Chapter 8: Interplanetary Bodies

A photograph of Halley's comet taken in 1986.

In ancient times, before science explained the phenomena, people interpreted comets as evil omens. The mysterious appearance of a visitor in the night sky was as frightening as the sudden darkness of a solar eclipse. For example, Halley's Comet has been recorded and linked to grave events throughout history. In 66 A.D., its arrival was said to have heralded the destruction of Jerusalem. Five orbits later, it was said to mark the defeat of Attila the Hun in 451. In 1066, it presided over the Norman conquest of England. In 1456, the appearance of a comet coincided with a threatened invasion of Europe by the Turks, who had already taken Constantinople three years before. Pope Calixtus III prayed for deliverance "from the devil, the Turk, and the comet."

 

The title page of Newton's "Principia".

Edmund Halley

These superstitions were eventually debunked by the English astronomer Edmond Halley. Halley had studied planetary orbits thoroughly, and he played a key role in the emergence of Newton's theory of gravity. One day in 1684, he was having lunch in a London pub with two other scientists. Halley felt sure that Kepler's elliptical orbits could be explained by a gravitational force that diminished with the inverse square of the distance from the Sun. They all made a wager over who could prove this assertion. They each failed. Halley then visited Newton in Cambridge and posed the same question to him. He found that Newton had already done the proof and urged him to publish it. From then on, Halley was the irresistible force that encouraged Newton to finish the Principia — perhaps the greatest scientific book ever written. Halley paid the publishing costs himself and sent copies to leading scientists and philosophers throughout Europe.

 

 

In 1704, Halley was working with Newton's law of gravity and some new methods of computing orbits. He discovered that comets travel on long, elliptical orbits around the Sun, and that certain comets reappear many times. Calculating the orbits of 24 well-documented comets, Halley found that what was believed to be four different comets (seen in 1456, 1531, 1607, and 1682) had the same orbit, and appeared approximately 75 years apart. Halley correctly inferred that these appearances were by a single comet. He even showed that slight irregularities in the time of appearance of the comet were caused by the gravitational influence of other planets, especially Jupiter. Halley predicted the comet would return late in 1758. It did so, on Christmas night of that year.

The discovery that comets are regular visitors on ordinary elliptical orbits, with predictable motions, helped dispel the superstition that comets are evil omens. Halley showed that Newton's law of gravity applied just as well to the elongated orbit of a comet as it did to the nearly circular orbit of a planet. His discovery extended the application of this simple physical law. It's also a classic example of how science works: before Halley, comets were viewed as random and mysterious visitors. Halley linked four separate events to a single object, and thus revealed a pattern in nature. He was then able to provide a mathematical description of the pattern, the comet's elliptical orbit, and make a successful prediction for the next appearance in the sequence. For many people in the 18th century, this cemented the idea that nature was comprehensible and predictable. Earlier in history, much the same thing happened with solar eclipses, which went from being signs of "angering the gods" to being repeatable and predictable phenomena.

Since the days of Halley, the brightest comets have been named for their discoverers. Many comets are not visible to the naked eye, so every comet is also given an anonymous, scientific name. Telescopes and computational techniques have advanced greatly since Halley's time. We don't have to wait for a return visit to recognize a comet. The orbits of most comets are calculated within a few weeks of their initial discovery. Halley's Comet renewed its fame in 1910 when the Earth passed through its tail. It was dimmer in 1986 because it did not pass as close to the Earth. Halley's comet will not appear again until 2061. More recent visitors to our nighttime skies were Hale-Bopp and Hyakutake, two of the brightest comets in the past half-century, both visible in the mid-1990s. Comet McNaught was even brighter in 2006, but only visible in the southern hemisphere.