fi (Mars)

  Meteoriticist Roberta Score of the U.S. Antarctic Program, in Denver, found the Mars rock known as ALH84001 on December 27, 1984. Scientists have successfully hunted meteorites in Antarctica since 1969. Analysis of ALH84001 began in mid-summer 1988, and tests confirming its Martian origin were completed by autumn 1993.

  The hills near the Mawson and Mackay Glaciers, where the Mars rock was found, were mapped in 1957–58 and named for Prof. R. S. Allan of the University of Canterbury, New Zealand.

  The so-called “Face on Mars,” a topographical feature widely perceived to resemble a human face, appeared in Viking orbiter photos from 1976. Several media promulgated the suggestion that the face was an alien artifact, until subsequent imaging by the Mars Global Surveyor destroyed the illusion.

  Giovanni Schiaparelli found what he called canali on Mars in 1877, eight years after the completion of the Suez Canal. Schiaparelli, trained as a hydraulic engineer, thought the straight lines no more the product of artificial intelligence than the English Channel, but later changed his mind. When Schiaparelli’s sight failed, Percival Lowell took over observations—and interpretations—of the canals.

  Johannes Kepler first imagined two moons for Mars in 1610, but the moons were not seen until August 1877, when Asaph Hall, working at the U.S. Naval Observatory in Washington, D.C., found them orbiting so close to the planet as to be nearly lost in its glare. He named them after two characters from Greek mythology, Phobos and Deimos, who were variously described by Homer as the sons of the war god Ares, or his attendants—or the horses that pulled his chariot.

  Astrology (Jupiter)

  Two surviving natal horoscopes drawn for (and probably by) Galileo are reproduced in vol. XIX of his complete works. An adept astrologer, he would not have classified individuals by Sun sign, as that practice arose in the twentieth century. Defining elements in the astrology of Galileo’s time included the horoscopus (rising sign), the mid-heaven, the immum coeli (opposite of the mid-heaven), and the descendant sign on the chart’s western horizon.

  My interpretation of Galileo’s natal chart is based on a reading by astrologer Elaine Peterson, August 14, 2003, and supplemented by listings in The Complete Astrological Handbook (see Bibliography.

  Galileo’s quote about “fate” is taken from his Starry Messenger, in which he described his telescopic discoveries. Remarks directed to Cosimo come from the dedicatory introduction to that same book. Galileo’s reference to the moons as “stars” is appropriate terminology for his time, when “the star of Jupiter” was seen as a rare “wandering star” among the more numerous “fixed stars” of the wider heavens.

  After Galileo identified four Jovian moons in January 1610, no more were discovered until 1892, when Edward Barnard of the Lick Observatory in California found Amalthea. Another twelve surfaced in the twentieth century, four of them detected by Voyager 2. Names for these and another forty-three satellites detected recently by astronomers at the University of Hawaii continue the theme of Jupiter’s intimates.

  Henry Cavendish discovered hydrogen in 1766. Its metallic form, first predicted in the 1930s, was created at the Lawrence Livermore National Laboratory in California in 1996, by subjecting a thin film of liquid hydrogen to two million atmospheres of pressure.

  The Sumerians of Mesopotamia recorded stellar observations as long ago as the eighteenth century B.C. Several of their constellation names, including Leo and Taurus, are still used. The fully realized western zodiac dates from the middle of the fifth century B.C.

  Although the Jovian satellite Europa holds out hope of another abode of life within the Solar System, scientists feel certain the planet Jupiter is devoid of life. The Galileo probe found no complex organic molecules in its atmosphere.

  Music of the Spheres (Saturn)

  The Saturn of Greek mythology, called Cronus, devoured his children for fear they would kill him, as he had killed his own father, Uranus, to wrest control of the heavens. The infant Zeus (Jupiter), who escaped being devoured, later overthrew Cronus.

  Saturn’s so-called classical rings—A, B, and C—extend to a distance of 85,000 miles from the center of the planet, or 170,000 miles across from tip to tip. These are the rings seen through a small telescope, and pictured in familiar images of Saturn. The narrow and twisted F ring, immediately exterior to the A, lies 2,000 miles beyond the perimeter of the A ring and its core is only 30 miles wide. The outlying diaphanous E ring, which begins a little more than a hundred thousand miles from the planet’s center, is itself nearly two hundred thousand miles in width, so that its ring span of 600,000 miles more than doubles the distance from Earth to the Moon. It encompasses the orbit of the moon Enceladus, and consists of icy debris that the shiny satellite sheds in its wake.

  The D and E rings were detected by ground-based telescopes in 1966 and 1970, respectively. (E was actually discovered first, but astronomers questioned its reality for years, while D met a ready welcome.) Pioneer 11 found the contorted F ring in 1979 and Voyager 1 the G ring in 1980.

  The Roche limit applies to objects held together by gravity. The Cassini spacecraft can dip safely inside Saturn’s Roche zone because its parts are held together by nuts, bolts, and the crystal cohesion of its metal molecules.

