The Planets

by Dava Sobel

  Areographer—one who makes maps of Mars (Ares).

  Asteroid—a minor planet, generally small and rocky, some one hundred thousand of which orbit the Sun in the wide gap between Mars and Jupiter.

  Cartouche—in cartography, a decorative emblem bearing text such as the title of the map, or the scale, and often including symbols of the regions represented.

  Coma—the fuzzy envelope surrounding the nucleus of a comet.

  Comet—a small icy body orbiting the Sun in a highly elliptical orbit, changing its appearance on close solar approach by emitting jets of gas and dust.

  Coronae—(singular: corona) sets of concentric rings surrounding features such as domes and depressions, unique to Venus, found where her surface crust is thinnest.

  Duricrust—loosely cemented dust seen on the surface of Mars, thought to be formed by the deposition and evaporation of water and carbon dioxide.

  Eccentricity—the degree to which a body’s orbit deviates from a circle. (The orbit of Pluto is highly eccentric—an exaggerated ellipse, while the orbits of Venus and Neptune appear virtually circular.)

  Eclipse—the disappearance of a part or all of one heavenly body behind another, or in the other’s shadow. (In a solar eclipse, the Moon blocks the Sun from view; in a lunar eclipse, Earth’s shadow falls on the Moon.)

  Ecliptic—the apparent path of the Sun, Moon, and planets as seen from Earth, so named for the eclipses that occur here; the plane of the Zodiac and of Earth’s orbit.

  Electromagnetic Radiation—light, in all its guises, ranging from high-energy gamma rays and X-rays through ultraviolet radiation, visible light, and infrared, to microwaves and radio waves.

  Elongation—the most favorable time to view Mercury or Venus, the planets interior to Earth, when they achieve their greatest apparent distance east or west of the Sun. The greatest possible elongation for Mercury is 28 degrees, and for Venus, 47 degrees.

  Ephemeris—a published table of predictions of heavenly bodies’ positions, especially those of planets and comets.

  Escape velocity—the speed a rocket (or any object) must attain to break free of the pull of gravity at a planet’s surface, and rise into space.

  Extremophile—any inhabitant of an extreme environment that would be toxic or otherwise unfit for all but properly adapted life forms.

  Galaxy—a collection of billions of stars, all gravitationally bound, as in the Solar System’s home galaxy, the Milky Way.

  Igneous—a term used to describe rocks formed from once-molten magma or lava.

  Kuiper Belt—a donut-shaped region beyond the orbit of Neptune, named for Gerard Kuiper, containing hundreds of thousands of icy planetoids. Some of these objects, when deflected by gravity or collisions into orbits that carry them close to the Sun, become the comets that return on regularly repeating schedules.

  Magnetic Field—the region around a magnet, throughout which the magnet affects charged particles or other magnets. Many planets, such as Jupiter and Earth, behave as giant magnets and generate their own magnetic fields.

  Magnetosphere—the invisible bubble of a planet’s magnetic field, defining the limits of the field’s sphere of influence.

  Magnitude—the brightness of a heavenly body, expressed as a number; apparent magnitude (the body’s relative brightness as seen from Earth) may differ significantly from its absolute magnitude, or intrinsic brightness.

  Mantle—the middle substance of a planet, filling the space between the surface crust and the core of a terrestrial world, or the upper atmosphere and solid center of a gaseous one.

  Meteor—a “falling” or “shooting” star, i.e., the light from a space rock or bit of comet dust descending through Earth’s atmosphere and becoming incandescent from the heat of friction.

  Meteorite—a landed piece of a meteoroid.

  Meteoroid—a space rock or chunk of a planet adrift in space.

  Methane—also known as marsh gas, the simplest compound of hydrogen and carbon.

  Moon—Earth’s natural satellite, and, by extension, a body in orbit around any planet or asteroid.

  Nebula—a blurry-looking heavenly object, such as the disk in which a star is born.

  Oort Cloud—a spherical region of the outer Solar System, beyond the Kuiper Belt, named for Dutch astronomer Jan Oort (1900–1992). Comets from the Oort Cloud follow extremely long-period orbits, and may leave the Solar System after one swing around the Sun.

