The Planets

by Dava Sobel

  Mapmakers decorate the blank expanses of ocean with ships, with whales and sea monsters, with puff-cheeked cherubs exhaling gales, and also with map titles and legends framed in elaborate cartouches as large as some countries.

  At least one compass rose, a flower-like emblem often rendered in gold leaf, indigo, and cochineal, now orients each map, with thirty-two painted petals pointing in every possible direction of wind and headway. The rose realizes all the logbook shorthand of exploration’s zigzag course—ENE, SSW, NW by N—and mirrors the face of the magnetic compass that dictates those notations.

  The magnetic compass, indispensable to mariners since at least the thirteenth century, helps them find the North Star even when clouds obscure it—even when their ship has sailed so far south as to plunge that guiding light below the horizon. Many think the compass needle must be attracted to the Pole Star, if not to some invisible celestial point close by it.

  But no, the earth itself is the magnet that draws all compass needles to its iron heart. William Gilbert, an English doctor, discovers this truth through experimentation in 1600, and demonstrates the effect for Queen Elizabeth by using a small spherical magnet to model the earth. Furthermore, Gilbert scorns the universal prohibition against garlic on shipboard, by showing that neither garlic fumes nor garlic smeared on a compass needle can diminish its magnetic power.

  The magnetic nature of the earth leads Gilbert and others to suspect magnetism as the force that keeps the planets in their orbits. Newton’s universal gravity trumps Gilbert’s interplanetary magnetism in 1687, but still the magnetic earth holds promise for navigation. Although compass needles generally tend north, a magnetic compass points slightly east of north in one part of the world, and slightly west of north in another. Columbus had noticed this shift on his outward voyage, and feared his instrument was failing him. By the seventeenth century, however, cumulative experience suggests the phenomenon may be exploitable. Perhaps the degree of “variation” of the compass can be measured from place to place, and the featureless oceans resolved into magnetic zones to help sailors establish their whereabouts during weeks or months at sea. This possibility launches the first purely scientific voyage, under the command of Edmond Halley, the only Astronomer Royal ever to win a commission as captain in the Royal Navy.

  Between 1698 and 1700, Halley leads two expeditions across the Atlantic Ocean, and also to the Atlantic’s northern and southern limits until stopped by icebergs in fog. Off the coast of Africa and again near Newfoundland, Halley’s specially designed flat-bottomed vessel, the Paramore, draws friendly fire from English merchantmen and colonial fishermen who mistake her for a pirate ship.

  The map Halley publishes in color in 1701 fills the ocean with curving lines of varying lengths and widths describing degrees of magnetic variation east and west. The continents bordering the Atlantic serve merely to anchor the all-important lines, and to bear the cartouches, whose palm trees, muses, and naked natives have been bumped from the busy waters to the empty lands.

  Halley concludes with honesty that magnetic variation will be of no real use to sailors as a means of determining longitude. What’s more, he predicts his carefully drawn lines will shift over time as a result of motions deep within the earth. Halley (presciently) envisions the interior of the planet in alternating shells of solid and molten material that control its magnetic behavior.

  Meanwhile Halley’s map of magnetic variation, though a disappointment to him and to his fellow seamen, foments a revolution in cartography. Its curved lines connecting points of equal values (to be hailed as Halleyan lines for a hundred years) add a third dimension to printed maps. Other maps of Halley’s—of the stars of the southern hemisphere, the Trade Winds, the predicted path of the 1715 solar eclipse—also gain notoriety for their innovations. For his part, he would chart the whole Solar System if only he could gauge the mileage from the earth to the Sun.*

  Halley discerns a way to make this key measurement on the special occasion of a transit of Venus: By watching and timing the event from widely separated points on the globe, scientists could triangulate the sky to calculate the distance from the earth to Venus, then deduce the earth’s distance from the Sun. Halley predicts two transits, for 1761 and 1769, but he will have to live to the age of 105 to see even the first of the pair. For although Venus passes between the Sun and the earth five times every eight years, her tilted orbit usually carries her above or below the Sun, from our perspective. In order for Venus to be seen crossing the Sun’s face, she must intersect the plane of the earth’s orbit—within two days of the earth’s intersecting Venus’s orbital plane. These stringent requirements permit two transits to follow within eight years of one another, but only a single such pair per century.