  Resonant orbits, such as the 2:1 relationship between the Cassini Division and the moon Mimas, were first proposed in 1866 by Daniel Kirkwood, an American astronomer who used the resonance concept to explain gaps in the distribution of orbits in the Asteroid Belt.

  Rotation periods of the giant planets were originally gauged by timing the reappearance of distinctive storms. Now they are determined by the rotation rate of each planet’s magnetosphere, as measured by Voyager 2. Since a planet’s magnetic field arises deep in the interior, scientists assume the two spin together at the same rate.

  Discovery (Uranus and Neptune)

  The epigraph in italics is taken from one of Maria Mitc
hell’s lectures, published posthumously by her sister Phebe Mitchell Kendall.

  For this chapter, I assumed Maria Mitchell wrote of her 1847 find to the only other woman in the world who had discovered a comet, Caroline Herschel (1750–1848). In composing Miss Herschel’s reply, I “fictionalized” only the form, not the factual material. Miss Herschel was her brother’s assistant when he discovered Uranus. At the time of Neptune’s discovery, she was still active and intellectually engaged, despite her ninety-six years, and received word of the new planet from explorer Alexander (Baron von) Humboldt. Miss Herschel’s correspondence put her in touch with most leading figures in this phenomenal epoch in the history of astronomy, and she met many of them in person, including King George III, his royal family, and three of his Astronomers Royal, as well as Giuseppe Piazzi (discoverer of the first asteroid), Carl Friedrich Gauss, and Johann Encke.

  The autumn 1847 discovery of Comet Mitchell preceded Miss Herschel’s death by three months. Miss Mitchell worked then as librarian of Nantucket Island, and lived with her family in an apartment over the bank, of which her father was president. William Mitchell, a serious amateur astronomer, had built an observatory on the bank’s roof, where he and she spent much time. In recognition of her discovery, Miss Mitchell won a gold medal from the King of Denmark, a $100 prize from the Smithsonian Institution, and election to honorary membership in the American Academy of Arts & Sciences. Later she became the first professor of astronomy at Vassar College, and led student expeditions to view two total solar eclipses. On her 1857–58 trip to Europe, when she stayed at the home of Sir John and Margaret Herschel, they gave her a page from one of the notebooks “Aunt Caroline” had used to record Sir William’s observations.

  The biographical footnotes giving astronomers’ life dates indeed support Miss Mitchell’s prescription of “night air” for longevity.

  Whenever Sir William polished a telescope mirror, Caroline Herschel says in her Memoir, “by way of keeping him alive I was constantly obliged to feed him by putting the victuals by bits into his mouth.” She did not mind such tasks: “When I found that a hand was sometimes wanted when any particular measures were to be made with the lamp micrometer, &c., or a fire to be kept up, or a dish of coffee necessary during a long night’s watching, I undertook with pleasure what others might have thought a hardship.” Her labors sometimes proved arduous: “The mirror was to be cast in a mould of loam prepared from horse dung, of which an immense quantity was to be pounded in a mortar and sifted through a fine sieve. It was an endless piece of work, and served me for many an hour’s exercise.”

  The first five known moons of Uranus are Sir William’s Oberon and Titania, the slightly dimmer Ariel and Umbriel, first seen by William Lassell from Liverpool in 1851, and Miranda, the nearest to Uranus, as well as the brightest and smallest, discovered in 1948 by Gerard Kuiper, and named by him for the heroine of The Tempest.

  Sir John Herschel must have been thinking generally of sprites and sylphs in English literature when he named the first four Uranian moons, for Umbriel (like the later Belinda) belongs to “The Rape of the Lock” by Alexander Pope. After Kuiper added Miranda, Shakespeare dominated subsequent choices. Five moons, spotted since 1997 with the Hale Telescope in California, honor Miranda’s father, Prospero, and Tempest characters Caliban, Stephano, Sycorax, and Setebos.

  The planetary interiors of Uranus and Neptune evoke the “hot ice and wondrous strange snow” in A Midsummer Night’s Dream (V, i):

  A tedious brief scene of young Pyramus,

  And his love Thisbe; very tragical mirth.

  Merry and tragical! tedious and brief!

  That is, hot ice and wondrous strange snow.

  How shall we find the concord of this discord?

  Following the discovery of Uranus’s rings in 1977 by James Elliot of MIT and his colleagues aboard the Kuiper Airborne Observatory, Voyager 1 saw evidence of faint rings at Jupiter in March 1979. Its sister ship, Voyager 2, confirmed the discovery three months later.

  Neptune’s ring arcs are named for Adams, Leverrier, Galle, Lassell, and François Arago (the leading French astronomer who urged Leverrier to study Uranus), but there is none for Airy.

  UFO (Pluto)

  A heavenly body’s motion against the background of the fixed stars reveals the object to be a wanderer of some kind, whether a planet, a comet, or an asteroid. The day-to-day shift in position, as noted in written records or caught on a sequence of photographic plates, is a parallax effect created by the Earth’s motion. Tombaugh studied his photographic plates with a blink comparator—an instrument that automatically blinked back and forth between magnified views of the same region of space taken at different times.

