Saturn
To show the advantages of large telescopes over small ones, Mr. C. Roberts
says that "with the 25-inch refractor of the Cambridge Observatory the
view of the planet Saturn is indescribably glorious; everything I had ever
seen before was visible at a glance, and an enormous amount of detail that
I had never even glimpsed before, after a few minutes' observation."[156]
Chacornac found that the illumination of Saturn's disc is the reverse of
that of Jupiter, the edges of Saturn being brighter than the centre of the
disc, while in the case of Jupiter--as in that of the sun--the edges are
fainter than the centre.[157] According to Mr. Denning, Saturn bears
satisfactorily "greater magnifying power than either Mars or
Jupiter."[158]
At an occultation of Saturn by the moon, which occurred on June 13, 1900,
M. M. Honorat noticed the great contrast between the slightly yellowish
colour of the moon and the greenish tint of the planet.[159]
In the year 1892, when the rings of Saturn had nearly disappeared, Prof.
L. W. Underwood, of the Underwood Observatory, Appleton, Wisconsin
(U.S.A.), saw one of Saturn's satellites (Titan) apparently moving along
the needlelike appendage to the planet presented by the rings. "The
apparent diameter of the satellite so far exceeded the apparent thickness
of the ring that it gave the appearance of a beautiful golden bead moving
very slowly along a fine golden thread."[160]
In 1907, when the rings of Saturn became invisible in ordinary telescopes,
Professor Campbell, observing with the great Lick telescope, noticed
"prominent bright knots, visible ... in Saturn's rings. The knots were
symmetrically placed, two being to the east and two to the west." This was
confirmed by Mr. Lowell, who says, "Condensations in Saturn's rings
confirmed here and measured repeatedly. Symmetric and permanent." This
phenomenon was previously seen by Bond in the years 1847-56. Measures of
these light spots made by Prof. Barnard with the 40-inch Yerkes telescope
show that the outer one corresponded in position with the outer edge of
the middle ring close to the Cassini division, and the inner condensation,
curious to say, seemed to coincide in position with the "crape ring."
Prof. Barnard thinks that the thickness of the rings "must be greatly
under 100 miles, and probably less than 50 miles," and he says--
"The important fact clearly brought out at this apparition of _Saturn_
is that the bright rings are not opaque to the light of the sun--and
this is really what we should expect from the nature of their
constitution as shown by the theory of Clerk Maxwell, and the
spectroscopic results of Keeler."[161]
Under certain conditions it would be theoretically possible, according to
Mr. Whitmell, to see the globe of Saturn through the Cassini division in
the ring. But the observation would be one of great difficulty and
delicacy. The effect would be that, of the arc of the division which
crosses the planet's disc, "a small portion will appear bright instead of
dark, and may almost disappear."[162]
A remarkable white spot was seen on Saturn on June 23, 1903, by Prof.
Barnard, and afterwards by Mr. Denning.[163] Another white spot was seen
by Denning on July 9 of the same year.[164] From numerous observations of
these spots, Denning found a rotation period for the planet of about
10{h} 39{m} 21{s}.[165] From observations of the same spots Signor Comas
Sola found a period 10{h} 38{m}·4, a close agreement with Denning's
result. For Saturn's equator, Prof. Hill found a rotation period of 10{h}
14{m} 23{s}·8, so that--as in the case of Jupiter--the rotation is faster
at the equator than in the northern latitudes of the planet. A similar
phenomenon is observed in the sun. Mr. Denning's results were fully
confirmed by Herr Leo Brenner, and other German astronomers.[166]
Photographs taken by Prof. V. M. Slipher in America show that the spectrum
of Saturn is similar to that of Jupiter. None of the bands observed in the
planet's spectrum are visible in the spectrum of the rings. This shows
that if the rings possess an atmosphere at all, it must be much rarer than
that surrounding the ball of the planet. Prof. Slipher says that "none of
the absorption bands in the spectrum of _Saturn_ can be identified with
those bands due to absorption in the earth's atmosphere," and there is no
trace of aqueous vapour.[167]
In September, 1907, M. G. Fournier suspected the existence of a "faint
transparent and luminous ring" outside the principal rings of Saturn. He
thinks that it may possibly be subject to periodical fluctuations of
brightness, sometimes being visible and sometimes not.[168] This dusky
ring was again suspected at the Geneva Observatory in October, 1908.[169]
M. Schaer found it a difficult object with a 16-inch Cassegrain reflector.
Prof. Stromgen at Copenhagen, and Prof. Hartwig at Bamberg, however,
failed to see any trace of the supposed ring.[170] It was seen at
Greenwich in October, 1908.
A "dark transit" of Saturn's satellite Titan across the disc of the planet
has been observed on several occasions. It was seen by Mr. Isaac W. Ward,
of Belfast, on March 27, 1892, with a 4·3-inch Wray refractor. The
satellite appeared smaller than its shadow. The phenomenon was also seen
on March 12 of the same year by the Rev. A. Freeman, Mr. Mee, and M. F.
Terby; and again on November 6, 1907, by Mr. Paul Chauleur and Mr. A. B.
Cobham.[171]
The recently discovered tenth satellite of Saturn, Themis, was discovered
by photography, and has never been seen by the eye even with the largest
telescopes! But its existence is beyond all doubt, and its orbit round the
planet has been calculated.
Prof. Hussey of the Lick Observatory finds that Saturn's satellite Mimas
is probably larger than Hyperion. He also finds from careful measurements
that the diameter of Titan is certainly overestimated, and that its
probable diameter is about 2500 miles.[172]
The French astronomer, M. Lucien Rudaux, finds the following variation in
the light of the satellites of Saturn:--
Japetus from 9th magnitude to 12th
Rhea " 9 " 10·6
Dione " 9·5 " 10·5
Tethys " 9·8 " 10·5
Titan " 8 " 8·6
The variation of light is, he thinks, due to the fact that the period of
rotation of each satellite is equal to that of their revolution round the
planet; as in the case of our moon.[173]
The names of the satellites of Saturn are derived from the ancient heathen
mythology. They are given in order of distance from the planet, the
nearest being Mimas and the farthest Themis.
1. Mimas was a Trojan born at the same time as Paris.
2. Enceladus was son of Tartarus and Ge.
3. Tethys was wife of Oceanus, god of ocean currents. She became mother of
all the chief rivers in the universe, as also the Oceanides or sea nymphs.
4. Dione was one of the wives of Zeus.
5. Rhea was a daughter of Uranus. She married Saturn, and became the
mother of Vesta, Ceres, Juno, and Pluto.
6. Titan was the eldest son of Uranus.
7. Hyperion was the god of day, and the father of sun and moon.
8. Japetus was the fifth son of Uranus, and father of Atlas and
Prometheus.[174]
9. Phoebe was daughter of Uranus and Ge.
10. Themis was daughter of Uranus and Ge, and, therefore, sister of
Phoebe.
In a review of Prof. Comstock's _Text Book of Astronomy_ in _The
Observatory_, November, 1901, the remark occurs, "We are astonished to see
that Mr. Comstock alludes with apparent seriousness to the _nine_
satellites of Saturn. As regards the ninth satellite, we thought that all
astronomers held with Mrs. Betsy Prig on the subject of this astronomical
Mrs. Harris." This reads curiously now (1909) when the existence of the
ninth satellite (Phoebe) has been fully confirmed, and a tenth satellite
discovered.