  “I strongly urge diligent searchers of the heavens (for whom, when I shall have ended my days, these sights are being kept in store) to bear in mind this injunction of mine,” writes Halley in 1716 of the coming Venus transits, “and to apply themselves actively and with all their might to making the necessary observations.”

  When the time of the first transit comes, in June of 1761, Halley’s followers face all manner of disasters—hostile armies, monsoons, dysentery, floods, severe cold—to cover prime observing sites in Africa, India, Russia, and Canada, as well as several European cities. Clouds foil most of the expeditions, however, and astronomers’ indeterminate results focus even greater attention on the next opportunity, in 1769, which dispatches 151 official observers to 77 locations around the world.

  Each group must time the four crucial moments of the transit, called “contacts,” when Venus and the Sun touch rim to rim. The first occurs as Venus appears to attach herself to the outside of the Sun’s circle. Second contact soon follows, as Venus enters fully inside the Sun’s embrace, but it takes hours for her to achieve third contact on the far side of the solar disk. By fourth contact she has already exited the Sun, and stands on the brink of separation.

  Responsibility for the Royal Society’s all-important observations at King George III Island (Tahiti) falls to Lieutenant James Cook. He sets out from England the year before, in August of 1768, so as to arrive in time to make preparations that include the building of a secure observatory, Fort Venus.

  “Saturday 3rd June [1769]. This day prov’d as favourable to our purpose as we could wish, not a Clowd was to be seen the whole day and the Air was perfectly clear, so that we had every advantage we could desire in Observing the whole of the passage of the Planet Venus over the Suns disk: we very distinctly saw an Atmosphere or dusky shade round the body of the Planet which very much disturbed the times of the Contacts particularly the two internal ones. Dr Solander observed as well as Mr Green and my self, and we differ’d from one another in observing the times of the Contacts much more than could be expected. Mr Greens Telescope and mine were of the same Mag[n]ifying power but that of the Dr was greater then ours.”

  Through no one’s fault, astronomers everywhere encounter the same difficulties as Cook’s men in judging the exact moments of Venus’s entry into and exit from the Sun’s disk. The limitations of even the best available optics undermine everyone’s results, and the international astronomical community must be content with merely narrowing the earth-Sun distance to something between 92 and 96 million miles.

  Cook turns his attention from Venus to the second, secret part of his instructions—a sortie through the icy sea in search of the great southern Terra Incognita. Failing to find it on this quest, he returns home, but mounts a second discovery attempt in 1772. Through three cold years of effort Cook, now made Captain, becomes adept at turning his ship frequently into the wind to shake the snow from her sails.

  “Monday 6th February [1775]. We continued to steer to the South and SE till noon at which time we were in the Latitude of 58° 15′ S Longitude 21′ 34′ West and seeing neither land nor signs of any, I concluded that what we had seen which I named Sandwich Land was either a group of Isles &ca or else a point of the Continent, for I fi
rmly believe that there is a tract of land near the Pole, which is the source of most of the ice which is spread over this vast Southern Ocean….I mean a land of some considerable extent….It is however true that the greatest part of this Southern Continent (supposing there is one) must lay within the Polar Circle where the Sea is so pestered with ice that the land is thereby inaccessible.”

  Cook’s reckoning of latitude and longitude surpasses the accuracy of all who preceded him in such pursuits. By tracking the motion of the Moon against the stars—a method Halley helped to develop—and with the aid of a new timekeeper that keeps up with the master clock back home at the Greenwich Observatory, Cook knows exactly where he is. His maps show others the way from Success Bay in Tierra del Fuego, his source of wood and water, to Botany Bay in Australia, which he named for its abundance of new plant species, and Poverty Bay, New Zealand, where Cook found “no one thing we wanted.”

