Astronomy for Everybody/Part 4/Chapter 3

Of all the star-like objects in the heavens the planet Venus is the most brilliant. The sun and moon are the only heavenly bodies outshining it. In a clear and moonless evening it may be seen to cast a shadow. If an observer knows exactly where to look for it, and has a well-focused eye, it can be seen in the daytime when near the meridian, provided that the sun is not in its immediate neighbourhood. When it is east of the sun it may be seen in the west, faintly before sunset and growing continually brighter as the light diminishes. When west of the sun it rises in the morning before the sun, and may then be seen in the east. Under these circumstances it has been called the evening and morning star respectively. The ancients called it Hesperus when an evening star, and Phosphorus when a morning star. It is said that, in the early history of our race, Hesperus and Phosphorus were not known to be the same body.

If Venus is examined with the telescope, even one of low power, it will be seen to exhibit phases like those of the moon. This fact was ascertained by Galileo when he first directed his telescope toward the planet, and afforded him strong evidence of the truth of the Copernican System. In accordance with a custom of the time he published this discovery in the form of an anagram—a ​collection of letters which, when subsequently put together would state the discovery. Translated Into English the anagram read, "The mother of the loves emulates the phases of Cynthia."

What we have said of the synodic motion of Mercury applies in principle to Venus, and need not therefore be repeated. In the following cut the apparent size of the planet is shown in various parts of its synodic orbit. As the planet passes from superior to inferior conjunction its globe continually grows larger in apparent size,

though we cannot see its entire outline. But the fraction of the disk illuminated continually becomes smaller, first having the shape of a half moon, and then the shape of a crescent, which grows thinner and thinner up to the time of inferior conjunction. In the latter position the dark hemisphere is turned toward us and the planet is invisible. Venus Is at its greatest brightness about halfway between Inferior conjunction and greatest elongation. It then sets about two hours after the sun, if east of it, and rises about two hours before the sun, if west of it.

The question of the rotation of Venus has interested astronomers and the public ever since the time of Galileo. But the difficulty of learning anything certain on the subject is very great, owing to the peculiar glare of the planet. When seen through a telescope no sharp and well-defined markings are visible. Instead of this there is a glare on the surface, varying by gentle gradations from one region to another, as if we were looking upon a globe of polished but slightly tarnished metal. Nevertheless, various observers have supposed that they could distinguish bright or dark spots. As far back as 1667 Cassini concluded from these seeming spots that the planet revolved on its axis in a little less than twenty-four hours. During the next century Blanchini, an Italian observer, published an extensive treatise on the subject, illustrated with many drawings of the planet. His conclusion was that Venus required more than twenty-four days to revolve on its axis. Cassini, the son, defended his father's conclusion by claiming that the planet had always made one revolution and a little more between the times of Blanchini's observations on successive evenings. Thus the Italian astronomer would naturally see the spots on successive evenings a little farther advanced, and estimated the motion by this advance, not being aware that a whole revolution had been made during the interval. At the end of twenty-four days the same hemisphere of the planet would be presented to the earth as before, the number of revolutions in the meantime being twenty-five. ​Schröter tried to decide the question for Venus in the same way that he supposed himself to have decided it for Mercury. He directed his attention especially to the fine sharp horns of the crescent, when the planet was nearly between the earth and the sun. At certain intervals he supposed one of them to be a little blunted. Ascribing this appearance to the shadow of a high mountain, he concluded that the time of rotation was twenty-three hours twenty-one minutes.

From the time of Schröter no one professed to throw any more light on the question until 1832. Then De Vico, of Rome, announced that he had rediscovered the markings found by Blanchini. He concluded that the planet rotated in twenty-three hours twenty-one minutes, in agreement with Schröter's result.

This close agreement between the results of observations by four distinguished observers led to the very general acceptance of twenty-three hours twenty-one minutes as the time of rotation of the planet. But there was much to be said on the other side. The great Herschel, with the most powerful telescopes that had ever been made, was never able to make out any permanent markings on Venus. If anything like a spot appeared, it varied and disappeared again so rapidly that no evidence of rotation could be afforded by it. This negative result has always been reached by the large majority of observers.

But a new and surprising theory has been recently put forth by Schiaparelli, and maintained by Lowell. This is that Venus rotates on its axis in the same period that it revolves around the sun; in other words both Mercury ​and Venus always present the same face to the sun, as the moon presents the same face to the earth. Schiaparelli reached this conclusion by noticing that a number of exceedingly faint spots could be seen on the southern hemisphere of Venus for several days in succession in the same position day after day. He could observe the planet through several hours on each day, and the constancy of the spots precluded the idea that the planet made one rotation and a little more in the course of a day. Lowell was led to the same conclusion by careful study of the planet at his Arizona observatory.

