Uranus and Neptune
From observations of Uranus made in 1896, M. Leo Brenner concluded that
the planet rotates on its axis in about 8-1/2 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, 373-1/2 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 [Greek: k] Geminorum. The supposed planet would
be faint, its brightness being from 11-1/2 to 13-1/2, 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]