Uranus and Neptune

From observations of Uranus made in 1896, M. Leo Brenner concluded that the planet rotates on its axis in about 8½ hours (probably 8^h 27^m). This is a short period, but considering the short periods of Jupiter and Saturn there seems to be nothing improbable about it.

Prof. Barnard finds that the two inner satellites of Uranus are difficult objects even with the great 36-inch telescope of the Lick Observatory! They have, however, been photographed at Cambridge (U.S.A.) with a 13-inch lens, although they are “among the most difficult objects known.”[175]

Sir William Huggins in 1871 found strong absorption lines (six strong lines) in the spectrum of Uranus. One of these lines indicated the presence of hydrogen, a gas which does not exist in our atmosphere. Three of the other lines seen were situated near lines in the spectrum of atmospheric air. Neither carbonic acid nor sodium showed any indications of their presence in the planet’s spectrum. A photograph by Prof. Slipher of Neptune’s spectrum “shows the spectrum of this planet to contain many strong absorption bands. These bands are so pronounced in the part of the spectrum between the Fraunhofer lines F and D, as to leave the solar spectrum unrecognizable.... Neptune’s spectrum is strikingly different from that of Uranus, the bands in the latter planet all being reinforced in Neptune. In this planet there are also new bands which have not been observed in any of the other planets. The F line of hydrogen is remarkably dark ... this band is of more than solar strength in the spectrum of Uranus also. Thus free hydrogen seems to be present in the atmosphere of both these planets. This and the other dark bands in these planets bear evidence of an enveloping atmosphere of gases which is quite unlike that which surrounds the earth.”[176]

With the 18-inch equatorial telescope of the Strasburgh Observatory, M. Wirtz measured the diameter of Neptune, and found from forty-nine measures made between December 9, 1902, and March 28, 1903, a value of 2·303 at a distance of 30·1093 (earth’s distance from sun = 1). This gives a diameter of 50,251 kilometres, or about 31,225 miles,[177] and a mean density of 1·54 (water = 1; earth’s mean density = 5·53). Prof. Barnard’s measures gave a diameter of 32,900 miles, a fairly close agreement, considering the difficulty of measuring so small a disc as that shown by Neptune.

The satellite of Neptune was photographed at the Pulkown Observatory in the year 1899. The name Triton has been suggested for it. In the old Greek mythology Triton was a son of Neptune, so the name would be an appropriate one.

The existence of a second satellite of Neptune is suspected by Prof. Schaeberle, who thinks he once saw it with the 36-inch telescope of the Lick Observatory “on an exceptionally fine night” in 1895.[178] But this supposed discovery has not yet been confirmed. Lassell also thought he had discovered a second satellite, but this supposed discovery was never confirmed.[178]

The ancient Burmese mention eight planets, the sun, the moon, Mercury, Venus, Mars, Jupiter, Saturn, and another named RÂhu, which is invisible. It has been surmised that “RÂhu” is Uranus, which is just visible to the naked eye, and may possibly have been discovered by keen eyesight in ancient times. The present writer has seen it several times without optical aid in the West of Ireland, and with a binocular field-glass of 2 inches aperture he found it quite a conspicuous object.When Neptune was visually discovered by Galle, at Berlin, he was assisted in his observation by Prof. d’Arrest. The incident is thus described by Dr. Dreyer, “On the night of June 14, 1874, while observing Coggia’s comet together, I reminded Prof. d’Arrest how he had once said in the course of a lecture, that he had been present at the finding of Neptune, and that ‘he might say it would not have been found without him.’ He then told me (and I wrote it down the next day), how he had suggested the use of Bremiker’s map (as first mentioned by Dr. Galle in 1877) and continued, ‘We then went back to the dome, where there was a kind of desk, at which I placed myself with the map, while Galle, looking through the refractor, described the configurations of the stars he saw. I followed them on the map one by one, until he said: “And then there is a star of the 8th magnitude, in such and such a position,” whereupon I immediately exclaimed: “That star is not on the map.”’”[179] This was the planet. But it seems to the present writer that if Galle or d’Arrest had access to Harding’s Atlas (as they probably had) they might easily have found the planet with a good binocular field-glass. As a matter of fact Neptune is shown in Harding’s Atlas (1822) as a star of the 8th magnitude, having been mistaken for a star by Lalande on May 8 and 10, 1795; and the present writer has found Harding’s 8th magnitude stars quite easy objects with a binocular field-glass having object-glasses of two inches diameter, and a power of about six diameters.

Supposed Planet beyond Neptune.—The possible existence of a planet beyond Neptune has been frequently suggested. From considerations on the aphelia of certain comets, Prof. Forbes in 1880 computed the probable position of such a body. He thought this hypothetical planet would be considerably larger than Jupiter, and probably revolve round the sun at a distance of about 100 times the earth’s mean distance from the sun. The place indicated was between R.A. 11^h 24^m and 12^h 12^m, and declination 0° 0' to 6° 0' north. With a view to its discovery, the late Dr. Roberts took a series of eighteen photographs covering the region indicated. The result of an examination of these photographs showed, Dr. Roberts says, that “no planet of greater brightness than a star of the 15th magnitude exists on the sky area herein indicated.” Prof. W. H. Pickering has recently revived the question, and has arrived at the following results: Mean distance of the planet from the sun, 51·9 (earth’s mean distance = 1); period of revolution, 373½ years; mass about twice the earth’s mass; probable position for 1909 about R.A. 7^h 47^m, north declination 21°, or about 5° south-east of the star ? Geminorum. The supposed planet would be faint, its brightness being from 11½ to 13½, according to the “albedo” (or reflecting power) it may have.[180]

Prof. Forbes has again attacked the question of a possible ultra-Neptunian planet, and from a consideration of the comets of 1556, 1843 I, 1880 I, and 1882 II, finds a mean distance of 105·4, with an inclination of the orbit of 52° to the plane of the ecliptic. This high inclination implies that “during the greatest part of its revolution it is beyond the zodiac,” and this, Mr. W. T. Lynn thinks, “may partly account for its not having hitherto been found by observation.”[181]

From a consideration of the approximately circular shape of the orbits of all the large planets of the solar system, Dr. See suggests the existence of three planets outside Neptune, with approximate distances from the sun of 42, 56, and 72 respectively (earth’s distance = 1), and recommends a photographic search for them. He says, “To suppose the planetary system to terminate with an orbit so round as that of Neptune is as absurd as to suppose that Jupiter’s system terminates with the orbit of the fourth satellite.”[182]

According to Grant, even twenty years before the discovery of Neptune the error of Prof. Adams’ first approximation amounted to little more than 10°.[183]