| 1666-7. | Cassini | 23 hrs. 21 min. |
| 1726-8. | Bianchini | 24 days 8 hrs.[34] |
| 1811. | Schröter | 23 hrs. 21 min. 8 sec.[35] |
| 1840-1. | Di Vico | 23 hrs. 21 min. 22 sec. |
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.
1881, Mar. 22, 6h.1881, Mar. 26, 7h. 1881, Mar. 28, 6½h.
Venus as an evening star. (10-inch Reflector; power 212.)
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.