Part 6 (1/2)

Halley's span of existence did not permit his living to see even the first of these now historic verifications. But we in our day may emphatically term the epoch of the third verified return _Annus Halleia.n.u.s_.

Says Turner, Halley's successor in the Savilian chair at Oxford to-day: ”There can be no more complete or more sensational proof of a scientific law, than to predict events by means of it. Halley was deservedly the first to perform this great service for Newton's Law of Gravitation, and he would have rejoiced to think how conspicuous a part England was to play in the subsequent prediction of the existence of Neptune.”

Halley rose rapidly among the chief astronomical figures of his day. But he had little veneration for mere authority, and the significant veering of his religious views toward heterodoxy was for years an obstacle to his advance.

Still Halley the astronomer was great enough to question any contemporary dicta that seemed to rest on authority alone. Everyone called the stars ”fixed” stars; but Halley doubting this, made the first discovery of a star's individual motion--proper motion, as astronomers say. To-day, two hundred years after, every star is considered to be in motion, and astronomers are ascertaining their real motions in the celestial s.p.a.ces to a nicety undreamed of by even the exacting Halley.

The moon, of priceless service to the early navigator, was regarded by all astronomers as endowed with an average rate of motion round the earth that did not vary from age to age. But Halley questioned this too; and on comparing with the ancient value from Chaldean eclipses, he made another discovery--the secular acceleration of the moon's mean motion, as it is technically termed. This was a colossal discovery in celestial dynamics; and the reason underlying it lay hidden in Newton's law for yet another century, till the keener mathematics of Laplace detected its true origin.

With Newton, Halley laid down the firm foundations of celestial mechanics, and they pushed the science as far as the mathematics of their day would permit. Halley, however, was not content with elucidating the motion of bodies nearest the earth, and pressed to the utmost confines of the solar system known to him. Here, too, he made a signal discovery of that mutual disturbance of the planets in their motion round the sun, called the great inequality of Jupiter and Saturn.

Halley's versatile genius attacked all the great problems of the day.

His observation of the sun's total eclipse in 1715 is the earliest reliable account of such a phenomenon by a trained astronomer. He described the corona minutely and was the first to see that other interesting phenomenon which only an alert observer can detect, which a great astronomer of a later day compared to the ”ignition of a fine train of gunpowder,” and which has ever since borne the name of ”Baily's beads.”

Besides being a great astronomer, Halley was a man of affairs as well, which Newton, although the greater mathematician, was not. Without Halley, Newton's superb discovery might easily have been lost to the age and nation, for the latter was bent merely on making discoveries, and on speculative contemplation of them, with never a thought of publis.h.i.+ng to the world.

Halley, more practical and businesslike, insisted on careful writing out and publication. Newton was then only forty-two, and Halley fully fourteen years his junior. But the philosophers of that day were keenly alive to the mystery of Kepler's laws, and Halley was fully conscious of the grandeur and far-reaching significance of Newton's great generalization which embodied all three of Kepler's laws in one.

Newton at last yielded, though reluctantly, and the ”Principia” was given to the world, though wholly at Halley's private charges.

But Halley was far from being completely engrossed with the absorbing problems of the sky; things terrestrial held for years his undivided attention. Imagine present-day Lords Commissioners of the Admiralty intrusting a s.h.i.+p of the British navy to civilian command. Yet such was their confidence in Halley that he was commissioned as captain of H.

M.'s pink _Paramour_ in 1698, with instructions to proceed to southern seas for geographical discoveries, and for improving knowledge of the longitude problem, and of the variations of the compa.s.s. Trade winds and monsoons, charts of magnetic variation, tides and surveys of the Channel coast, and experiments with diving bells were practical activities that occupied his attention.

Halley in 1720 became Astronomer Royal. He was the second inc.u.mbent of this great office, but the first to supply the Royal Observatory with instruments of its own, some of which adorn its walls even to-day. His long series of lunar observations and his magnetic researches were of immense practical value in navigation.

Halley lived to a ripe old age and left the world vastly better than he found it. His rise from humblest obscurity was most remarkable, and he lived to gratify all the ambitions of his early manhood. ”Of attractive appearance, pleasing manners, and ready wit,” says one of his biographers, ”loyal, generous, and free from self-seeking, he was one of the most personally engaging men who ever held the office of Astronomer Royal.”

He died in office at Greenwich in 1742.

”Halley was buried,” says Chambers, ”in the churchyard of St.

Margaret's, Lee, not far from Greenwich, and it has lately been announced that the Admiralty have decided to repair his tomb at the public expense, no descendants of his being known.” There is no suitable monument in England to the memory of one of her greatest scientific men.

In any event the collection and republication of his epoch-making papers would be welcomed by astronomers of every nation.

CHAPTER XVII

BRADLEY AND ABERRATION

Living at Kew in London early in the 18th century was an enthusiastic young astronomer, James Bradley. He is famous chiefly for his accurate observations of star places which have been invaluable to astronomers of later epochs in ascertaining the proper motions of stars.

The lat.i.tude of Bradley's house in Kew was very nearly the same as the declination of the bright star Gamma Draconis, so that it pa.s.sed through his zenith once every day. Bradley had a zenith sector, and with this he observed with the greatest care the zenith distance of Gamma Draconis at every possible opportunity. This he did by pointing the telescope on the star and then recording the small angle of its inclination to a fine plumb line. So accurate were his measures that he was probably certain of the star's position to the nearest second of arc.

What he hoped to find was the star's motion round a very slight orbit once each year, and due to the earth's motion in its...o...b..t round the sun. In other words, he sought to find the star's parallax if it turned out to be a measurable quant.i.ty.

It is just as well now that his method of observation proved insufficiently delicate to reveal the parallax of Gamma Draconis; but his a.s.siduity in observation led him to an unexpected discovery of greater moment at that time. What he really found was that the star had a regular annual orbit; but wholly different from what he expected, and very much larger in amount. This result was most puzzling to Bradley.