Page:The American Cyclopædia (1879) Volume XVI.djvu/324

 304 VENUS 21 min. 19'2 sec. They were made on a moun- tain (or what he took to be such) near the southern horn of Venus. He remarked that while the northern horn always preserved its proper pointed figure, the southern sometimes appeared rounded, which circumstance he con- sidered due to the presence of a mountain whose shadow fell on the place where the horn should have been. Beyond this place he observed a luminous point which he regard- ed as the summit of another mountain, illu- minated by the sun. The proper position for showing these appearances cannot be long maintained, as the planet rotates, and thus their recurrence affords a means of determining the rate of its rotation. By taking as many as 160 rotations, Schroter deduced the rotation period mentioned above. De Vico at Rome, in 1839- '41, made a series of observations, confirming Bianchini's drawings in a remarkable degree. He deduced the rotation period 23 h. 21 min. 22 sec. Accepting this result, we find for the three planets Venus, the earth, and Mars, in this order (the order of their distances), the rotation periods 23 h. 21 min. 22 sec., 23 h. 56 min. 4 sec., and 24 h. 37 min. 23 sec., increas- ing with distance from the sun in a nearly uniform manner. While, as we have said, it seems impossible to reject the evidence afford- ed by the observations of Schroter, De Vico, and others, in favor of the existence of recog- nizable marks on Venus, yet as many of the ablest observers, using the finest telescopes of our day, have failed to recognize such spots, we must adopt Sir John Herschel's explanation, who says that "the most natural conclusion from the very rare appearance and want of permanence of the spots is, that we do not see, as in the moon, the real surface of the planet, but only its atmosphere, much loaded with clouds, serving to mitigate the otherwise in- tense glare of the sunshine." It is clear from other circumstances that Venus has an atmos- phere. During her transits over the sun's disk in 1761 and 1769 a sort of penumbral light was observed round her disk. "Wargentin ob- served that the part of the disk off the sun could be recognized by a faint light bordering it, during almost the whole time of emersion. Bergman, who observed the transit of 1761 at Upsal, states that at ingress the part of Venus still off the sun could be seen, being bounded by a crescent of light. At the egress this ap- pearance was even more remarkable. As more of the planet's disk passed off the sun's, the part of the crescent of light furthest from the sun grew fainter and ultimately vanished, so that at last only the horns could be seen. Many other accounts of these two transits con- tain similar statements, and during the recent transits the same appearances were seen by many observers. It is readily perceived that such appearances might be expected in the case of a planet surrounded by an atmospheric envelope. The sun would be raised by atmos- pheric refraction precisely as our own sun is raised above the horizon after he has really passed below it. When we look at the part of Venus furthest from the sun, before immersion or after emersion (at a transit), we are looking in the same direction as an observer of Venus at that part who should direct his gaze sun- ward. As he would see the sun raised above the horizon by atmospheric refraction, though really below, so we see the sun doubly raised because the line of sight passes through the hither half as well as the further half of the atmosphere above that part of Venus. In other words, though the outline of the sun's disk is really (that is, in a geometrical sense) behind the disk of Venus, we receive actual sunlight round even the part of Venus's disk remotest from the sun ; a fortiori then is sun- light received round the remaining part of Venus's disk which lies outside the sun's. Hence an arc of light, brightest near the cusps of Venus, but visible (soon after immersion begins and again till near the end of emersion) at the part of Venus remotest from the sun. Prof. Lyman of Yale college has even seen this light at the part remotest from the sun, when the whole disk of Venus has been off the sun's, as at inferior conjunctions where there has been no transit. Irom such an ob- servation it may be inferred that the atmos- phere of Venus is deeper than our own. For we can infer from Bergman's observations a horizontal refraction scarcely less than that of our own atmosphere ; and Lyman's obser- vation would imply an atmospheric refraction nearly twice as effective. The atmosphere of Venus has been analyzed with the spectroscope by Huggins, Vogel, and others, and the pres- ence of aqueous vapor is held to have been de- monstrated by the observations ; yet it was not until the transit of 1874 that this point was in reality established. On that occasion Tac- chini's observations seemed to demonstrate the fact that there is water on Venus. A curious question is raised by the apparently strong evidence obtained during the 18th century to show that Venus is accompanied by a satellite. It is quite certain now that no such satellite exists, yet several skilful observers not only imagined that they perceived such a satellite, but even assigned to it a definite period of revolution around its primary. During the transits of 1761, 1769, and 1874, it was con- clusively shown that no such body exists, but the difficulty of accounting for the apocryphal observations remains as great as ever. Thir- teen sidereal revolutions of Venus are com- pleted in a period very nearly equal to eight sidereal years. Hence at every fifth conjunc- tion the planets return nearly to the same longitude. Accordingly the perturbing effects taking place at one conjunction are repeated at the fifth conjunction thereafter, and so on. Consequently there is an accumulation of per- turbations, resembling the great inequality of Saturn and Jupiter, though far less remarkable in amount. It has for its period about 240