  Ships laden with surveying instruments—ships that not only cross oceans but can course close to the land all along coastlines and into the mouths of rivers—now start to reexamine the New World with new precision. This is the mission of H.M.S. Beagle in 1831, whose captain carries twenty-two of the best available chronometers—timekeepers of the type Cook praised on his second voyage. Bound for a detailed survey of South America and then home the long way around via the East Indies, Captain Robert FitzRoy seeks a gentleman companion who shares his interests in geology and natural history, and who will pay his own way. Charles Darwin, a twenty-two-year-old recent college graduate unsure of his life’s vocation, signs on.

  The Beagle tortures Darwin with seasickness. Although he may freely, legally abandon ship at any port, he stays the full tour of duty, which lasts five years. He copes by spending as much time as possible engaged ashore while FitzRoy coasts the whole of Argentina, Chile, and the Falkland and Galápagos Islands to make maps.

  “I stayed ten weeks at Maldonado, in which time a nearly perfect collection of the animals, birds, and reptiles, was procured,” Darwin reports of the summer of 1832. “I will give an account of a little excursion I made as far as the river Polanco, which is about 70 miles distant, in a northerly direction. I may mention, as proof of how cheap everything is in this country, that I paid only two dollars a day, or eight shillings, for two men, together with a troop of about a dozen riding-horses. My companions were well armed with pistols and sabres; a precaution which I thought rather unnecessary; but the first piece of news we heard was, that, the day before, a traveler from Monte Video had been found dead on the road, with his throat cut. This happened close to a cross, the record of a former murder.”

  Despite the dangers of the local wars, Darwin still prefers the land to the sea:

  “August 11th [1833]—Mr Harris, an Englishman residing at Patagones, a guide, and five Gauchos, who were proceeding to the army on business, were my companions on the journey…. Shortly after passing the first spring we came in sight of a famous tree, which the Indians reverence as the altar of Walleechu…. About two leagues beyond this curious tree we halted for the night: at this instant an unfortunate cow was spied by the lynx-eyed Gauchos. Off they set in chase, and in a few minutes she was dragged in by the lazo, and slaughtered. We here had the four necessaries of life ‘en el campo,’—pasture for the horses, water (only a muddy puddle), meat, and firewood. The Gauchos were in high spirits at finding all these luxuries; and we soon set to work at the poor cow. This was the first night which I had ever passed under the open sky, with the gear of the recado [saddle] for my bed. There is high enjoyment in the independence of the Gaucho life—to be able at any moment to pull up your horse, and say, ‘Here we will pass the night.’ The deathlike stillness of the plain, the dogs keeping watch, the gypsy-group of Gauchos making their beds round the fire, have left in my mind a strongly marked picture of this first night, which will not soon be forgotten.”

  There will be time enough, after Darwin returns to England, for him to marry and put the concerns of his children ahead of his own, to wander in circles for years of private thought as the souvenir birdskins and other mementos from the Galápagos help him divine the secret of life’s diversity.

  For now, hunting fossils, “geologizing,” climbing the Andes, he ponders the forces that can uplift such massive mountains over ages, or grind them to gravel, or make them tremble.

  “February 20th [1835]—The day has been memorable [here] in…Valdivia, for the most severe earthquake experienced by the oldest inhabitant. I happened to be on shore, and was lying down in the wood to rest myself. It came on suddenly, and lasted two minutes; but the time appeared much longer. The rocking of the ground was most sensible. The undulations appeared to my companion and myself to come from due east; whilst others thought they proceeded from south-west; which shows how difficult it is in all cases to perceive the direction of these vibrations. There was no difficulty in standing upright, but the motion made me almost giddy. It was something like the movement of a vessel in a little cross ripple….”

