Much progress has been made during the last hundred years in our knowledge of the planets. In fact, the study of Mercury only dates from the commencement of the nineteenth century. Our knowledge of the vicinity of the Sun is very limited, and Mercury is difficult of observation. So limited, in fact, is our knowledge of the Sun’s surroundings, that it is not yet known for certain whether there is a planet, or planets, between Mercury and the Sun. Perturbations in the motion of the perihelion of Mercury’s orbit led Le Verrier in 1859 to the belief that a planet of about the size of Mercury, or else a zone of asteroids, existed between Mercury and the Sun. It was, however, obvious that such a planet could only be seen when in transit across the Sun’s disc, or during a total eclipse. Meanwhile a French doctor, Lescarbault, informed Le Verrier that he had seen a round object in transit over the Sun’s disc. Le Verrier, certain that this was the missing planet, named it “Vulcan,” and calculated its orbit, assigning it a revolution period of twenty days. But it was never seen again. Transits of “Vulcan” were fixed for 1877 and 1882, but nothing was seen on these dates. During the total eclipse of July 29, 1878, two observers—James Watson (1838-1880), the well-known astronomer, and Lewis Swift (born 1820)—believed themselves to have discovered two separate planets, and ultimately claimed two planets each, which were never heard of again. During the total eclipse of 1883 an active watch for “suspicious objects” was kept, but with no result. At the eclipses of 1900 and 1901 respectively, photographs were exposed by the American astronomers, W. H. Pickering and Charles Dillon Perrine (born 1867), but on none of these plates could any trace of “Vulcan” be found. At the total eclipse of August 30, 1905, plates were again exposed, but no announcement has been made of an intra-Mercurial planet; and the prevalent opinion among astronomers is that no planet comparable with Mercury in size exists between that planet and the Sun.
The study of the physical appearance of Mercury was inaugurated by Schröter, who in 1800 noticed that the southern horn of the crescent presented a blunted appearance, which he attributed to the existence of a mountain eleven miles in height. From observations of this mountain he came to the conclusion that the planet rotated in 24 hours 4 minutes. This was afterwards reduced by Friedrich Wilhelm Bessel (1784-1846) to 24 hours 53 seconds.
After the time of Schröter there was no astronomer who paid much attention to either Mercury or Venus until the arrival on the scene of the most persistent planetary observer and one of the foremost astronomers of the nineteenth century. Giovanni Virginio Schiaparelli was born at Savigliano, in Piedmont, in 1835, and graduated at Turin in 1854. Called to Milan as assistant in the Brera Observatory in 1860, he became director in 1862, and there for thirty-eight years he studied astronomy in all its aspects, making a great name for himself in various branches of the science. In 1900 he retired from the post of director, and pursues his astronomical researches in his retirement.
In 1882 Schiaparelli took up the study of Mercury in the clear air of Milan. Instead of observing the planet through the evening haze, like Schröter and others, he examined it by day, and was enabled to follow it hourly instead of looking at it for a short period when near the horizon. At length, after seven years’ observation, he announced, on December 8, 1889, that Mercury performs only one rotation during its revolution round the Sun—in fact, that its day and year coincide. As a consequence, the planet keeps the same face towards the Sun, one side having everlasting day and the other perpetual night; but owing to the libratory movement of Mercury—the result of uniform motion on its axis and irregular motion in its orbit—the Sun rises and sets on a small zone of the planet’s surface. Schiaparelli’s observations indicated that Mercury is a much spotted globe, with a moderately dense atmosphere, and he was enabled to form a chart of its surface-markings.
Schiaparelli’s conclusions remained until 1896 unconfirmed and yet not denied, although most astronomers were sceptical on the subject. In 1896 the subject was taken up by the American astronomer, Percival Lowell (born 1855), who, in the clear air of Arizona, confirmed Schiaparelli’s conclusions, fixing 88 days as the period of rotation. He remarked, however, that no signs of an atmosphere or clouds were visible to him. The surface of Mercury, he says, is colourless,—“a geography in black and white.” The determination of the rotation period by Schiaparelli and Lowell is now generally accepted, and is confirmed by the theory of tidal friction. It is only right to add that William Frederick Denning (born 1848) in 1881 suspected a rotation period of 25 hours, but this remains unconfirmed. In April 1871 the spectrum of Mercury was examined by Hermann Carl Vogel (born 1842) at Bothkamp. He suspected traces of an atmosphere similar to ours, but was not certain. Of more interest are the photometric observations of Zöllner in 1874. These observations indicated that the surface of Mercury is rugged and mountainous, and comparable with the Moon,—a conclusion supported by Lowell’s observations in 1896.
