A Century's Progress in Astronomy

by Hector Macpherson

On completing his famous 40-foot reflector, Herschel, on August 28, 1789, turned it on Saturn and its five known satellites. Near the planet, and in the plane of the ring, was seen another object, which Herschel believed to be a sixth satellite. To settle the question, he watched the planet for several hours to see if the object would partake in the planet’s motion. Finding that it did, he announced it as a new satellite, which he found to revolve round Saturn in 1 day 8 hours. About three weeks later, on September 17, Herschel discovered another satellite yet closer to Saturn, revolving round the planet in about 22 hours. These two satellites were not seen by any astronomers except Herschel; and after his death they could not be observed. His son, however, rediscovered them.

The eighth satellite, Japetus, was shown by Herschel to rotate on its axis in a period equal to that of its revolution, and his observations were confirmed by modern observers. “I cannot,” Herschel said, “help reflecting with some pleasure on the discovery of an analogy which shows that a certain uniform plan is carried on among the secondaries of our Solar System; and we may conjecture that probably most of the satellites are governed by the same law.” In April 1805 Herschel observed the globe of Saturn to present not a spherical but a “square-shouldered” aspect. It was for long believed that this was an optical illusion; but Proctor and others have shown that it is quite possible for storms in Saturn’s atmosphere to cause the planet’s apparent distortion in shape.

Herschel paid much attention to the planet Uranus, which he discovered on March 13, 1781. The discovery of Uranus, which was mentioned in a previous chapter, was in a sense the most striking of Herschel’s achievements. Uranus was the first planet discovered within the memory of man: besides, the discovery enlarged the diameter of the Solar System from 886 to 1772 millions of miles. Throughout his lifetime Herschel referred to the planet as the “Georgium Sidus,” out of gratitude to George III. for appointing him King’s Astronomer; but the astronomers of France and Germany, who, as Sir Robert Ball remarks, “saw no reason why the King of England should be associated with Jupiter and Saturn,” opposed this term. Lalande called the planet “Herschel,” but Herschel’s countrymen, the Germans, named it Uranus, in keeping with the custom of designating the planets from the Greek mythology. The name of Uranus ultimately prevailed.

In January 1787 Herschel discovered two satellites to Uranus, with the aid of his 20-foot telescope. These satellites he believed to revolve round Uranus in 8 days and 13 days respectively, and accordingly he made a drawing of what their positions should be on February 10. On that day he found them in their predicted places. In 1797 he announced that the satellites revolved round Uranus in orbits at right angles to the ecliptic, and in a retrograde direction. In subsequent years Herschel believed that he had discovered other four satellites to Uranus, but he was unable to confirm his belief. As Mr Gore says, some of the satellites “must, therefore, have been either optical ‘ghosts’ or else small fixed stars which happened to be near the planet’s path at the time of observation. Herschel also suspected that he could see traces of rings round Uranus like those round Saturn, but his observation was never confirmed, either by himself or other observers.”

Although Herschel made several important observations on the Moon, and measured the heights of the lunar mountains, he was not a devoted student of our satellite. Caroline Herschel remarks in her memoirs that if it had not been for clouds or moonlight, neither her brother nor herself would have got any sleep; adding that Herschel on the moonlight nights prepared his papers or made visits to London. However, he did make some investigations, and in 1783 and 1787 believed himself to have witnessed the eruption of three lunar volcanoes. He afterwards concluded, however, that what he believed to be eruptions was really the reflexion of earth-shine from the white peaks of the lunar mountains. Herschel never discovered a comet, leaving that branch of astronomy to his sister, who discovered eight of these objects. He was, however, much interested in comets, and attentively studied them, introducing the terms of “head,” “nucleus,” and “coma.” Herschel anticipated the view that comets are not lasting, but are partly disintegrated at their perihelion passages. He was of opinion that they travelled from star to star. The extent of their tails and appendages he thought to be a test of their age.

We have now completed our sketch of Herschel’s important labours regarding our Solar System. As Miss Clerke says, “A whole cycle of discoveries and successful investigations began and ended with him.” But through observing the stars he made a further discovery in connection with the Solar System; indeed, one of the greatest discoveries in the history of astronomy—the movement through space of the Sun, carrying with it planets and comets.