  The Lowell Observatory withheld announcement of Planet X’s detection until March 13, 1930, to coincide with what would have been Percival Lowell’s 75th birthday, as well as the 149th anniversary of the discovery of Uranus. Mrs. Lowell, the former Constance Savage Keith, selected the name “Zeus” for the new planet, then changed her mind to “Percival,” and finally to “Constance,” but the Observatory staff preferred the name suggested by eleven-year-old Venetia Burney of Oxford, England, and communicated to them by cable. “Pluto” not only fit the mythological scheme of planetary names (and had figured in the staff’s top three picks even before the cable arrived), but also commemorated the founder’s initials, “P. L.”

  Counting the Earth-Sun distance as one astronomical unit (AU), Jupiter is stationed at 5 AU and Neptune at 30, while Pluto and more than one hundred other members of the Kuiper Belt travel between 30 and 50 AU. The 17-degree tilt of Pluto’s orbit carries it by turns 8 AU above the plane of the Solar System and 13 AU below. The actual distance between Pluto and Neptune remains at least 17 AU at all times because of the stable resonance of their orbits.

  James W. Christy and Robert S. Harrington, of the U.S. Naval Observatory in Washington, D.C., deduced the presence of Charon from images of Pluto taken at Flagstaff, Arizona, only a short distance from Mars Hill. Christy named the moon for his wife, Char (short for Charlene), and also for the boatman Charon who ferried dead souls across the River Styx into Pluto’s underworld. Two tiny moons of Pluto, discovered in 2005 with the Hubble Telescope, are “Nix” and “Hydra.”

  David Jewitt (Institute of Astronomy, Hawaii) and Jane Luu (University of Leiden), while working together at the University of Hawaii’s telescope on Mauna Kea, discovered the first Kuiper Belt Object, which they called “Smiley,” after the spy in the novels of John LeCarré, though its official name remains 1992 QB1. Quaoar, Varuna, and Ixion, as well as the controversial 2003 UB313, have all been discovered from Mount Palomar in California by the team of Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale), who chose their approved KBO names, following IAU guidelines, from among the worldwide catalog of underworld deities.

  Gerard Kuiper based his prediction of what is now called the Kuiper Belt on the motions of short-period comets such as Comet Halley and Comet Encke. Calculated orbits for these bodies suggested they originated in the Kuiper Belt region, and returned to it whenever they disappeared from view. In 1950, the same year Kuiper published this idea, Dutch astronomer Jan Oort used a similar argument to predict another, more distant reservoir of comets at 50,000 AU. While the Kuiper Belt is shaped as a torus (donut), the “Oort Cloud” forms a spherical shell. The orbits of short-period comets from the Kuiper Belt rarely incline more than twenty degrees from the plane of the ecliptic. Long-period comets from the Oort Cloud, on the other hand, may travel paths of any inclination, even perpendicular to the ecliptic.

  In Lowell’s day, the Observatory on Mars Hill owned a cow, named Venus. After the ninth planet was discovered, Walt Disney appropriated the name Pluto for the cartoon dog he introduced in 1936. Clyde Tombaugh understandably chose that same name for his cat.

  Bibliography

  Titles listed here are sources of scientific, historical, and literary background. Current inform
ation about the planets unfolds as news reports posted in scientific journals and on the Internet, including the Web pages of NASA (www.nasa.gov), The Planetary Society (www.planetary.org), the Space Telescope Science Institute (www.stsci.edu), and the United States Geological Survey (http://planetarynames.wr.usgs.gov).

  Abrams, M. H., with E. Talbot Donaldson, Hallett Smith, Robert M. Adams, Samuel Holt Monk, George H. Ford, and David Daiches, eds. The Norton Anthology of English Literature. 2 volumes. New York: Norton, 1962.

  Ackerman, Diane. The Planets: A Cosmic Pastoral. New York: William Morrow, 1976.

  Albers, Henry, ed. Maria Mitchell: A Life in Journals and Letters. Clinton Corners, N.Y.: College Avenue Press, 2001.

  Andrewes, William J. H., ed. The Quest for Longitude. Cambridge, Mass.: Collection of Historical Scientific Instruments (Harvard University Press), 1996.

  Asimov, Isaac. Asimov’s Biographical Encyclopedia of Science and Technology. New York: Doubleday, 1972.

  Aveni, Anthony. Conversing with the Planets. New York: Times Books, 1992.

  Barnett, Lincoln. The Universe and Dr. Einstein. 2nd revised edition. New York: William Morrow, 1957.

  Beatty, J. Kelly, with Carolyn Collins Petersen and Andrew Chaikin, eds. The New Solar System. Fourth edition. Cambridge, Mass.: Sky Publishing, and Cambridge, England: Cambridge University Press, 1999.

  Bedini, Silvio A., Wernher von Braun, and Fred L. Whipple. Moon: Man’s Greatest Adventure. New York: Abrams, 1970.

  Bennett, Jeffrey, with Megan Donahue, Nicholas Schneider, and Mark Voit. The Cosmic Perspective. 3rd Edition. San Francisco: Pearson/Addison Wesley, 2004.