Beauty of Venus.—Brilliancy.—Period &c.—Venus as a telescopic object.—Surface-markings on the planet.—Rotation-period.—Faintness of the markings.—Twilight on Venus.—Alleged Satellite.—Further observations required.—Transits of Venus.—Occultations of Venus.

Beauty of Venus.—This planet has an expressive name, and it naturally leads us to expect that the object to which it is applied is a beautiful one. The observer will not be disappointed in this anticipation: he will find Venus the most attractive planet of our system. No such difficulties are encountered in finding Venus as in detecting Mercury; for the former recedes to a distance of 47° from the Sun, and sometimes remains visible 4½ hours after sunset, as in February 1889. But Venus owes her beauty not so much to favourable position as to surpassing lustre. None of the other planets can compare with her in respect to brilliancy. The giant planet Jupiter is pale beside her, and offers no parallel. Ruddy Mars looks faint in her presence, and does not assume to rivalry.

This planet alternately adorns the morning and evening sky, as she reaches her W. and E. elongations from the Sun. The ancients styled her Lucifer (“the harbinger of day”) when a morning star and Hesperus when an evening star.

Brilliancy.—Her brightness is such as to lead her to occasionally become a conspicuous object to the naked eye in daytime, and at night she casts a perceptible shadow. This is specially the case near the epoch of her maximum brilliancy, which is attained when the planet is in a crescent form, with an apparent diameter of about 40, and situated some 5 weeks from inferior conjunction. Though only a fourth part of the disk is then illuminated, it emits more lustre than a greater phase, because the latter occurs at a wider distance from the Earth and when the diameter is much less. Her appearance is sometimes so striking that it is not to be wondered at that people, not well informed as to celestial events, have attributed it to causes of unusual nature. When the planet was visible as a morning star in the autumn of 1887, an idea became prevalent in the popular mind that the “Star of Bethlehem” had returned, and there were many persons who submitted to the inconvenience of rising before daylight to gaze upon a spectacle of such phenomenal import. And they were not disappointed in the expectancy of beholding a star of extreme beauty, though altogether wrong in surrounding it with a halo of mystery and wonder.

At intervals of eight years the elongations of Venus are repeated on nearly the same dates as before, and the planet is presented under very similar conditions. This is because five synodical periods (nearly = 13 sidereal periods) of Venus are equal to eight terrestrial years. Thus very favourable E. elongations occurred on May 9, 1860, May 7, 1868, May 5, 1876, and May 2, 1884; and on April 30, 1892, there will be a similar elongation.

Period &c.—Venus moves round the Sun in an orbit of slight eccentricity, and completes a revolution in 224^d 16^h 49^m 8^s. Her mean distance from that luminary is 67,000,000 miles. The apparent diameter of the planet varies from 9·5 at superior to 65 at inferior conjunction, and it averages 25 at elongations. Her real diameter is 7500 miles. The polar compression is very slight—in fact, not sufficiently decided for measurement; this is also true of Mercury.

Venus as a Telescopic Object.—When the telescope is directed to Venus it must be admitted that the result hardly justifies the anticipation. Observers are led to believe, from the beauty of her aspect as viewed with the unaided eye, that instrumental power will greatly enhance the picture and reveal more striking appearances than are displayed on less conspicuous planets. But the hope is illusive. The lustre of Venus is so strong at night that her disk is rarely defined with satisfactory clearness; there is generally a large amount of glare surrounding it, and our instruments undergo a severe ordeal when their capacities are tested upon this planet. Observations should be undertaken in the daytime, or near the times of sunrise or sunset, when the refulgence of this object does not exert itself in extreme degree. But putting aside the question of definition for the moment, there are other circumstances which conspire to render the view a somewhat unattractive one. There are no dark spots, of bold outline, such as we may plainly discern on Mars, visible on her surface. There is no wonderful arrangement of luminous rings, such as encircle Saturn. There are no signs of dark variegated belts, similar to those which gird both Jupiter and Saturn; nor is there any system of attendant satellites, such as accompany each of the superior planets. But though Venus is wanting in these respects, she may yet boast an attraction which the outer planets can never display to us, namely, the beautiful crescented phase, which, tradition says, was predicted by Copernicus, and, when afterwards observed in Galilei’s telescope, justly considered a convincing fact in support of the Copernican system. The phases are best seen in strong twilight, whenever Venus is favourably situated. It has been asserted that the crescent of this planet has been distinguished with the naked eye; but the statement is undoubtedly erroneous. Any small glass will show it, however, as it is sometimes well visible when subtending an angle of 50 or 55.

