With reference to the suggestion, recently made, that a portion, at least, of the sun’s heat may be due to radium, and the experiments which have been made with negative results, Mr. R. T. Strutt – the eminent physicist – has made some calculations on the subject and says, “even if all the sun’s heat were due to radium, there does not appear to be the smallest possibility that the Becquerel radiation from it could ever be detected at the earth’s surface.”10
The eminent Swedish physicist Arrhenius, while admitting that a large proportion of the sun’s heat is due to contraction, considers that it is probably the chemical processes going on in the sun, and not the contraction which constitute the chief source of the solar heat.11
As the centre of gravity of the sun and Jupiter lies at a distance of about 460,000 miles from the sun’s centre, and the sun’s radius is only 433,000 miles, it follows that the centre of gravity of the sun and planet is about 27,000 miles outside the sun’s surface. The attractions of the other planets perpetually change the position of the centre of gravity of the solar system; but in some books on astronomy it is erroneously stated that the centre of gravity of the system is always within the sun’s surface. If all the planets lay on the same side of the sun at the same time (as might possibly happen), then the centre of gravity of the whole system would lie considerably more than 27,000 miles outside the sun’s surface.
With reference to the sun’s great size, Carl Snyder has well said, “It was as if in Vulcan’s smithy the gods had moulded one giant ball, and the planets were but bits and small shot which had spattered off as the glowing ingot was cast and set in space. Little man on a little part of a little earth – a minor planet, a million of which might be tumbled into the shell of the central sun – was growing very small; his wars, the convulsions of a state, were losing consequence. Human endeavour, human ambitions could now scarce possess the significance they had when men could regard the earth as the central fact of the universe.”12
With reference to the late Prof. C. A. Young (U.S.A.) – a great authority on the sun – an American writer has written the following lines: —
“The destined course of whirling worlds to trace,
To plot the highways of the universe,
And hear the morning stars their song rehearse,
And find the wandering comet in his place;
This is the triumph written in his face,
And in the gleaming eye that read the sun
Like open book, and from the spectrum won
The secrets of immeasurable space.”13
CHAPTER II
Mercury
As the elongation of Mercury from the sun seldom exceeds 18°, it is a difficult object, at least in this country, to see without a telescope. As the poet says, the planet —
“Can scarce be caught by philosophic eye
Lost in the near effulgence of its blaze.”
Tycho Brahé, however, records several observations of Mercury with the unaided vision in Denmark.
It can be occasionally caught with the naked eye in this country after sunset, when it is favourably placed for observation, and I have so seen it several times in Ireland. On February 19, 1888, I found it very visible in strong twilight near the western horizon, and apparently brighter than an average star of the first magnitude would be in the same position. In the clear air of the Punjab sky I observed Mercury on November 24-29, 1872, near the western horizon after sunset. Its appearance was that of a reddish star of the first magnitude. On November 29 I compared its brilliancy with that of Saturn, which was some distance above it, and making allowance for the glare near the horizon in which Mercury was immersed, its brightness appeared to me to be quite equal to that of Saturn. In June, 1874, I found it equal to Aldebaran, and of very much the same colour. Mr. W. F. Denning, the famous observer of meteors, states that he observed Mercury with the naked eye about 150 times during the years 1868 to 1905.14
He found that the duration of visibility after sunset is about 1h 40m when seen in March, 1h 30m in April, and 1h 20m in May. He thinks that the planet is, at its brightest, “certainly much brighter than a first magnitude star.”15 In February, 1868, he found that its brightness rivalled that of Jupiter, then only 2° or 3° distant. In November, 1882, it seemed brighter than Sirius. In 1876 it was more striking than Mars, but the latter was then “faint and at a considerable distance from the earth.”
In 1878, when Mercury and Venus were in the same field of view of a telescope, Nasmyth found that the surface brightness (or “intrinsic brightness,” as it is called) of Venus was at least twice as great as that of Mercury; and Zöllner found that from a photometric point of view the surface of Mercury is comparable with that of the moon.
With reference to the difficulty of seeing Mercury, owing to its proximity to the sun, Admiral Smyth says, “Although Mercury is never in opposition to the earth, he was, when in the house of Mars, always viewed by astrologers as a most malignant planet, and one full of evil influences. The sages stigmatized him as a false deceitful star (sidus dolosum), the eternal torment of astronomers, eluding them as much as terrestrial mercury did the alchemists; and Goad, who in 1686 published a whole folio volume full of astro-meteorological aphorisms, unveiling the choicest secrets of nature, contemptuously calls Mercury a ‘squinting lacquey of the sun, who seldom shows his head in these parts, as if he was in debt.’ His extreme mobility is so striking that chemists adopted his symbol to denote quicksilver.”16
Prof. W. H. Pickering thinks that the shortness of the cusps (or “horns”) of Mercury’s disc indicates that the planet’s atmosphere is of small density – even rarer than that of Mars.
The diameter of Mercury is usually stated at about 3000 miles; but a long series of measures made by Prof. See in the year 1901 make the real diameter about 2702 miles. This would make the planet smaller than some of the satellites of the large planets, probably smaller than satellites III. and IV. of Jupiter, less than Saturn’s satellite Titan, and possibly inferior in size to the satellite of Neptune. Prof. Pickering thinks that the density of Mercury is about 3 (water = 1). Dr. See’s observations show “no noticeable falling off in the brightness of Mercury near the limb.” There is therefore no evidence of any kind of atmospheric absorption in Mercury, and the observer “gets the impression that the physical condition of the planet is very similar to that of our moon.”17
Schröter (1780-1815) observed markings on Mercury, from which he inferred that the planet’s surface was mountainous, and one of these mountains he estimated at about 11 miles in height!18 But this seems very doubtful.
To account for the observed irregularities in the motion of Mercury in its orbit, Prof. Newcomb thinks it possible that there may exist a ring or zone of “asteroids” a little “outside the orbit of Mercury” and having a combined mass of “one-fiftieth to one-three-hundredth of the mass of Venus, according to its distance from Mercury.” Prof. Newcomb, however, considers that the existence of such a ring is extremely improbable, and regards it “more as a curiosity than a reality.”19
M. Léo Brenner thinks that he has seen the dark side of Mercury, in the same way that the dark side of Venus has been seen by many observers. In the case of Mercury