An occultation of Mars by the moon is recorded by the Chinese, on February 14, B.C. 69, and one of Venus, on March 30, A.D. 361. These have also been verified by Hind, and his calculations show the accuracy of these old Chinese records.
It has been suggested that the moon may possibly have a satellite revolving round it, as the moon itself revolves round the earth. This would, of course, form an object of great interest. During the total lunar eclipses of March 10 and September 3, 1895, a careful photographic search was made by Prof. Barnard for a possible lunar satellite. The eclipse of March 10 was not very suitable for the purpose owing to a hazy sky, but that of September 3 was “entirely satisfactory,” as the sky was very clear, and the duration of totality was very long. On the latter occasion “six splendid” photographs were obtained of the total phase with a 6-inch Willard lens. The result was that none of these photographs “show anything which might be taken for a lunar satellite,” at least any satellite as bright as the 10th or 12th magnitude. It is, of course, just possible that the supposed satellite might have been behind the moon during the totality.
With reference to the attraction between the earth and moon, Sir Oliver Lodge says —
“The force with which the moon is held in its orbit would be great enough to tear asunder a steel rod 400 miles thick, with a tenacity of 30 tons to the square inch, so that if the moon and earth were connected by steel instead of gravity, a forest of pillars would be necessary to whirl the system once a month round their common centre of gravity. Such a force necessarily implies enormous tensure or pressure in the medium. Maxwell calculates that the gravitational stress near the earth, which we must suppose to exist in the invisible medium, is 3000 times greater than what the strongest steel can stand, and near the sun it should be 2500 times as great as that.”88
With reference to the names given to “craters” on the moon, Prof. W. H. Pickering says,89 “The system of nomenclature is, I think, unfortunate. The names of the chief craters are generally those of men who have done little or nothing for selenography, or even for astronomy, while the men who should be really commemorated are represented in general by small and unimportant craters,” and again —
“A serious objection to the whole system of nomenclature lies in the fact that it has apparently been used by some selenographers, from the earliest times up to the present, as a means of satisfying their spite against some of their contemporaries. Under the guise of pretending to honour them by placing their names in perpetuity upon the moon, they have used their names merely to designate the smallest objects that their telescopes were capable of showing. An interesting illustration of this point is found in the craters of Galileo and Riccioli, which lie close together on the moon. It will be remembered that Galileo was the discoverer of the craters on the moon. Both names were given by Riccioli, and the relative size and importance of the craters [Riccioli large, and Galileo very small] probably indicates to us the relative importance that he assigned to the two men themselves. Other examples might be quoted of craters named in the same spirit after men still living… With the exception of Maedler, one might almost say, the more prominent the selenographer the more insignificant the crater.”
The mathematical treatment of the lunar theory is a problem of great difficulty. The famous mathematician, Euler, described it as incredibile stadium atque indefessus labor.90
With reference to the “earth-shine” on the moon when in the crescent phase, Humboldt says, “Lambert made the remarkable observation (14th of February, 1774) of a change of the ash-coloured moonlight into an olive-green colour, bordering upon yellow. The moon, which then stood vertically over the Atlantic Ocean, received upon its night side the green terrestrial light, which is reflected towards her when the sky is clear by the forest districts of South America.”91 Arago said, “Il n’est donc pas impossible, malgré tout ce qu’un pareil résultat exciterait de surprise au premier coup d’œil qu’un jour les météorologistes aillent puiser dans l’aspect de la Lune des notions précieuses sur l’etat moyen de diaphanité de l’atmosphère terrestre, dans les hemisphères qui successivement concurrent à la production de la lumière cendrée.”92
The “earth-shine” on the new moon was successfully photographed in February, 1895, by Prof. Barnard at the Lick Observatory, with a 6-inch Willard portrait lens. He says —
“The earth-lit globe stands out beautifully round, encircled by the slender crescent. All the ‘seas’ are conspicuously visible, as are also the other prominent features, especially the region about Tycho. Aristarchus and Copernicus appear as bright specks, and the light streams from Tycho are very distinct.”93
Kepler found that the moon completely disappeared during the total eclipse of December 9, 1601, and Hevelius observed the same phenomenon during the eclipse of April 25, 1642, when “not a vestige of the moon could be seen.”94 In the total lunar eclipse of June 10, 1816, the moon during totality was not visible in London, even with a telescope![95]
The lunar mountains are relatively much higher than those on the earth. Beer and Mädler found the following heights: Dörfel, 23,174 feet; Newton, 22,141; Casatus, 21,102; Curtius, 20,632; Callippus, 18,946; and Tycho, 18,748 feet.95
Taking the earth’s diameter at 7912 miles, the moon’s diameter, 2163 miles, and the height of Mount Everest as 29,000 feet, I find that
From which it follows that the lunar mountains are proportionately about three times higher than those on the earth.
According to an hypothesis recently advanced by Dr. See, all the satellites of the solar system, including our moon, were “captured” by their primaries. He thinks, therefore, that the “moon came to earth from heavenly space.”96
CHAPTER VI
Mars
Mars was called by the ancients “the vanishing star,” owing to the long periods during which it is practically invisible from the earth.97 It was also called πυρόεις and Hercules.
I have seen it stated in a book on the “Solar System” by a well-known astronomer that the axis of Mars “is inclined to the plane of the orbit” at an angle of 24° 50′! But this is quite erroneous. The angle given is the angle between the plane of the planet’s equator and the plane of its orbit, which is quite a different thing. This angle, which may be called the obliquity of Mars’ ecliptic, does not differ much from that of the earth. Lowell finds it 23° 13′ from observations in 1907.98
The late Mr. Proctor thought that Mars is “far the reddest star in the heavens; Aldebaran and Antares are pale beside him.”99 But this does not agree with my experience. Antares is to my eye quite as red as Mars. Its name is derived from two Greek words implying “redder than Mars.” The colour of Aldebaran is, I think, quite comparable with that of the “ruddy planet.” In the telescope the colour of Mars is, I believe, more yellow than red, but I have not seen the planet very often in a telescope. Sir John Herschel suggested that the reddish colour of Mars may possibly be due to red rocks, like those of the Old Red Sandstone,