The most ancient astronomical observation we have of Venus is a Babylonian record of the year 685 B. C. It is written on a brick and preserved in the British Museum.
The best hours for examining Venus in a telescope are those of daylight. In the night the irradiation produced by the brilliant light of this beautiful planet prevents us from distinguishing clearly the outlines of its phases.
When Venus occupies the region of its orbit behind the sun, with reference to us—which is called the point of superior conjunction—it is at its greatest distance, and is reduced to a disk of 9½ seconds in diameter. It comes imperceptibly toward us, and when it passes its quadrature, at its mean distance, it presents the aspect of a half-moon. It soon attains its most brilliant light, at the epoch when it shines at a distance of 39° from the sun, and shows the third phase 69 days before its inferior conjunction. Its apparent diameter is then 40 seconds, and the width of its illuminated part is scarcely 10 seconds. In this position we see the fourth of the disk illuminated; but this quarter emits more light than the more complete phases. Finally, when it reaches the region of its orbit nearest to the earth, it shows us nothing more than an excessively thin crescent, since it is then between the sun and us, and presents to us, so to say, its dark hemisphere. This is the position where its apparent size is greatest, and it then measures 62 seconds in diameter. After passing its inferior conjunction the phases are reproduced, in inverse order, as a morning star.
Venus is constantly visible in full daylight in astronomical instruments, even at the moment of its superior conjunction. It is then round and quite small. At the epochs of its inferior conjunction it presents itself under the form of a very thin crescent.
We sometimes notice that the interior of the crescent of Venus, the remainder of the disk, is less black than the background of the sky. This has been called the ashy light (lumière cendrée) of Venus, although it has no satellite to produce it. It seems to me that this visibility, rather subjective than objective, arises from clouds on the planet, which whiten its disk and vaguely reflect the stellar light scattered through space. The eye instinctively continues the outline of the crescent, and imagines, rather than sees, the rest.
The revolution of Venus round the sun is performed in an orbit almost exactly circular, and without perceptible eccentricity (0.0068), in a period of 224 days, 16 hours, 49 minutes, 8 seconds.
The days of Venus, also, are a little more rapid than ours, but not much. Since the year 1666 attentive observation of the planet led Cassini to conclude that it turns on itself in 23 hours, 15 minutes. This observation is extremely difficult, on account of the brightness of the planet and the faintness of the irregularities visible on its disk.
The year of Venus, composed of 224 terrestrial days, consequently contains 231 of its own, since the day is a little shorter there than here.
These same observations show that the axis of rotation of this planet is much more inclined than ours, and that this inclination is 55 degrees. It follows that the seasons, although each lasting but 56 terrestrial days, or 58 Venusian days, are much more intense on this world than on ours. They pass, without transition, from summer to winter.
The inclination of the world of Venus being more than twice as great as ours, we have only to take a terrestrial globe and incline it by the same quantity to understand the climates and seasons which will result. We may easily see that the torrid zone extends, in this case, up to the frigid zone, and even beyond it; and, reciprocally, the frigid zone extends to the torrid zone, and even encroaches on it; so that no place remains for a temperate zone. There is not, then, on Venus any temperate climate, but all latitudes are both tropical and arctic.