  Perigee—that part of the Moon’s (or an artificial satellite’s) orbit that brings it closest to Earth, at which point it travels fastest.

  Perihelion—a planet’s or comet’s (or Sun-orbiting spacecraft’s) closest approach to the Sun, and therefore the time of its greatest orbital velocity.

  Planet—a heavenly body, generally but not necessarily expected to be larger than a thousand miles in diameter, and orbiting a star.

  Planetesimal—a chunk of material smaller than a planet, and which may join with other like pieces to become a planet or moon.

  Regolith—dusty and rocky debris coating the surface of a terrestrial planet or satellite, similar to soil but lacking any live components.

  Roche zone—the region close to a planet where tidal forces prohibit the build-up of planetesimals into satellites, named for French mathematician Edouard Roche (1820–1883), who first described it.

  Satellite—a natural satellite is a moon; an artificial satellite is a spacecraft in orbit around a planet.

  Solstice—either of the two days each year (in June and December) when the Sun reaches its farthest distance above or below the equator, resulting in the shortest or the longest day.

  Star—a ball of gas, mostly hydrogen and helium, massive enough to ignite thermonuclear fusion at its core, and shine by its own emitted light.

  Syzygy—the all-in-a-line arrangement of heavenly bodies, such as the Sun, Moon, and Earth during an eclipse, or the Sun, Venus, and Earth during a Transit of Venus.

  Tessera (plural tesserae)—extremely deformed and fault-scarred areas that constitute the second most common land form on Venus (after volcanic plains), from the Russian word for “tiled.”

  Transit—the passage of one heavenly body in front of another, as when Mercury or Venus is seen passing across the disk of the Sun. The satellites of Jupiter and Saturn can also be observed in transit across their parent planets.

  Zodiac—the circle of twelve constellations through which the Sun seems to pass as the Earth makes its annual journey. These constellations correspond to the astrological signs of the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces.

  A Note About the Illustrations

  4. My own shoebox diorama of the Solar System fell far short of Lynette Cook’s rendering here, though I love the way she recaptured the experience. Even better is the view out the window, showing that the child at work on this project lives on some other planet—an exoplanet of another star.

  21. In this astronomical “Garden of Eden,” planets are taking shape in the topiary bushes, and the surface of the sundial is the face of our own Sun, sunspots and all. In the sky, the stages of partial and total solar eclipse parade behind the branches of a tree of life.

  32. Mercury, the god mentioned most frequently in the Greco-Roman myths, is captured on a Grecian urn that depicts some of the key events of his personal mythology. The urn divides the twilight landscape into the realms of life and death, while the planet Mercury hovers overhead.

  67. Traditional symbols of feminine beauty perch on the parched landscape of Venus, shown here in the fiery colors astronomers chose for rendering their radar images. The Magellan spacecraft buzzes by the cloud-swathed planet, depicted as the pull on the window shade. On the shade itself, Venus shows her phases above a temple dedicated to her, and also poses with the Moon.

  83. Many intellectual and actual voyages of discovery revealed the Earth to be a planet in orbit about the Sun and not, as had been thought for ages, the hub
of the universe. Today the exploration of Earth continues out through the atmosphere to the place where space begins, and into the unseen core that drives the restless surface.

  109. Moon dust and the Moon illusion illuminate this earthly Moon garden, lighting a glimmer path on its water. On the dome of the temple honoring the Apollo astronauts is decorative ironwork in the shape of the lunar landing module, and the plaque at its base repeats their hopeful message, “We came in peace for all mankind.”

  131. More science fiction has been written about Mars than about any other planet, so that our view of that world is shaped as much by artistic imagination as by science. Meteorites and aliens converge here. The small whitish globe in the red dust is Lowell’s Mars, covered with canals, and the bluish one the wet Mars we now believe existed eons ago.

  154. The natal chart superimposed on this image of Jupiter belongs to the Galileo spacecraft, whose ghostly outline appears at lower right. The four Galilean satellites flank the planet along a straight diagonal line, while the comet approaching Jupiter’s lightning-studded cloud tops seems poised to be engulfed or torn to pieces at any moment.

