Alleged Satellite.—Cassini, Short, Montaigne, and others, in the 17th and 18th centuries, observed small crescents near Venus and inferred the existence of a satellite; but no such object has presented itself in more recent times. It is extremely probable that the observers were mistaken. In some cases the duplicate image may have been formed by reflection in the eyepiece; in others a small star or planet situated near Venus gave rise to the deception. M. Stroobant has fully investigated this astronomical myth, and disposed of many of the observations, without having recourse to the apocryphal satellite named “Neith” by M. Niesten, who has discussed the question from an affirmative point of view.
Further Observations required.—From the foregoing summary amateurs will notice that several difficult and more or less evanescent features on this brilliant member of our system stand in need of confirmation. Certain disputed forms require also to be looked for. The faint dusky patches, the bright spots at the horns, and the inequalities in the curve of the terminator will sure to be re-observed in future years; and it is necessary that such details should be precisely noted in regard to their positions and outlines as often as possible. A series of reliable observations of this character might enable a fresh value of the rotation-period to be deduced from them; and this is desirable, for though Cassini, Schröter, and Di Vico give periods which are in close harmony, there are elements of uncertainty attached to their results. A new determination of the period would be valuable, and especially so if based on really trustworthy data obtained by one of the best modern telescopes. With the planet situated near inferior conjunction, the crescent (reduced at such a time to a mere thread of light) should be brought into the field, and the observer should look for the extension of a faint glow over the interior parts of the surface, and make comparisons between the relative brightness of the planet’s dark limb and of the sky on which it is projected. The telescopic images of Venus are often excellent in daylight, and those who possess means of readily finding the planet at such times will be very likely to gain some useful materials. As to the presumed satellite, that may be relegated to the care of observers who have the leisure and inclination to pursue an ignis fatuus; but should any doubtful object appear in the field with Venus at any time, it ought to be fully recorded and identified, if possible.
Transits of Venus.—Those who were prevented by circumstances of weather or otherwise from witnessing either of the transits of Venus which occurred in 1874 and 1882 lost a spectacle of great rarity, and one which they can never have another chance to behold. The next transit occurs in the year 2004, and its phenomena will doubtless be watched with avidity by the astronomers of a future generation. The transit of 1882 was seen by many observers in England, though in some parts of the country the Sun was obscured by clouds. The planet was distinctly visible to the naked eye as a black circular spot in gradual motion across the solar disk. The most important result of the telescopic observations was of course the re-determination of the Sun’s distance; but amongst the physical features noted, one of the most interesting was the appearance of a silver arc of light on that portion of the planet’s edge which was outside the Sun. This is assumed to have been caused by the refraction of an atmosphere on Venus. The phenomenon was seen by several observers, including Prof. Langley in America and Messrs. Prince and Brodie in England.
Occultations of Venus.—An occupation of this planet by the Moon appears to have been recorded by the Chinese on March 19, 361 A.D. Tycho Brahe witnessed a similar phenomenon on May 23, 1587. Mœstlin observed Venus occult Regulus on Sept. 16, 1574; and on Oct. 2, 1590, this planet appears to have passed over Mars. Visible occultations of Venus are somewhat rare; they usually occur in daylight. A phenomenon of this kind was witnessed on Dec. 8, 1877, over all the W. part of the United States; and Prof. Pritchett, of Missouri, says:—“The interest taken in it was shared alike by the educated and the illiterate, and even by children.” The evening was cloudless, and many persons noted the time of disappearance of Venus as seen by the unassisted eye. With a 12½-inch refractor, power 275, Prof. Pritchett noted that “when the bright limb of Venus was within 8″ or 10″ of the Moon’s dark limb, a border of wavering light, several seconds in width, seemed to precede the planet. Its general effect was such as to place in doubt the moment of external contact.” A full description of this event, and of the partial occultation of Venus on Oct. 12, 1879, is given in No. 1 of the ‘Publications’ of the Morrison Observatory, Missouri, U.S.A.
Venus is said, by the Arabian astronomer Ibn-Jounis, to have occulted Regulus on Sept. 9, 885 A.D.; and Hind has examined the observations, by means of Le Verrier’s tables of the Sun and planets. He finds that on Sept. 9 in the year mentioned, at 16h 43m mean time, Venus approached the star within 1´·7; so that to the naked eye the latter would appear to be occulted, being overpowered in the glare of the planet.
[CHAPTER IX.]
MARS.
Appearance of the planet.—Period &c.—Phase.—Surface Configuration.—Charts and Nomenclature of Mars.—Discovery of two Satellites and of Canal-shaped markings.—Summary of Observations.—Rotation of Mars.—Further Observations required.—Changes on Mars.—The two Satellites.—Occultations of Mars.
Appearance of the Planet.—Mars is the fourth planet in the order of distance from the Sun. He revolves in an orbit outside that of the Earth, and is the smallest of the superior planets. His brilliancy is sometimes considerable when he occupies a position near to the Earth, and he emits an intense red light[36], which renders his appearance all the more striking. Ordinarily his lustre does not equal that of Jupiter, though when favourably placed he becomes a worthy rival of that orb. In 1719 he shone so brightly and with such a fiery aspect as to cause a panic. The superstitious notions and belief in astrological influences prevailing at that time no doubt gave rise to the popular apprehension that the ruddy star was an omen of disaster, and thus it was regarded with feelings of terror. Fortunately the light of science has long since removed such ideas from amongst us, and celestial objects, in all their various forms, are contemplated without misgiving. They are rather welcomed as affording the means of advancing our knowledge of God’s wonderful works as displayed in the heavens.
Period &c.—Mars revolves round the Sun in 686d 23h 30m 41s, and his mean distance from that luminary is 141,500,000 miles. The orbit is one of considerable eccentricity, the distance varying between 154,700,000 and 128,360,000 miles. The apparent diameter of the planet when in conjunction with the Sun is only 4″; but this may augment to 30″·4 at an opposition, when the Earth and Mars occupy the least distant parts of their orbits. The real diameter of Mars is nearly 5000 miles.