Edmond Halley Successfully Predicts the Return of the Great Comet of 1682

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Edmond Halley Successfully Predicts the Return of the Great Comet of 1682

Overview

On Christmas night, 1758, a comet appeared in the skies over Europe. First seen by German amateur astronomer Johann Georg Palitzsch, its appearance was a landmark event in the history of both astronomy and physics. For the first time, using Isaac Newton's (1642-1727) laws of gravitation and motion, a comet's appearance had been successfully predicted. By so doing, Edmond Halley (1656-1742) had not only launched a new era in predictive astronomy, but had also proven the accuracy and value of Newton's revolutionary new physics.

Background

For millennia, comets were viewed as harbingers of change, both good and bad. A comet appeared when Julius Caesar was murdered; another comet was said to have marked the death of Alexander the Great. Throughout history, comets were seen as messengers, portents, or omens.

During these same millennia, arguments raged over where comets were located. Some, including the Greek philosopher Aristotle (384-322 b.c.), felt that comets were part of the Earth's atmosphere while others thought they existed in the realm of the planets. This debate was finally settled in the sixteenth century, when Ptolemy (c. 100-170 a.d.) proved that comets were at least as distant as the moon, proving that they must lie beyond the atmosphere.

However, even in the early seventeenth century, little more was known about comets than in previous times. As late as 1619, Johannes Kepler (1571-1630) thought that comets moved in straight lines across the heavens, and it was not until Newton's 1687 publication of the Principia that this was realized to be impossible. In fact, Newton found that his laws of gravity and motion required all bodies to move in paths that belonged to a family of shapes called "conic sections," the shapes one gets from cutting a cone at different angles. Conic sections are circles, ellipses, parabolas, and hyperbolas, but do not include straight lines. For Newton to be right, either Kepler was wrong or comets obeyed a different set of rules.

Shortly after publishing the Principia, Newton used the observed positions of the Great Comet of 1680 to calculate its orbit as a parabola. A few years later, Newton's friend, Edmond Halley, took Newton's calculations to the next level, calculating the orbits of no fewer than 24 comets, assuming in advance that they were parabolic. In so doing, he noticed that one comet in particular, the Great Comet of 1682, seemed to have an orbit identical to that of two previous comets, the comets of 1531 and 1607. Intrigued, Halley also noticed that the periods of these three comets were almost identical. However, Halley also realized that it was impossible for a comet following a parabolic orbit to return to the inner solar system; parabolic orbits are not "closed," they continue on forever without returning.

Halley eventually realized that, contrary to expectation, many comets follow elliptical orbits that are so elongated that they appear parabolic at their closest approach to the Sun, which is all that was visible from the Earth. Casting aside the assumption that his orbits were parabolas, Halley recalculated the orbital parameters of these three comets, finding them to be a nearly exact match. Based on this, in 1705 he predicted that another great comet would appear in the skies in 1758. Unfortunately, Halley died in 1742 without seeing the return of "his" comet.

Halley was proven correct on Christmas night in 1758 when a large new comet was sighted. Coming close to the time predicted and in the correct location in the sky, there was little doubt that this was the same comet for which Halley had calculated an orbit. Its reappearance marked the start of astronomy as a predictive science, able to use universal laws to predict the positions of planets, stars, moons, and comets across the sky and through the solar system.

Impact

Halley's prediction resonated on several levels, both scientifically and within society. First, and perhaps most important, he had shown the predictive and descriptive power of Newton's physics and mathematics in a dramatic fashion. This would not be equaled for nearly a century, when Neptune was discovered from mathematical calculations based on Newton's laws of gravity and motion. Indeed, ironically, the hunt for Neptune was launched following the 1835 appearance of Halley's comet, when astronomers noticed that it failed to follow the predicted orbit during that return.

In addition to confirming the accuracy of Newtonian mechanics, Halley's successful prediction made it possible to finally resolve the place of comets in the solar system. And, from the standpoint of society, this prediction was extremely important because it helped demonstrate the power of the human intellect. Using nothing more than a pencil, Newton and Halley had described how gravity held the solar system together, forcing planets and comets alike to follow the paths laid out for them since the solar system first formed. This was heady knowledge that gave people pause. The next time that such a fundamental reassessment of our ability to describe the universe would occur was in the early 1900s, when Albert Einstein's (1879-1955) theory of relativity was confirmed by noting changes in apparent star positions during a solar eclipse. In both cases, the consensus seemed to be that such predictions were a triumph of science.

The scientific implications of Halley's successful prediction were immense. Although the accuracy of Newton's laws were not doubted with respect to objects on earth, and they appeared to govern the movements of the observed planets, this was the first time they had been used to predict the motions of an object that was no longer visible. In a sense, the comet had vanished from human senses and was "visible" only mathematically. Its dramatic reappearance proved, beyond any doubt, that the solar system and, indeed, the whole of the universe, operated according to specific laws that had been elucidated by Newton. The success of Newtonian mathematics and physics helped encourage scientists in these and other disciplines that the universe was, fundamentally, logical and understandable. While this is hardly a surprising statement today, it was almost heretical during the seventeenth and previous centuries.

It is also difficult to comprehend today that scientists have not always been able to make predictions like the one made by Halley. And many point to the Egyptians, Maya, the builders of Stonehenge, and their intellectual kin, noting that they made astronomy into a predictive science with elaborate and highly accurate calendars. However, these early astronomers, for all of their successes, lacked a real understanding of the objects of their study. They could, for example, predict the location of planets in the sky with uncanny accuracy, but they didn't know what the planets were, where they were located in space, or what rules led them to appear in their designated places year after year. In a sense, they simply developed rules to explain when a particular point of light would appear in a particular place in the sky, based on watching the sky for centuries.

Before Newton, Galileo, Copernicus, and others came to the realization that the planets were other worlds, like the Earth, in orbit around the Sun. Newton explained how they remained in orbit, made more accurate calculations of their predicted locations over the years, and helped place planetary motions into a solid, consistent physical framework. He also showed that gravity's influence extended from the Earth's surface throughout the solar system, and that it worked the same everywhere. And, most importantly, unlike ancient astronomers, he was able to erect a theoretical structure that allowed the tracking of the motions of invisible objects, as well as calculating future motions of asteroids, comets, and other bodies, based only on a few observations rather than on years or centuries of studying the heavens. Once confirmed by Halley's successful prediction, this firmly established astronomy and physics as legitimate, predictive, theoretically based fields of science rather than collections of observations made over the years.

In addition, Halley's successful prediction helped give comets their rightful place in science and in the solar system. No longer were they seen as extraordinary portents of terrestrial calamity or as celestial bodies obeying their own set of laws. Instead, they were now known to be simply a different kind of object in orbit around the Sun, and obeying the same laws of nature that ruled the rest of the solar system. This is not to say that superstitious thinking about comets ceased. However, at least scientists and the scientifically aware public began looking at comets as ordinary objects following well-understood laws of nature. In the case of Halley's comet, this culminated in the 1986 flotilla of scientific spacecraft rendezvousing with it during its return to the inner solar system in that year.

Finally, this was a philosophical breakthrough. Coming as it did during the Enlightenment, when human thought was taking center stage in man's intellectual universe, this scientific triumph served to reinforce the emerging vision of the universe as being something that, eventually, could be fully understood by man. This was one stage in a revolution in human thought.

P. ANDREW KARAM