The practical conclusion from this observation is, that, at the superior conjunction of this planet, when its distance from the sun’s margin is not less than 58´, its polar and equatorial diameter may be measured by a micrometer, when it will be determined whether or not Venus be of a spheroidal figure. The Earth, Mars, Jupiter and Saturn are found to be not spheres but spheroids, having their polar shorter than their equatorial diameters. But the true figure of Venus has never yet been ascertained, because it is only at the superior conjunction that she presents a full enlightened hemisphere, and when both diameters can be measured, except at the time when she transits the sun’s disk, which happens only twice in the course of 120 years.[42]
The following conclusions are deduced from the observations made on Venus.
1. That this planet may be seen distinctly, with a moderate degree of magnifying power, at the moment of its superior conjunction with the sun, when its geocentric latitude, either north or south, at the time of conjunction, is not less than 1° 14´, or, when the planet is about 58´ from the sun’s limb. This conclusion is deduced from the observation of Oct. 2, 1843,[45] stated above.
2. Another conclusion is—that during the space of 583 days, or about 19 months—the time this planet takes in moving from one conjunction with the sun to a like conjunction again—when its latitude at the time of its superior conjunction exceeds 1° 14´, it may be seen with an equatorial telescope every clear day without interruption, except about the period of its inferior conjunction, when its dark hemisphere is turned towards the earth, and a short time before and after it. When its geocentric latitude is less than 1° 14´, it will be hid only about four days before, and the same time after its superior conjunction. During the same period it will be invisible to the naked eye, and consequently no observations can be made upon it with a common telescope, for nearly six months, and sometimes more, according as its declination is north or south, namely about two or three months before, and the same time after its superior conjunction, except where there is a very free and unconfined horizon. In regard to the time in which this planet can be hid about the period of its inferior conjunction, I have ascertained from observation, that it can never be hid longer than during a space of 2 days 22 hours; having seen Venus, about noon, like a fine slender crescent, only 35 hours after she had passed the point of her inferior conjunction; and in a late instance she was seen when little more than a day from the period of conjunction. The longest time, therefore, that this planet can be hid from view during a period of 583 days, is only about 10 days; and when its latitude at the time of the superior conjunction, equals or exceeds 1° 14´, it can be hid little more than two days. This is a circumstance which cannot be affirmed of any other celestial body, the sun only excepted.
3. That every variation of the phases of this planet—from a slender crescent to a full enlightened hemisphere—may, on every clear day, be conveniently exhibited by means of the equatorial telescope. This circumstance renders this instrument peculiarly useful in the instruction of the young in the principles of astronomy. For, if the phase which Venus should exhibit at any particular time be known, the equatorial telescope may be directed to the planet, and its actual phase in the heavens be immediately exhibited to the astronomical pupil.
4. Since it is only at the period of the superior conjunction that this planet presents a full enlightened hemisphere, and since it is only when this phase is presented that both its diameters can be measured—it is of some importance that observations be made on it at the moment of conjunction, by means of powerful telescopes furnished with micrometers, so as to determine the difference (if any) between its polar and equatorial diameters.
5. Another conclusion from the observations on Venus, is, that a moderate diminution of the aperture of the object-glass of the telescope is useful, and even necessary in viewing this planet when near the sun. Its effect is owing in part to the direct solar rays being thereby more effectually excluded; for when these rays enter directly into the tube of the telescope, it is very difficult, and almost impossible to perceive this planet, or any other celestial body when in the vicinity of the sun.
Observations on Jupiter and other planets.
This planet is very easily distinguished in the day-time with a very moderate magnifying power, when it is not within 30° or 35° of the sun. The following extract from my memorandums may serve as a specimen. May 12, 1813, 1h 40m, P.M. Saw Jupiter with a power of 15 times, the aperture not contracted. The planet appeared so distinct with this power, that I have reason to believe, it would have been perceived with a power of 6 or 7 times. When the aperture was contracted 9/10 inch, and afterwards to half an inch, there was little perceptible difference in its appearance. It was then about 58° in longitude, east of the sun.
Though Jupiter when at a considerable distance from the sun, and near his opposition, appears to the naked eye with a brilliancy nearly equal to that of Venus, yet there is a very striking difference between them, in respect of lustre, when viewed in day-light. Jupiter, when viewed with a high magnifying power, in the day-time, always exhibits a very dull cloudy appearance; whereas Venus appears with a moderate degree of splendour. About the end of June 1813, between 5 and 6 in the evening, having viewed the planet Venus, then within 20° of the sun, and which appeared with a moderate degree of lustre, I directed the telescope to Jupiter, at that time more than 32° from the sun, when the contrast between the two planets was very striking, Jupiter appearing so faint as to be just discernible, though his apparent magnitude was nearly double that of Venus. In this observation a power of 65 was used. In his approach towards the sun, about the end of July, I could not perceive him when he was within 16° or 17° of his conjunction with that luminary.—These circumstances furnish a sensible and popular proof, independently of astronomical calculations, that the planet Jupiter is placed at a much greater distance from the sun than Venus; since its light is so faint as to be scarcely perceptible when more than 20 degrees from the sun, while that of Venus is distinctly seen amidst the full splendour of the solar rays, when only about a degree from the margin of that luminary. With a power of 65 I have been enabled to distinguish the belts of Jupiter before sun-set, but could never perceive any of his satellites till the sun was below the horizon. There are no observations which so sensibly and strikingly indicate the different degrees of light emitted by the different planets as those which are made in the day-time. To a common observer, during night, Jupiter and Venus appear, in a clear sky, nearly with equal brilliancy, and even Mars, when about the point of his opposition to the sun, appears with a lustre somewhat similar, though tinged with a ruddy hue; but when seen in day-light their aspect is very dissimilar. This circumstance evidently indicates, 1. that these planets are placed at different distances from the sun, and consequently are furnished with different degrees of light proportional to the square of their distances from that luminary;—and 2. that there are certain circumstances connected with the surfaces and atmospheres of the planetary bodies, which render the light they emit more or less intense, independently of their different distances from the central luminary. For Mars, though much nearer to the sun than Jupiter, is not so easily distinguished in the day-time, and, even in the night-time, appears with a less degree of lustre.