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Edmond Halley, Father of Geophysics


Edmond Halley

Edmond Halley in his forties.

Royal Society

Scientists don't reach greatness under their own power alone. Isaac Newton, one of the very greatest, said that he could see so far only because he had stood on the shoulders of giants. In that sense scientists are like trees in a forest—the tallest ones grow where other tall trees surround them. Where would Newton have been without his colleague Edmond Halley, the father of geophysics?

Early Success

Halley (1656–1742) was a bold and restless prodigy, born to a wealthy family, who published three papers on astronomical subjects while still a college undergraduate. He left school at age 20—to spend a year on the remote South Atlantic island of St. Helena, observing the skies of the southern hemisphere. He returned to England and published the first catalog of the southern stars.

That achievement earned him membership in the Royal Society at age 22. It also illustrates a secret of successful scientists, which is skill in finding the frontier, the edge of current knowledge. In his case it was finding a part of the sky that no astronomer had studied yet. But Halley was lucky, too, in that St. Helena, the nearest English possession in the southern hemisphere, had come under English rule only two years before. He made a point of getting there first.

Another trick used by great scientists is to put existing data into a novel form. By doing so Halley launched another entire discipline. From many years worth of careful records of the deaths in the German city of Breslau (now called Wroclaw, in Poland), he created the first useful actuarial table of mortality in 1693 and analyzed how to finance life annuities. This is the basis for the life insurance industry, and the profession still remembers Halley.

The Deep Past and Future

Another distinctive thing he did was to reinterpret old data. In the case of Halley's Comet, he realized that the comet of 1682 had the same orbit as the great comets of 1607 and 1531. That was what led to his sensational announcement, in 1705, that the comet of 1682 was in fact a permanent heavenly object—a new member of the solar system—that would return in 1758, 1835, and 1910, over 200 years in the future.

But Halley looked even farther back in time. In 1710 he used the ancient star catalog of Ptolemy, from the second century, to show that in the 15 centuries since then, some stars had changed position. With this single study, he abolished the idea that the stars were fixed in space and proved that all of the sky, not just the solar system, is an active, dynamic place.

His interest in planetary motions led him to Isaac Newton, whose new mathematics of the calculus, laws of physics, and theory of gravitation were being widely scorned and disputed. Halley was so convinced that he financed the publication of Newton's book Principia Mathematica in 1687, which put physics, astronomy, and science itself on a foundation that endured until Einstein's modifications more than 200 years later.

Edmond Halley is not known as the father of geophysics for these achievements. But he earned the title by doing similar things.

Halley and Geophysics

For instance, in 1698 he got himself a commission as a captain in the royal navy—the only time a civilian has ever done that—and the following year he sailed HMS Paramore around the South Atlantic Ocean, taking readings of magnetic north wherever he went. That cruise was the first voyage ever made for a purely scientific purpose. From the data he collected, Halley produced the first map showing the variation of the Earth's magnetic field in 1700. So with that voyage he established not just new science, but a new way of doing science.

The main benefit of his map was for navigation—mariners could correct their compass readings—but it was profoundly significant for studies of the deep Earth, too. For many years it had been known that the magnetic field was slowly changing, and the map was a valuable baseline for measuring that change. Just as Halley turned the heavens into a lively place, so did he help show that the deep Earth had motion too. He had unearthed a new problem—understanding the geomagnetic field—and while we have made great progress in 300 years, it is still a central mystery of geophysics.

In 1692 Halley proposed that the geomagnetic field moved because Earth had a magnetic "nucleus or inner globe included within ours, with a fluid medium in between." And that was the first theory of geomagnetism. (Read the paper here.) Today, we believe that the Earth's magnetic field is generated as the inner core, a sphere of solid iron inside the liquid outer core, spins out of phase with the rest of the planet. Essentially, Halley was right.

Similarly Halley made the first map of the world's winds, setting the baseline for another great field of geophysics, atmospheric dynamics. And he applied Newton's new physics to the problem of predicting the tides from the Sun and Moon's gravity. Thus he did his part for the Newtonian revolution, which united the physical laws of heaven and Earth. Along those lines, he suggested in 1694 that comets might strike the Earth, a subject he said he would "leave to be discussed by the studious of physical matters." And so we discuss cosmic impacts to this day.

PS: Halley's first geophysical voyage was the model for scientific research cruises of later years. Captain James Cook's first voyage in 1769 was made to observe a transit of Venus. Halley had proposed these observations as the best method of measuring the true distances of the Sun and planets, at the time one of science's most vexing problems. The last one was in 2004, the first since 1882; the next one is in 2012.

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