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 Africaand 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 mapsof 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 withineight 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,
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 Africaand 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 mapsof 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 withineight 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,
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