Venus, the nearest planet to the Earth, has been attentively studied for three centuries, and still comparatively little is known regarding it. This is due to its remarkable brilliancy, combined with its proximity to the Sun. The great problem at the beginning of the nineteenth century was the rotation of the planet. In 1779 the subject was taken up by Schröter at Lilienthal. Nine years later, from a faint streak visible on the disc, he concluded that rotation was performed in 23 hours 28 minutes, and in 1811 this was reduced by seven minutes; but as Herschel was unable to observe the markings seen by Schröter, many astronomers were inclined to be sceptical regarding the accuracy of the Lilienthal observers results. Schröter also observed the southern horn of Venus when in the crescent form to be blunted, and he ascribed this to the existence of a great mountain, five or six times the elevation of Chimborazo; while he observed irregularities along the terminator, which he considered to be more strongly marked than those on the Moon. Schröter’s opinion on this point, although rejected by Herschel, was confirmed by Mädler, Zenger, Ertborn, Denning, and by the Italian astronomer Francesco Di Vico (1805-1848), director of the Observatory of the Collegio Romano. In 1839 Di Vico attacked the problem of the rotation, and his results were confirmatory of those of Schröter. He estimated that the axis of Venus was inclined at an angle of 53° to the plane of its orbit. Meanwhile a series of important observations had been made on Venus by the Scottish astronomer and theologian, Thomas Dick (1772-1857), who suggested daylight observations on Venus to solve the problem of the rotation.
In 1877 the question was attacked by Schiaparelli, who commenced a series of observations on Venus at Milan in that year. The results of his studies were summed up in 1890 in five papers contributed to the Milan Academy. He came to the conclusion that the markings observed by Schröter, Di Vico, and others were not really permanent, and concentrated his attention on round white spots, which remained fixed in position. Instead of observing Venus in the evening, Schiaparelli followed it by day, watching it continuously on one occasion for eight hours. But the markings remained fixed. Schiaparelli accordingly concluded that the planet’s rotation was performed in probably 225 days, equal to the time of revolution. One face is turned towards the Sun continually, while the other is perpetually in darkness.
The announcement was so startling that, as Miss Clerke says, “a clamour of contradiction was immediately raised, and a large amount of evidence on both sides of the question has since been collected.” Perrotin at Nice, Tacchini at Rome, Cerulli at Teramo, Mascari at Catania and Mount Etna, and Lowell in Arizona, all in favourable climates, confirmed Schiaparelli’s results, as also did a second series of observations by the Milan astronomer himself in 1895. On the other hand, Neisten, Trouvelot, Camille Flammarion (born 1842), and others, under less favourable climatic conditions, arrived at a period of 24 hours. Aristarch Bélopolsky (born 1854), from spectroscopic observations at Pulkowa, by means of Doppler’s principle, found a period of 12 hours. Lowell, by the same principle, found, in 1901-03, a period of 225 days, in agreement with Schiaparelli’s results. This is the last word on the subject. Schiaparelli’s rotation period, confirmed by the theory of tidal friction, is generally accepted.
That Venus has an atmosphere was one of the conclusions reached by Schröter in 1792; and in this at least he was correct, as the atmosphere of Venus, illuminated by the solar rays, has been seen extending round the entire disc of the planet. Spectroscopic observations by Tacchini, Ricco, and Young, during the transits of 1874 and 1882, indicated the existence of water-vapour in the planet’s atmosphere. Very little has been discovered regarding the “geography” of Venus. White patches at the supposed “poles” of the planet were observed in 1813 by Franz von Gruithuisen, and in 1878 by the French astronomer Trouvelot (1827-1895). The secondary light of Venus, similar to the “old Moon in the new Moon’s arms,” was repeatedly observed since the time of Schröter by Vogel, Lohse, Zenger, and others. Vogel attributed it to twilight, and Lamp, a German observer, to electrical processes analogous to our auroræ. In 1887 a Belgian astronomer, Paul Stroobant, submitted to a searching examination all the supposed observations of a satellite of Venus, and was enabled to explain nearly all the supposed satellites as small stars which happened to lie near the planet’s path in the sky at the time of observation.
The study of our own planet can hardly be said to belong to the realm of astronomy. Nevertheless, it is through astronomical observation that the motion of the North Pole has been discovered. For many years it has been a problem whether there is a variation of latitude resulting from the motion of the pole. Euler had declared, from theoretical investigation, that, were there such a motion, the period must be 10 months. The question was revived in 1885 by the observations of Seth Carlo Chandler (born 1846) at Cambridge, Mass., with his newly-invented instrument, the “almucantar,” which indicated an appreciable variation of latitude. This was confirmed by Friedrich Küstner (born 1856), now director of the Observatory at Bonn. The idea now occurred to Chandler to search through the older records to discover if there was any trace of the variation of latitude, with the result that he brought out a period of 14 months instead of 10. This aroused much interest, and many prominent astronomers denied Chandler’s results, which were announced in 1891. As a well-known astronomer has expressed it, “Euler’s work had shown what period the motion must have, and any appearance of another period must be due to some error in the observations. Chandler replied to the effect that he did not care for Euler’s mathematics: the observations plainly showed 14 months, and if Euler said 10, he must have made the mistake. I do not exaggerate the situation in the least; it was a deadlock: Chandler and observation against the whole weight of observation and theory.” It was now shown by Newcomb that Euler had assumed the Earth to be an absolutely rigid body, while modern investigations show that it is not so. Chandler’s discovery is now accepted, and proves that the North Pole is not fixed in position, but has a small periodic motion, though never twelve yards from its mean position. That the small resulting variation in the position of the stars has been noticed at all is a striking illustration of the accuracy of astronomical observation.