Page:Popular Science Monthly Volume 75.djvu/522

518 companion. That has seemed to me a characteristic of such great men of science as I have chanced to meet. They are always face to face with the transcendent mysteries of nature. . . . Such labours produce a sublime calm, and it was that which seemed always to pervade Lord Kelvin. Surely in an age fertile in distinction, but not lavish of greatness, he was truly great."

Edmund Halley, born in 1656, Savilian professor of geometry at Oxford and later Flamsteed's successor as astronomer royal, made notable contributions to astronomy and cosmical physics. He was the first to catalogue the stars of the southern sky; he studied the orbits of Jupiter and Saturn; he detected the acceleration of the moon's mean motion; he used the transit of Venus to determine the solar parallax; he discovered the proper motion of the fixed stars; he suggested the magnetic origin of the aurora borealis; he studied terrestrial magnetism and located magnetic poles; he surveyed the tides and coasts of the British Channel; he cooperated with Newton in the publication of the "Principia."

But outside the circle of astronomers, Halley's name is known because it is attached to a comet whose orbit he calculated and whose return he predicted. This was in 1682, when Halley computed its parabolic orbit, and comparing this with the imperfect observations of comets which had appeared in 1456, 1531 and 1607, concluded that each was the same body returning from the outer region of the solar system beyond the furthest known planet. He wrote: "Wherefore, if it should return according to our predictions about the year 1758, impartial posterity will not refuse to acknowledge that this was first discovered by an Englishman."

This was not only a great advance in astronomy and important in its relation to the theory of gravitation, but was a forward movement in the conception of the orderliness of the universe. Comets had been portents of war, pestilence and famine. It was indeed Halley's comet which appeared in 1066 at the time of the invasion of William the Conqueror and again in 1456 when Constantinople was besieged by the Turks and the crescent-shaped tail was a mighty omen.

Halley's comet duly appeared in 1759, somewhat retarded by the attraction of Jupiter and Saturn, its perturbations having been accurately calculated by the French astronomer, Clairaut. It appeared again in 1835 and is now once more rapidly approaching the earth and the sun, having passed the orbit of Jupiter in April last. It has been observed by Professor Max Wolf, of Heidelberg, and we are able to give here photographs taken by Mr. Oliver J. Lee with a two-foot reflector at the Yerkes Observatory. These are printed by the courtesy of Dr. Edwin B. Frost, director of the observatory and editor of the Astrophysical Journal, where they are also printed.

The plate of September 16 was taken with an exposure of 180 minutes, standard central time of mid-exposure being 14h 5m (2:45 Sept. 17). The comet's position, as measured on the plate, was R.A. 6h 18m 56s, Dec. + 17° 9′ 23″. The plate of September 17 was exposed for 130 minutes, mid-exposure at (central standard time) 14h 10m (2:10 a.m. Sept. 18). The right ascension had increased by 4s, and the declination had decreased by 23″. The arrows indicate the position, and also the components of the direction of the comet's motion.

The original negatives are magnified about ten times, and the scale of the I pictures here is about 8″.5 to the milli, meter, or 3.5 minutes of arc to the inch; in other words, the width of each picture is about one seventh of a degree. The new Lumière "Sigma" plates were used, a fresh and clean