Astronomical Curiosities Part 6

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 (t.i.tan) 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 Ca.s.sini division, and the inner condensation, curious to say, seemed to coincide in position with the "c.r.a.pe 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 const.i.tution 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 Ca.s.sini 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 lat.i.tudes 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 princ.i.p.al 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 Ca.s.segrain 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 t.i.tan 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 43-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...o...b..t 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 t.i.tan 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:--

j.a.petus from 9th magnitude to 12th Rhea " 9 " 106 Dione " 95 " 105 Tethys " 98 " 105 t.i.tan " 8 " 86

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 Ocea.n.u.s, G.o.d 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 Ura.n.u.s. She married Saturn, and became the mother of Vesta, Ceres, Juno, and Pluto.

6. t.i.tan was the eldest son of Ura.n.u.s.

7. Hyperion was the G.o.d of day, and the father of sun and moon.

8. j.a.petus was the fifth son of Ura.n.u.s, and father of Atlas and Prometheus.[174]

9. Phbe was daughter of Ura.n.u.s and Ge.

10. Themis was daughter of Ura.n.u.s and Ge, and, therefore, sister of Phbe.

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 (Phbe) has been fully confirmed, and a tenth satellite discovered.

CHAPTER X

Ura.n.u.s and Neptune

From observations of Ura.n.u.s made in 1896, M. Leo Brenner concluded that the planet rotates on its axis in about 8 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 Ura.n.u.s 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 Ura.n.u.s. 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 p.r.o.nounced 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 _Ura.n.u.s_, 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 Ura.n.u.s 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 301093 (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 154 (water = 1; earth's mean density = 553). 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. La.s.sell 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 Rahu, which is invisible.

It has been surmised that "Rahu" is Ura.n.u.s, 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-gla.s.s of 2 inches aperture he found it quite a conspicuous object.

When Neptune was _visually_ discovered by Galle, at Berlin, he was a.s.sisted 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-gla.s.s. 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-gla.s.s having object-gla.s.ses 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, 519 (earth's mean distance = 1); period of revolution, 373 years; ma.s.s about twice the earth's ma.s.s; 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 11 to 13, 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 1054, 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,"