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153. It has been already mentioned how closely Galilei was followed by other astronomers (if not in some cases actually anticipated) in most of his telescopic discoveries. To his rival Christopher Scheiner (chapter VI., §§ 124, 125) belongs the credit of the discovery of bright cloud-like objects on the sun, chiefly visible near its edge, and from their brilliancy named faculae (little torches). Scheiner made also a very extensive series of observations of the motions and appearances of spots.
The study of the surface of the moon was carried on with great care by John Hevel of Danzig (1611-1687), who published in 1647 his Selenographia, or description of the moon, magnificently illustrated by plates engraved as well as drawn by himself. The chief features of the moon—mountains, craters, and the dark spaces then believed to be seas—were systematically described and named, for the most part after corresponding features of our own earth. Hevel’s names for the chief mountain ranges, e.g. the Apennines and the Alps, and for the seas, e.g. Mare Serenitatis or Pacific Ocean, have lasted till to-day; but similar names given by him to single mountains and craters have disappeared, and they are now called after various distinguished men of science and philosophers, e.g. Plato and Coppernicus, in accordance with a system introduced by John Baptist Riccioli (1598-1671) in his bulky treatise on astronomy called the New Almagest (1651).
Hevel, who was an indefatigable worker, published two large books on comets, Prodromus Cometicus (1654) and Cometographia (1668), containing the first systematic account of all recorded comets. He constructed also a catalogue of about 1,500 stars, observed on the whole with accuracy rather greater than Tycho’s, though still without the use of the telescope; he published in addition an improved set of tables of the sun, and a variety of other calculations and observations.
154. The planets were also watched with interest by a number of observers, who detected at different times bright or dark markings on Jupiter, Mars, and Venus. The two appendages of Saturn which Galilei had discovered in 1610 and had been unable to see two years later (chapter VI., § 123) were seen and described by a number of astronomers under a perplexing variety of appearances, and the mystery was only unravelled, nearly half a century after Galilei’s first observation, by the greatest astronomer of this period, Christiaan Huygens (1629-1695), a native of the Hague. Huygens possessed remarkable ability, both practical and theoretical, in several different directions, and his contributions to astronomy were only a small part of his services to science. Having acquired the art of grinding lenses with unusual accuracy, he was able to construct telescopes of much greater power than his predecessors. By the help of one of these instruments he discovered in 1655 a satellite of Saturn (Titan). With one of those remnants of mediaeval mysticism from which even the soberest minds of the century freed themselves with the greatest difficulty, he asserted that, as the total number of planets and satellites now reached the perfect number 12, no more remained to be discovered—a prophecy which has been abundantly falsified since (§ 160; chapter XII., §§ 253, 255; chapter XIII., §§ 289, 294, 295).
Using a still finer telescope, and aided by his acuteness in interpreting his observations, Huygens made the much more interesting discovery that the puzzling appearances seen round Saturn were due to a thin ring (fig. 64) inclined at a considerable angle (estimated by him at 31°) to the plane of the ecliptic, and therefore also to the plane in which Saturn’s path round the sun lies. This result was first announced—according to the curious custom of the time—by an anagram, in the same pamphlet in which the discovery of the satellite was published, De Saturni Luna Observatio Nova (1656); and three years afterwards (1659) the larger Systema Saturnium appeared, in which the interpretation of the anagram was given, and the varying appearances seen both by himself and by earlier observers were explained with admirable lucidity and thoroughness. The ring being extremely thin is invisible either when its edge is presented to the observer or when it is presented to the sun, because in the latter position the rest of the ring catches no light. Twice in the course of Saturn’s revolution round the sun (at B and D in fig. 66), i.e. at intervals of about 15 years, the plane of the ring passes for a short time through or very close both to the earth and to the sun, and at these two periods the ring is consequently invisible (fig. 65). Near these positions (as at Q, R, S, T) the ring appears much foreshortened, and presents the appearance of two arms projecting from the body of Saturn; farther off still the ring appears wider and the opening becomes visible; and about seven years before and after the periods of invisibility (at A and C) the ring is seen at its widest. Huygens gives for comparison with his own results a number of drawings by earlier observers (reproduced in fig. 67), from which it may be seen how near some of them were to the discovery of the ring.