“If the proper motion of the stars be admitted,” said Herschel, “who can deny that of our Sun?” Of course it was plain that the motion of the Sun could only be detected through the resulting apparent motion of the stars. Thus, if the Sun is moving in a certain direction, the stars in front will appear to open out, while those behind will close up. But the problem is by no means so easy as this. The stars are also in motion, and, before the solar motion can be discovered, the proper motions of the stars—themselves very minute—have to be decomposed into two parts, the real motion of the star, and the apparent motion, resulting from the movement of the Solar System. To any astronomer but Herschel the problem would have been insoluble. Only sixty years had elapsed since Halley had announced the proper motions of the brighter stars which had been previously supposed to be immovable—hence the name of “fixed stars.” Herschel did not deal with the motions of many stars. Only a few proper motions were known with accuracy when he attacked the problem in 1783. Making use of the proper motions of seven stars, and separating the real from the apparent motion, he found that the Solar System was moving towards a point in the constellation Hercules, the “apex” being marked by the star λ Herculis. The rate of the solar motion, Herschel thought, was “certainly not less than that which the Earth has in her annual orbit.” This extraordinary discovery was one of Herschel’s greatest works. “Its directness and apparent artlessness,” Miss Clerke remarks, “strike us dumb with wonder.” In 1805 Herschel again attacked the subject, utilising the proper motions of thirty-six stars. His second inquiry, on the whole, confirmed his previous result, the “apex” being again situated in Hercules; but the determination of 1783 was probably the more accurate of the two.

Herschel was far in advance of his time regarding the solar motion. The two greatest astronomers of the next generation, Bessel and Sir John Herschel, rejected the results reached by Sir William Herschel. But in 1837 Argelander, after a profound mathematical discussion, confirmed Herschel’s views, and proved the solar motion to be a reality. Since that date the problem has been attacked by various methods by Otto Struve, Gauss, Mädler, Airy, Dunkin, Ludwig Struve, Newcomb, Kapteyn, Campbell, and others, with the result that the reality of the solar motion and of the direction fixed by Herschel has been proved beyond a doubt. As Sir Robert Ball well remarks, mathematicians have exhausted every refinement, “but only to confirm the truth of that splendid theory which seems to have been one of the flashes of Herschel’s genius.”

In his volume ‘Herschel and his Work,’ Mr James Sime writes: “To Herschel belongs the credit not merely of having suspected the revolution of sun around sun in the far-distant realms of space, but also of actually detecting that this was going on among the stars.” Throughout his career double stars were favourite objects of observation. The study of double stars was commenced by Herschel while a musician in Bath. Before his day, of course, double stars had been discovered and studied, but it was believed that the proximity of two stars was merely an optical accident, the brighter star being much nearer to us than the other. Herschel, at first sharing the general view, observed double stars in the hope of measuring their relative parallaxes; assuming one star to be much farther away from the Solar System than another, he attempted to measure the parallactic displacement of the brighter star relatively to the position of the fainter. “This,” he afterwards wrote, “introduced a new series of observations. I resolved to examine every star in the heavens with the utmost attention, that I might fix my observations upon those that would best answer my end. I took some pains to find out what double stars had been recorded by astronomers; but my situation permitted me not to consult extensive libraries, nor, indeed, was it very material; for as I intended to view the heavens myself, Nature, that great volume, appeared to me to contain the best catalogue.”

Herschel, on January 10, 1782, submitted to the Royal Society a catalogue of 269 double stars: of these he himself discovered 227. In December 1784 he forwarded another catalogue, containing 434 stars. He soon found that he was unable to measure stellar parallax, and the idea dawned on him that the double stars were physically connected by the law of gravitation, though he made no announcement to that effect for many years. On July 1, 1802, Herschel informed the Royal Society that “casual situations will not account for the multiplied phenomena of double stars.... I shall soon communicate a series of observations, proving that many of them have already changed their situation in a progressive course, denoting a periodical revolution round each other.” In 1803 he showed that many stars were revolving round their centres of gravity, proving them, in his own words, to be “intimately held together by the bond of mutual attraction.” In other words, Herschel discovered that the law of gravitation prevailed in the Stellar Universe, as well as in our Solar System—that the law which Newton ascertained to prevail in the Solar System extended throughout the depth of space.