  176. The song of Saturn, à la Gustav Holst, figures as the wallpaper motif here. Kepler’s shorter Saturn song fills the staff inside the doorway, its notes modeled after the moon Titan. Below that snippet of heavenly harmony, Huygens descends into Titan’s cloud tops. On the tiled floor, Huygens icons alternate with Cassini’s round white antenna.

  200. The telescope that Herschel used to spot Uranus stands in the foreground of a wintry landscape with views of ice giants looming. In the fractured picture frame is a comet—the mistaken identity of Uranus at first sight. The larger frame holds Voyager 2’s view of the planet, before the current change of season made Uranus look more like blue Neptune.

  222. Pluto and Charon occupy the upper right corner, with Sedna and all manner of yet-to-be-discovered worlds on the floor below. The oddly shaped window, which looks out on Ellis Island, repeats the outline of the Lowell Observatory building that housed Clyde Tombaugh’s telescope. That’s his cat, also named Pluto, underneath.

  235. Each of these planets is painted, using traditional techniques, from life—as seen through the best available telescopes and spacecraft imagery—and then superimposed on the photo of the green leaves. The resulting surreal scene of planets growing on plants expresses the hope that readers will come to feel more familiar with the other worlds of our Solar System.

  Details

  Model Worlds (Overview)

  Model Solar Systems big enough to walk or drive through can be visited in Aroostook County, Maine; Boston, Massachusetts; Boulder, Colorado; Flagstaff, Arizona; Ithaca, New York; Peoria, Illinois; Washington, D.C.; Stockholm, Sweden; York, England; and in the Alps near St.-Luc, Switzerland.

  The Soviet spacecraft Venera 4 made the first probe of the Venusian atmosphere in 1967; Venera 7 landed on Venus in 1970, and Venera 8 in 1972. In November 1971, America’s Mariner 9 became the first Mars orbiter—the first spacecraft to orbit a planet beyond the Earth-Moon system. The Soviet Mars 3 lander arrived the following month, but survived only twenty seconds on the Martian surface.

  Michel Mayor and Didier Queloz of the Geneva Observatory made the first discovery of an exoplanet, and announced their findings about 51 Pegasi in October 1995. Two Americans—Geoffrey W. Marcy, University of California at Berkeley, and R. Paul Butler, now at the Carnegie Institution in Washington, D.C.—quickly confirmed the Swiss claims and went on to identify other extrasolar planets.

  Genesis (The Sun)

  The extraordinary phenomenon of hydrogen fusion requires the tremendous heat and pressure found inside stars. Under normal circumstances on Earth, two hydrogen nuclei would never unite with one another, because both carry positive charge, and the electromagnetic force that causes two positively charged particles to repel each other is stronger than gravity. Inside the Sun, in contrast, high temperature pushes particles together so hard and fast that they collide despite electromagnetic repulsion. And once the particles are that close together, they succumb to a third force—called the “strong force” because it is the strongest known in nature—which binds them together. The great power of the strong force, however, operates only over the tiniest distances, such as the size of an atomic nucleus.

  In a single second inside its core, the Sun converts 700 million tons of hydrogen to 695 million tons of helium. The five-million-ton difference between input and outcome is transformed into the energy of light. This is a great deal of energy, according to the formula that describes energy (E) as the equivalent (=) of a given mass (m), or 5 million tons in this case, multiplied by the speed of light (c) squared (2). Since the speed of light is a very high number to begin with (186,000 miles per second), squaring it—multiplying it by itself—yields a truly astronomical figure (34,596,000,000), which indicates the phenomenal power lurking inside even the tiniest amounts of matter.

  Helium, the second most common ingredient (after hydrogen) in the Sun and throughout the universe, accounts for 10 percent of the Sun’s makeup. All other elements detectable by analysis of the Sun’s light—carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron, taken together—total only 2 percent of the Sun’s mass.

  During periods of high solar activity, conglomerations of dark sunspots on the Sun dim its radiation by a few measurable tenths of a percent, but overall the Sun stays a constant source of steady light.

  The Moon at apogee (its greatest distance from Earth) cannot completely cover the Sun, but instead produces an “annular” eclipse, in which the Sun appears as a glowing ring around the Moon and the corona may not be visible.

