Part 22

The finished work contained a note to this effect: “The inverse law of gravity holds in all the celestial motions, as was discovered also independently by my countrymen Wren, Hooke, and Halley.”

The book was dedicated to the Royal Society, and to it was prefixed a set of Latin hexameters addressed by Halley to the author, ending with the well known line:

“Nec fac est propius mortali attingere divos.”

(“It is not given to a mortal to get in closer touch with the gods.”)

Halley was fifty years old when he made his famous prediction of the return of the Comet of 1682. This was in his “Synopsis of Comet Astronomy,” which ended with these words: “Hence I may venture to foretell that this Comet will return again in the year 1758.”

Besides being an astronomer of the first class, Halley was also a good navigator. In 1698 he was commissioned a captain in the Royal Navy and was put in command of the King’s ship, “The Paramour Pink.” With this vessel he set out on a long cruise to the Pacific for the purpose of making observations on the laws which govern magnetic variations. This task he accomplished in a voyage which lasted two years and extended to the fifty-second degree of southern latitude, when the ice compelled him to turn back. On the return voyage his crew mutinied and his lieutenant sided with the mutineers. Halley quelled the mutiny by sheer force of personality, and returning to England got rid of his lieutenant. The results of his voyage were published in his “General Chart of the Variation of the Compass” in 1701. Immediately afterwards Halley set out on another King’s ship and executed by royal command a careful survey of the tides and coasts of the British Channel, an elaborate chart of which he published in 1702.

Next Halley was sent by the King to Dalmatia, for the purpose of selecting and fortifying the port of Trieste.

On Halley’s return to England, he was made Savilian professor of geometry at Oxford, and received an honorary doctor’s degree. He filled two terms of eight years each as secretary to the Royal Society, and early in 1720 he succeeded Flamsteed as Astronomer Royal.

He died on January 14, 1742, at the age of eighty-five in the full possession of his faculties, the foremost astronomer of the day and a man universally beloved and respected. His gravestone stands at the Greenwich Observatory.

Halley’s works fill several shelves in the library of the Royal Society. His fame is kept green by the periodical return of the wandering star known by his name.

WHAT ARE COMETS?

The modern answer to the question “What are Comets made of?” is this:

Probably the heads are a mixture of solid and gaseous matter. The tails are gaseous—the result of the volatilisation of the solid matter of the heads.

The spectroscope shows that gases appear to be a constituent of all Comets. The spectra of Comets are very similar to those of a Bunsen flame. Recent spectroscopic photographs have revealed the presence of hydrocarbons, nitro-carbons, of cyanogen and of the vapours of sodium, iron and other metals.

The connection between Comets and Meteors implies the presence in Comets of solid matter. A modern theory, voiced by Schiaparelli, is that meteor showers are broken up Comets.

The tails of Comets appear to be composed of luminous gases ejected from the head of the Comet through a solar force held to be “Light Pressure,” which causes these tails to shoot off and disperse into space at the rate of 865,000 miles an hour.

The length of some Comets’ tails has been estimated at 125,000,000 miles, while the Comets’ heads themselves are generally much larger in size than our Earth. Halley’s Comet is more than ten-fold the size of our Earth.

E. W. Maunder, of the Royal Observatory of Greenwich, a modern astronomer, has thus summarized the latest theories of the substance of Comets:

“Though the bulk of comets is huge, they contain extraordinarily little substance. Their heads must contain some solid matter, but it is probably in the form of a loose aggregation of stones enveloped in vaporous material. There is some reason to suppose that Comets are apt to shed some of these stones as they travel along their paths, for the orbits of the meteors that cause some of our greatest ‘star showers’ are coincident with the paths of Comets that have been observed. But it is not only by shedding its loose stones that a Comet diminishes its bulk; it loses also through its tail. As the Comet gets close to the Sun its head becomes heated, and throws off concentric envelopes, much of which consists of matter in an extremely fine station of division.”

The orbits of Comets visible to human eyes are all governed by the Sun. In the words of C. L. Poor: “The attraction of the Sun is to the Comet like the flame to the moth. The Comet flutters for a moment about the Sun, and then swings back into outward space. But not unscathed; like the moth, the Comet has been singed. The fierce light of the Sun has beaten upon it, and spread out its particles and scattered them along its path.”

As a comet swings toward and away from the Sun, it travels at a tremendous rate of speed—over a million miles an hour. The distance covered from one end of the orbit to the other is 3,370,000,000 miles.

The great majority of Comets appear to travel in parabolas, open curves leading from infinite space to and around the Sun, and thence back into infinite space to some other fixed star invisible to us. As a matter of fact, though, the parabolic curves of Comets’ orbits through the gravitational attraction of the planets, whose orbits are crossed by it, may be changed into hyperbolic curves and ellipses by planetary perturbations. Hence the differences in time between the returns of certain Comets, like Halley’s, for instance.

In a general way, it may be said that every Comet comprises a nucleus, an envelope (called the “coma”) surrounding the nucleus and measuring from 20,000 to 1,000,000 miles in diameter, and a long tail which streams behind the nucleus from sixty to a hundred million miles or more.

Astronomers have decided that the nucleus is probably a heap of meteorites varying in size from a grain to masses weighing several tons each; a heap, moreover, so easily sundered that its elements are distributed gradually along the orbit. It follows that every Comet must eventually perish unless it restores its nucleus by collecting stray meteors. That disintegration does occur has been observed time and time again.

For example, Biela’s Comet, which was discovered in 1826, burst into two fragments, which drifted apart a distance of one million miles. Thus it became a twin Comet. Eventually it disappeared as a Comet, and in its stead we see a shoal of meteors whenever we cross its track every six and a half years.

It is possible that the Comets of 1668, 1843, 1880, 1882 and 1887, all travelling in approximately the same path, are fragments of a single large body which was broken up by the gravitational action of other bodies in the system, or through violent encounter with the Sun’s surroundings.

The luminous tail which streams behind the nucleus, which Shakespeare described so beautifully as “crystal tresses,” is startling, to say the least. Despite a length which may exceed a hundred million miles, it is so diaphanously light and subtle that it is difficult to compare it with any earthly fabric. The air that we breathe is a dense blanket in comparison. Several hundred cubic miles of the matter composing that wonderful luminous plume would not outweigh a jarful of air. By reason of its fairy lightness, it is possible for a tail occupying a volume thousands of times greater than the sun to sweep through our solar system without causing any perturbations in planetary movements.

No celestial phenomenon has caused more perplexity than the ghostly sheaf of light we call a Comet’s tail. In a day, in a few hours even, the form of that wonderful gossamer may change. Hence it is that periodic Comets are identified when they return, not by the length and arch of their tails, but by their orbits. These alone are permanent.