Testing Newton’s theory

In the first part of the 18th century, the inverse-square law was subjected to several dramatic tests. The first concerned Earth’s shape. Newton had argued that Earth’s rapid rotation on its axis must cause Earth to depart from perfect sphericity. Instead, Earth should be an oblate spheroid—that is, flattened at the poles like an onion. For evidence, Newton pointed to the example of Jupiter, which showed a noticeable flattening when seen through a telescope. Also, in 1672 the French scientist Jean Richer had carefully measured the rate of a pendulum clock near Earth’s Equator (by comparing it with the motion of the stars) and found that the clock ran slightly slower than an identical clock in Paris. Newton argued that if Earth was flattened at the poles, Paris would be a little closer to Earth’s centre than the Equator would be. If gravity varied as the inverse square of the distance, Earth’s gravity should then be stronger in Paris than at the Equator, and thus the Paris pendulum clock would run faster. But in 1718 Jacques Cassini announced results of a survey of the Paris meridian from Dunkirk to Collioure, made by his father, Gian Domenico Cassini, and himself, that seemed to show just the opposite—that Earth is elongated at the poles like a lemon. French natural philosophers, steeped in the vortex theory of Descartes, found ways of explaining this in terms of Cartesian physics. In the 1730s the French Academy of Sciences sponsored two expeditions—one to Lapland, led by mathematician Pierre-Louis Moreau de Maupertuis, and one to equatorial South America—just to settle this question. Careful geodetic and astronomical measurements were made to determine the length of a degree of the meridian for a place near the pole and a place near the Equator. The results of the Lapland expedition showed decisively that Earth was flattened at the poles, as Newton had maintained. Voltaire famously addressed his friend Maupertuis as “the flattener of the world and of Cassinis.”

Second, Newton had been unable to calculate the correct rate for the advance of the Moon’s perigee—that is, the movement of the point on the Moon’s orbit where it is closest to Earth. The reason for the advance of the perigee lies in the perturbing attraction of the Sun on the Moon, but Newton obtained a rate too small by half (a complete revolution of the perigee takes about 18 years instead of the observed 9). In the 18th century several leading mathematicians tried to solve the problem and failed. In 1747 French mathematician and physicist Alexis-Claude Clairaut proposed a modification of Newton’s law of gravity. Instead of a pure inverse-square law, Clairaut proposed adding a small term, proportional to the inverse fourth power of the distance, in order to get the motion of the Moon’s perigee to come out correctly. Clairaut later withdrew this proposal and showed in a new calculation that the inverse-square law was perfectly adequate for explaining the motion of the Moon’s perigee. The problem was too complex to be solved directly, and it was necessary to introduce approximations. Clairaut showed that the approximations made by Newton and those who followed had been too rash and that with more-careful approximations, the advance of perigee came out just right. This was, by far, the most precise test of the Newtonian theory to date.

Finally, as the time approached for the expected reappearance of Halley’s Comet, celestial mechanicians undertook a more-precise calculation of the date of return. Halley had argued that the comets of 1531, 1607, and 1682 were one and the same and predicted a return for late 1758 or early 1759, but he did not live to see it happen. When the comet, on its very elongated orbit, passes by massive planets, such as Jupiter, on its way out of and back into the inner solar system, the planets exert forces that perturb its motion. In Paris, Clairaut, astronomer Jérôme Lalande, and Nicole Lepauté, the wife of a well-known instrument maker, calculated the motion of the comet, including the perturbing forces. This was the most ambitious program of numerical integration ever undertaken up to that time. When the comet reappeared within their announced one-month window of error, it was seen by many as a triumph of calculation, as well as of the law of universal gravitation.