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155. To our countryman William Gascoigne (1612?-1644) is due the first recognition that the telescope could be utilised, not merely for observing generally the appearances of celestial bodies, but also as an instrument of precision, which would give the directions of stars, etc., with greater accuracy than is possible with the naked eye, and would magnify small angles in such a way as to facilitate the measurement of angular distances between neighbouring stars, of the diameters of the planets, and of similar quantities. He was unhappily killed when quite a young man at the battle of Marston Moor (1644), but his letters, published many years afterwards shew that by 1640 he was familiar with the use of telescopic “sights,” for determining with accuracy the position of a star, and that he had constructed a so-called micrometer95 with which he was able to measure angles of a few seconds. Nothing was known of his discoveries at the time, and it was left for Huygens to invent independently a micrometer of an inferior kind (1658), and for Adrien Auzout (?-1691) to introduce as an improvement (about 1666) an instrument almost identical with Gascoigne’s.
The systematic use of telescopic sights for the regular work of an observatory was first introduced about 1667 by Auzout’s friend and colleague Jean Picard (1620-1682).
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156. With Gascoigne should be mentioned his friend Jeremiah Horrocks (1617?-1641), who was an enthusiastic admirer of Kepler and had made a considerable improvement in the theory of the moon, by taking the elliptic orbit as a basis and then allowing for various irregularities. He was the first observer of a transit of Venus, i.e. a passage of Venus over the disc of the sun, an event which took place in 1639, contrary to the prediction of Kepler in the Rudolphine Tables, but in accordance with the rival tables of Philips von Lansberg (1561-1632) which Horrocks had verified for the purpose. It was not, however, till long afterwards that Halley pointed out the importance of the transit of Venus as a means of ascertaining the distance of the sun from the earth (chapter X., § 202). It is also worth noticing that Horrocks suggested the possibility of the irregularities of the moon’s motion being due to the disturbing action of the sun, and that he also had some idea of certain irregularities in the motion of Jupiter and Saturn, now known to be due to their mutual attraction (chapter X., § 204; chapter XI., § 243).
157. Another of Huygens’s discoveries revolutionised the art of exact astronomical observation. This was the invention of the pendulum-clock (made 1656, patented in 1657). It has been already mentioned how the same discovery was made by Bürgi, but virtually lost (see chapter V., § 98); and how Galilei again introduced the pendulum as a time-measurer (chapter VI., § 114). Galilei’s pendulum, however, could only be used for measuring very short times, as there was no mechanism to keep it in motion, and the motion soon died away. Huygens attached a pendulum to a clock driven by weights, so that the clock kept the pendulum going and the pendulum regulated the clock.96 Henceforward it was possible to take reasonably accurate time-observations, and, by noticing the interval between the passage of two stars across the meridian, to deduce, from the known rate of motion of the celestial sphere, their angular distance east and west of one another, thus helping to fix the position of one with respect to the other. It was again Picard (§ 155) who first recognised the astronomical importance of this discovery, and introduced regular time-observations at the new Observatory of Paris.
158. Huygens was not content with this practical use of the pendulum, but worked out in his treatise called Oscillatorium Horologium or The Pendulum Clock (1673) a number of important results in the theory of the pendulum, and in the allied problems connected with the motion of a body in a circle or other curve. The greater part of these investigations lie outside the field of astronomy, but his formula connecting the time of oscillation of a pendulum with its length and the intensity of gravity97 (or, in other words, the rate of falling of a heavy body) afforded a practical means of measuring gravity, of far greater accuracy than any direct experiments on falling bodies; and his study of circular motion, leading to the result that a body moving in a circle must be acted on by some force towards the centre, the magnitude of which depended in a definite way on the speed of the body and the size of the circle,98 is of fundamental importance in accounting for the planetary motions by gravitation.
159. During the 17th century also the first measurements of the earth were made which were a definite advance on those of the Greeks and Arabs (chapter II., §§ 36, 45, and chapter III., § 57). Willebrord Snell (1591-1626), best known by his discovery of the law of refraction of light, made a series of measurements in Holland in 1617, from which the length of a degree of a meridian appeared to be about 67 miles, an estimate subsequently altered to about 69 miles by one of his pupils, who corrected some errors in the calculations, the result being then within a few hundred feet of the value now accepted. Next, Richard Norwood (1590?-1675) measured the distance from London to York, and hence obtained (1636) the length of the degree with an error of less than half a mile. Lastly, Picard in 1671 executed some measurements near Paris leading to a result only a few yards wrong. The length of a degree being known, the circumference and radius of the earth can at once be deduced.
160. Auzout and Picard were two members of a
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