On the night of September 19, 1903, a star of magnitude 6½ was occulted by the disc of Jupiter. This curious and rare phenomenon was photographed by M. Lucien Rudaux at the Observatory of Donville, France.145 The star was Lalande 45698 (= BAC 8129).146
Prof. Barnard, using telescopes with apertures from 5 inches up to 36 inches (Lick), has failed to see a satellite through the planet’s limb (an observation which has been claimed by other astronomers). He says, “To my mind this has been due to either poor seeing, a poor telescope, or an excited observer.”147 He adds —
“I think it is high time that the astronomers reject the idea that the satellites of Jupiter can be seen through his limb at occultation. When the seeing is bad there is a spurious limb to Jupiter that well might give the appearance of transparency at the occultation of a satellite. But under first-class conditions the limb of Jupiter is perfectly opaque. It is quibbling and begging the question altogether to say the phenomenon of transparency may be a rare one and so have escaped my observations. Has any one said that the moon was transparent when a star has been seen projected on it when it ought to have been behind it?”
Prof. Barnard and Mr. Douglass have seen white polar caps on the third and fourth satellites of Jupiter. The former says they are “exactly like those on Mars.” “Both caps of the fourth satellite have been clearly distinguished, that at the north being sometimes exceptionally large, covering a surface equal to one-quarter or one-third of the diameter of the satellite.”148 This was confirmed on November 23, 1906, when Signor J. Comas Sola observed a brilliant white spot surrounded by a dark marking in the north polar region of the third satellite. There were other dark markings visible, and the satellite presented the appearance of a miniature of Mars.149
An eighth satellite of Jupiter has recently been discovered by Mr. Melotte at the Greenwich Observatory by means of photography. It moves in a retrograde direction round Jupiter in an orbit inclined about 30° to that of the planet. The period of revolution is about two years. The orbit is very eccentric, the eccentricity being about one-third, or greater than that of any other satellite of the solar system. When nearest to Jupiter it is about 9 millions of miles from the planet, and when farthest about 20 millions.150 It has been suggested by Mr. George Forbes that this satellite may possibly be identical with the lost comet of Lexell which at its return in the year 1779 became entangled in Jupiter’s system, and has not been seen since. If this be the case, we should have the curious phenomenon of a comet revolving round a planet!
According to Humboldt the four bright satellites of Jupiter were seen almost simultaneously and quite independently by Simon Marius at Ausbach on December 29, 1609, and by Galileo at Padua on January 7, 1610.151 The actual priority, therefore, seems to rest with Simon Marius, but the publication of the discovery was first made by Galileo in his Nuncius Siderius (1610).152 Grant, however, in his History of Physical Astronomy, calls Simon Marius an “impudent pretender”! (p. 79).
M. Dupret at Algiers saw Jupiter with the naked eye on September 26, 1890, twenty minutes before sunset.153
Humboldt states that he saw Jupiter with the naked eye when the sun was from 18° to 20° above the horizon.154 This was in the plains of South America near the sea-level.
CHAPTER IX
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.”155
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.156 According to Mr. Denning, Saturn bears satisfactorily “greater magnifying power than either Mars or Jupiter.”157
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.158
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.”159
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.”160
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.”161
A remarkable white spot was seen on Saturn on June 23, 1903, by Prof. Barnard, and afterwards by Mr. Denning.162 Another white spot was seen by Denning on July 9 of the same year.163 From numerous observations of these spots, Denning found a rotation period for the planet of about 10h 39m 21s.164 From observations of the same spots Signor Comas Sola found a period 10h 38m·4, a close agreement with Denning’s result. For Saturn’s equator, Prof. Hill found a rotation period of 10h 14m 23s·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.165
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