The latest conclusion has been reached by the spectroscope. We have already explained how, with this instrument, it can be determined whether a heavenly body is moving toward us or from us. The principle applies to a planet which we see by the reflected light of the sun as well as to a star. Hence, if Venus rotates, one part of its disk will be moving toward us, and the other from us. By comparing the dark lines of the spectrum shown by the two edges of the disk of Venus it can then be determined how various points of the disk are moving with respect to the earth. It was thus found by Belopolsky that the planet was affected by a quite rapid rotation. The observation is so difficult, and the displacement of the lines so small, that it was not possible to state a very certain result, although the general fact was made very probable. On the whole we must regard this conclusion as the most likely that has yet been reached, although it is at variance with the observations of Schiaparelli, as well as those of the Lowell Observatory. But the spectro​scopic observations have not yet been made with sufficient precision to teach us the exact time of revolution. Recent discoveries as to the nature of the atmosphere of Venus make it almost certain that all the observers who supposed that they saw markings on the planet were mistaken.

It is now well established that Venus is surrounded by an atmosphere which is probably denser than that of the earth. This was shown in a remarkable and interest-

ing way during the transit of Venus over the sun's disk in 1882, which was observed by the writer at the Cape of Good Hope. When the planet was a little more than halfway on the disk, its outer edge appeared illuminated, as shown on the figure. This illumination, however, did not commence at the middle point of the arc, as it ​should have done had it been caused by regular refraction, but commenced at a point quite near one end of the arc. This appearance was explained by Russell, of Princeton, who showed that the atmosphere is so full of vapour that we cannot see the light of the sun by direct refraction through it. What we see is an illuminated stratum of clouds or vapour floating in an atmosphere. Such being the case, it is not at all likely that astronomers on the earth can ever see the solid body of the planet through these clouds. Hence the supposed spots could only have been temporary clouds, continually changing.

To illustrate the illusions to which the sight of even good observers may be subject, we may mention the fact that several such observers have supposed the whole hemisphere of Venus to be visible when the planet was near inferior conjunction. It then had the appearance familiarly known as "the new moon in the old moon's arms," with which everyone who observes our satellite when a narrow crescent is familiar. In the case of the moon it is well known that we thus see the dark hemisphere by the light reflected from the earth. But in the case of Venus there is no possibility of a sufficient reflection of light from the earth, or any other body. The appearance has sometimes been explained by a possible phosphorescence covering the whole hemisphere of Venus. But it is more likely due to an optical illusion. It has generally been seen in the daytime, when the sky is brightly illuminated, and when any faint light like that of phosphorescence would be completely in​visible. To whatever we might attribute the light, it ought to be seen far better after the end of twilight in the evening than during the daytime. The fact that it is not seen then seems to be conclusive against its reality.

The appearance illustrates a well-known psychological law, that the imagination is apt to put in what it is accustomed to see, even when the object is not there. We are so accustomed to the appearance on the moon that when we look at Venus the similarity of the general phenomena leads us to make this supposed familiar addition to it.

During the past two centuries several observers have from time to time thought that they saw a satellite of Venus. Countless observers, with good telescopes, have seen nothing of the sort. We may safely say that Venus has no satellite visible in the most powerful telescopes of our time. Quite likely these supposed satellites were seeming objects quite familiar to astronomers under the name of "ghosts." These are sometimes seen when a telescope is pointed at a bright object, and are due to a double reflection of light in the lenses either of the object-glass or the eyepiece.

A few years ago the writer received a letter from the owner of a very large telescope in England stating that, by great care, he could see a very faint, round, and well-defined aureole of light around the planet Mars. He desired to know whether the object could be real, or how the appearance was to be explained. In reply, he was informed that such an appearance would be produced ​by the double reflection of light between the two inner lenses of the object-glass, provided their curvatures were nearly, but not exactly the same. It was suggested that he point the telescope at Sirius and see if a similar appearance did not surround the star. He probably found that such was the case.

The transits of Venus across the sun's disk are among the rarest phenomena of astronomy, as they occur, on the average, only once in sixty years. For many centuries past and to come there will be a regular cycle, bringing about four transits in two hundred and forty-three years. The intervals between the transits are one hundred and five and a half years, eight years, one hundred and twenty-one and a half years, eight years; then one hundred and five and a half years again, and so on. The dates of the last six transits and the two next to come are as follows:

It will be seen that no person now living is likely to see this phenomenon, as the next transit does not occur until 2004. Yet, the time when Venus will appear upon the disk on June 8 of that year can now be predicted for any point on the earth's surface, within a minute or two.

​The interest which has attached to these transits during the past century arose from the fact that they were supposed to afford the best method of determining the distance of the sun from the earth. This fact and the rarity of the phenomenon led to the last four transits being observed on a large scale. In 1761, and again in 1769, the leading maritime nations sent observers to various parts of the world to note the exact time at which the planet entered upon and left the sun's disk. In 1874 and 1882, expeditions were fitted up on a large scale by the United States, Great Britain, France, and Germany. On the first of these occasions American parties occupied stations in China, Japan, and eastern Siberia on the north, and in Australia, New Zealand, Chatham Island, and Kerguelen Island in the south. In 1882 it was not necessary to send out so many expeditions, because the transit was visible in this country. In the southern hemisphere stations were occupied at the Cape of Good Hope and other points. The observations made by these expeditions proved of great value in determining the future motions of Venus, but it was found that other methods of determining the distance of the sun would lead to a more certain result.