Surface-markings.—In 1666 and the following year J. D. Cassini observed several bright spots on Venus and also two obscure markings; but the latter were extremely faint and of irregular extent, so that little could be gleaned from them. He watched these forms closely and remarked certain changes in their positions, which finally enabled him to determine the period of the planet’s rotation. In 1726 and 1727 Bianchini, at Rome, repeatedly observed dark spots, and their outlines seem to have been so consistent that he depicted them on a chart and gave them names. But J. Cassini, at Paris, failed to confirm these results, though he used telescopes of 82-and 114-feet focus; and it was supposed the climate of Paris was not suitable for such delicate observations. SchrÖter reviewed this planet in 1788 and later years, and succeeded in detecting various markings and irregularities in the terminator and cusps. He announced that he had seen the S. horn of the crescent truncated, so that a bright point was apparently isolated at its extremity. From this he concluded there must be mountains of great altitude on the planet, and the perpendicular height of one of these he computed at 22 miles, which is four times the height of the most lofty mountain on the Earth. If the surface of Venus were uniformly level, then her cusps would taper gradually away to points, and no such deformation as that described by SchrÖter could possibly be produced. And there is strong negative evidence among modern observations as to the existence of abnormal features; so that the presence of very elevated mountains must be regarded as extremely doubtful, if, indeed, the theory has not to be entirely abandoned. The detached point at the S. horn shown in SchrÖter’s telescope was probably a false appearance due to atmospheric disturbances or instrumental defects. Whenever the seeing is indifferent, this planet assumes some treacherous features which are very apt to deceive the observer, especially if his telescope is faulty. Spurious details are seen, which quite disappear from the sharp images obtained in steadier air with a good glass. I have never observed truncation in either of the horns of Venus; but on certain occasions, when the planet has been ill-defined in passing vapours, it was most easy to believe that a fragment became detached from the extremity of the cusp, just in the manner described by SchrÖter. But close attention has showed the effect to be false, and revealed its cause. It was the rippling of the image that gave rise to the apparently dissevered cusp, in the same way that passing air-waves and resulting quivers in the image of Saturn’s ring will sometimes produce displacements, so that the observer momentarily sees several black divisions, and the edges are multiplied and superimposed one on another. Refraction, exercised by heated vapours in crossing objects, is obviously the source of all this.

Sir W. Herschel frequently examined this planet between 1777 and 1793, but could not discern spots sufficiently definite and durable to enable him to fix the time of rotation. He dissented from SchrÖter as to the alleged mountains, and said, “No eye which is not considerably better than mine, or assisted by much better telescopes, will ever get a sight of them.”

MÄdler effected some observations of this planet in 1833 and some subsequent years. He detected spots on two occasions only, but noticed irregularities in the terminator and cusps. Di Vico and others at Rome, in 1840-1, devoted much attention to this object, and secured a large number of observations. They appear to have recovered the spots charted by Bianchini, and described them as of the last degree of faintness. The observers who saw the spots most readily were those who had the most difficulty in detecting the faint companion of a close double star. In the spring of 1841 Di Vico saw a marking on the northern cusp involved in an oval luminosity, and he likened it to a crater on the Moon viewed obliquely. This spot had a diameter of at least 4½, and it was seen to advance even into the obscure part of the disk.

Rotation-Period.—The following are the periods of rotation as given by the different authorities whose observations we have mentioned:—

Schiaparelli has recently discussed a large number of observations of this planet, and concludes that, like Mercury, she rotates on her axis in the same time that she completes a sidereal revolution round the Sun, viz. in 224·7 days! I merely mention this remarkable deduction, without quoting any facts in opposition to it.

From observations by Perrotin at Nice in 1890, including 74 observations, the rotation of this planet is very slow, and is made in such a way that the relative positions of the spots and terminator do not experience any notable change during many days.

Faintness of the Markings.—Several observers have noticed a slight blunting of the S. horn of Venus, and in recent years dusky spots have not unfrequently been seen, notably by Buffham, Langdon, and others. The only markings distinguishable with my 10-inch reflector are faint grey areas, without definite boundaries. These are sometimes so delicate that it is difficult to assign exact form and position to them, and occasionally I have regarded their very existence as of doubtful character. They appear to be mere inequalities in brightness of the surface, and may be due to different reflective power in parts of the dense atmosphere of this planet. Certainly the spots are nothing like those seen on the disks of Mars and Jupiter, many of which are extremely distinct and show sharply terminated outlines. Dawes, an observer endowed with very keen sight, could never succeed in finding any markings on Venus, and many others have failed. But the evidence affirming their reality is too weighty and too numerously attested to allow them to be set aside. Occasionally the disk appears speckled with minute shadings, and some observers have noticed crateriform objects near the terminator; but these are uncertain. Brilliant spots have also been recorded quite recently at the cusps.

Perhaps it may be advisable here to add a word of caution to observers not to be hastily drawn to believe the spots are visible in very small glasses. Accounts are sometimes published of very dark and definite markings seen with only 2 or 3 inches aperture. Such assertions are usually unreliable. Could the authors of such statements survey the planet through a good 10-or 12-inch telescope, they would see at once they had been deceived. Some years ago I made a number of observations of Venus with 2-, 3-, and 4-1/4-inch refractors and 4-and 10-inch reflectors, and could readily detect with the small instruments what certainly appeared to be spots of a pronounced nature, but on appealing to the 10-inch reflector, in which the view became immensely improved, the spots quite disappeared, and there remained scarcely more than a suspicion of the faint condensations which usually constitute the only visible markings on the surface. I believe, also, the serrated terminator is not a real feature of the object, but rather an effect either of the rippling contour of the image or of an imperfect or inadequate telescope.

An atmosphere of considerable density probably surrounds this planet, for at the limb the brightness of the disk is much intensified. A medium like this, that reflects and refracts light in extreme degree, is brighter under oblique vision, as at the limb of Venus.

Twilight on Venus.—When Venus is a slender crescent, near inferior conjunction, a feeble luminosity pervades the dark part of the disk similar to the “ashy light” or earthshine observed on the crescented Moon. On such occasions the unilluminated surface appears to be involved in a phosphorescence. Several observers have, however, described the unilluminated limb of Venus as darker than the background of sky. Zenger, at Prague, has noticed a brownish-red ring surrounding the planet, and he attributes the appearance to much the same cause as that which occasions the coppery colour of the Moon in a total eclipse.

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. Moestlin 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.