  Although it is safe to look at the Sun during totality, viewing the stages of partial eclipse preceding and following totality requires eye protection.

  Mythology (Mercury)

  Procrustes gained notoriety by lopping off his tall guests’ legs and stretching short visitors on a rack to make them fit his bed, thus lending his name to violently or arbitrarily enforced conformity.

  Mercury, traveling an elliptical orbit, reaches its peak velocity of thirty-five miles per second at perihelion, when it approaches within twenty-nine million miles of the Sun, and slows to twenty-four miles per second at the opposite orbital extreme, or aphelion, where the Mercury-Sun distance exceeds forty-three million miles.

  The first of several mentions of “Rosy-fingered dawn,” as Homer called the reddish morning sky, occurs in Book I of The Iliad, line 477.

  Transits of Mercury occur approximately thirteen times per century. Although the planet passes between the Earth and the Sun about four times a year, it most often travels above or below the Sun from our perspective, when no transit is seen.

  Mercury’s period of spin is exactly two-thirds its orbital period, “coupling” the two time intervals in a ratio of 3:2, or three rotations for every two orbits. (The discovery of the actual rotation rate was made by bouncing radar from the Arecibo Observatory in Puerto Rico off the surface of Mercury.) Most other tidally bound bodies in the Solar System display a 2:1 spin-orbit resonance. The most notable exception is the Moon, which completes one rotation per revolution about the Earth, giving it a 1:1 resonance.

  Beauty (Venus)

  William Blake wrote his ode to Venus in 1789, long before the discovery of the planet’s own westerly winds. His mention of “thy west wind” refers to evening breezes timed to her appearance.

  Former President Jimmy Carter, while serving as governor of Georgia, reported Venus to the state police. During World War II, a squadron of B-29 pilots mistook the planet for a Japanese plane and tried to shoot it from the sky.

  Donald W. Olson and Russell Doescher of Southwest Texas State University in San Marcos took their honors astronomy class to France in May, 2000, and successfully identified the building featured in “White House at Night” by using planetarium programs to recreate the sky over France in the summer of 1890, reading letters Van Gogh wrote during his last
weeks, and consulting archived weather reports.

  The duration of a solar day on Venus, measured from one noon to the next, is 117 Earth-days, so that periods of light and dark last nearly 59 Earth-days each. The sidereal day, or the time it takes the planet to rotate with respect to the background stars, is 243 Earth-days—longer than the Venus orbital year of 225 Earth-days. On Earth, as on Venus, the length of the solar day differs from the sidereal day; in Earth’s case the solar day is about four minutes longer than the sidereal.

  A complete Venus cycle—from morning star apparition to disappearance behind the Sun, through evening star apparition and disappearance in front of the Sun—lasts 584 days. This time period formed the foundation of the Mayan calendar. Since Venus makes eight orbits of the Sun in five Earth years, and passes between Earth and Sun five times in the process, there are five distinct 584-day Venus patterns in Earth’s sky. The Mayas had a name for each.

  Since 1919, authority for planetary nomenclature has been vested in the International Astronomical Union. Although discoverers may suggest names for new satellites or other bodies, the choices must be approved by task and working groups, and ultimately voted into effect by the IAU General Assembly, which meets every three years.

  Geography (Earth)

  Even before Ptolemy, mapmakers applied concepts of latitude and longitude to the heavenly sphere and the globe of the Earth. After Ptolemy introduced a uniform coordinate system expressed in degrees, the ability to determine longitude awaited the late seventeenth century, and remained a problem at sea for another hundred years.

  Ptolemy’s Geography survived in manuscripts copied by scribes. The oldest such extant manuscript dates to the thirteenth century.

  In 1828, in his History of the Life and Voyages of Christopher Columbus, American author Washington Irving popularized the romantic image of Columbus fighting for the roundness of the world. Medieval knowledge of the world’s shape is well documented, however, in texts such as the thirteenth-century Sphere of Sacrobosco, and the world globe Martin Behaim completed months before Columbus left Spain. The Ancients could have concluded a round world from the stars visible at different latitudes, or the curved shape of the Earth’s shadow on the Moon during a lunar eclipse.