  Indeed, the continents themselves are voyaging. They ride as passengers aboard great slabs of the earth’s crust in constant motion. In 1912, German geologist Alfred Wegener explains that the east coast of South America complements the western edge of Africa because the two continents are pieces of the same jigsaw puzzle. Once in a prehistoric era they lay cheek by jowl, part of a single land mass Wegener calls “Pangaea” (“All-earth”), surrounded by the waters of “Panthalassa” (“All-sea”), before geological forces pulled them apart.

  Today the Old World and the New continue to recede from each other along a still-widening rift in the mid-Atlantic, where molten material wells up from inside the Earth and lays down new ocean floor. As the Atlantic spreads, the Pacific shrinks. Under the restless coasts of Peru, Chile, Japan, and the Philippines, old, cold ocean floor is plunging back into Earth’s infernal interior, to the accompaniment of earthquakes and volcanoes, and sometimes catastrophic tsunamis.

  The ocean bottom undergoes constant recycling, and no part of it is older than two hundred million years. The continents, in contrast, stay topside through the ages, eroded but still intact after four billion years. Instead of sinking under each other, the continents wrinkle when the stress of contact deforms their crust: The Appalachian Mountains testify to an ancient collision between Africa and North America, while the ongoing pressure on the Himalayas continues, even now, to increase their altitude.

  Modern explorations conducted by submarine and spacecraft reveal the true, apolitical network of Earth’s borderlines, hidden underwater. Mid-ocean ridges and complementary coastal trenches divide the surface of the globe into a mosaic of some thirty plates, each one carrying a piece of a continent, a part of a sea floor. The mosaic pattern changes as plates separate, collide, or grind sidewise past one another, impelled by the pent-up residual heat of the Earth’s violent birth and ongoing radioactive decay.

  The seismic shocks that pierce the Earth during earthquakes permit the deepest possible introspection. They suggest the continents and ocean floors cast only a thin skin, or crust, around the planet. This crust slims to a slender mile under some ocean areas, while the continental crust averages a thickness of twenty miles plus, yet the crust in its entirety accounts for only one-half of one percent of Earth’s mass. The great bulk of the planet (about two-thirds of its mass) consists of the rocky yet fluid mantle roiling between the crust and the core. At the center of the Earth, part of the iron-nickel core has already cooled to a solid ball. Seismologists can hear it rotating independently inside the still-molten outer core, turning almost one second a day faster than the rest of the world.

  Like the hidden levels of the inner Earth, the invisible layers of Earth’s atmosphere have also been charted, from low in the troposphere, up through the stratosphere and mesosphere to the top of the thermosphere. The magnetic field and radiation belts surrounding Earth can be mapped from space. Also from space, a network of global positioning satellites can pinpoint locations—even indi
viduals—on the planet with centimeter accuracy, while laser beam reflectors planted on the Moon by Apollo astronauts gauge the exact Earth-Moon distance.

  Earth’s place in space is now known to such confident extremes of accuracy that the most recent transit of Venus, on June 8, 2004, was relegated to the status of a tourist attraction—a chance to see an anomaly unknown to any living soul, given the date of the previous transit, on December 6, 1882. In the interim between that transit and this, the extent of the known world had expanded to include additional planets of the Solar System, extrasolar planets in the Galaxy, and the configuration of the Milky Way itself, twirling through space with billions of stars in its spiral arms. A longer view into the infinite takes in the other galaxies of our Local Group, and the clusters and superclusters of galaxies stretching out in space and back in time to the birth of the universe. But even this sophisticated sense of our surroundings, like Ptolemy’s map, captures only the present moment’s self-awareness.

  *Since the Sun appears to circle the sphere of the earth, 360 degrees around, once every 24 hours, Ptolemy calculates each hour’s time difference as 360 divided by 24, or 15 degrees of longitude.

  *Kepler’s third law of planetary motion, published in 1609, expressed only the relative distances among the planets, based on the revolutionary period of each. No actual distances had yet been calculated.