Part 12

This remarkable photograph was made at the Yerkes Observatory by E. E. Barnard.]

She shows the full disc when farthest away. As she draws nearer she shows first the half moon and then the smaller crescent. She is nearest when she shows the larger crescent. She is brightest, though, when she shows the smaller crescent.]

VENUS’S LIKENESS TO THE EARTH

The fact that of all the planets Venus most resembles this good little earth on which our present lot is cast gives us a strong feeling of kinship with her, and a more lively interest in all her affairs than we might otherwise have. She and the earth are so nearly of one size that they are often referred to as twin sisters. There is a difference of less than three hundred miles in their diameters, the earth’s diameter measuring 7,917 miles, and that of Venus 7,629 miles. The surface of the planet is about ninety-three per cent. as extensive as that of the earth; its mass is a little more than eighty per cent., and its volume about ninety per cent. as great as the earth’s. Differing so little in these particulars, it follows that it must differ very little in density and gravity. The earth is the densest of all the planets, and Venus is only one-tenth less dense than the earth. Its force of gravity is not quite nine-tenths that of the earth. A removal from the earth to Venus would make just a comfortable reduction in one’s weight. A person weighing one hundred and seventy-five pounds here would weigh on Venus one hundred and fifty-four. If through strength of appetite and weakness of will one should take on two hundred pounds of too, too solid flesh here, transportation to Venus would bring about an instantaneous reduction to a solid one hundred and seventy-six pounds--as much of a reduction as would be compatible with health.

Venus must have begun her career in much the same way that the earth began its career. The nebula that formed her nucleus was probably nearly the same size (contained about the same amount of matter) as that with which the earth began its existence. The two bodies have succeeded in capturing about the same amount of loose material, and their gravity is such that they can hold within their bounds particles traveling at about the same rate of speed. No molecule of gas coming within the range of Venus’s attraction and traveling more slowly than six and thirty-seven hundredths miles per second can escape from Venus, and the earth can hold only such as move, when coming within its own attraction, with a less speed than six and ninety-five one-hundredths miles per second.

The earth has a moon, and Venus has none; but that may be because, like Mercury, Venus is too near the sun to be permitted to retain such a luxury. It is likely that if, in her earlier history, she had within the limit of her gravitative attraction the nucleus of a satellite, it would have been taken away from her by the stronger attraction of the sun. The same thing would have happened to us if we had been a little nearer the sun. And yet in 1645 a moon belonging to Venus was supposed to have been discovered, and it was thought to have been seen three times within the rest of that century, and four times within the first half of the following century. The last supposed view of it was in 1791; it has never been seen since. There is little doubt that it was an illusion of some kind. Perhaps, though, Venus has not the same need of a moon that we have.

ATMOSPHERE, DAY AND NIGHT, AND SEASONS

There is no doubt that Venus is in much better plight than Mercury, the other inferior planet, in regard to atmosphere. Until recently no one has questioned the belief that her atmosphere is very extensive--twice as heavy, perhaps, as that of the earth, dense, and full of clouds. The luminous ring about her, shown when she is making a transit across the face of the sun, points to a heavy atmosphere; and no less certain indications of it are given in the faint light which stretches beyond the termination of the horns when she is in the crescent phase, near inferior conjunction. Her very high reflecting power is also indicative of an atmosphere laden with clouds. White clouds form one of the most highly reflecting surfaces known, and the peculiar brilliancy of Venus is thought to be in great part due to the presence of large masses of clouds in her atmosphere. By the spectroscope, and in other ways, the water necessary to form clouds is shown to be abundant in her atmosphere. Even those astronomers who doubt the long-current belief in the large extent of her atmosphere concede an atmosphere of more or less density, though by one authority it is characterized as somewhat gauzy.

There is one vital point concerning the development of Venus upon which we have as yet no positive knowledge: the length of time in which she rotates on her axis. This is unfortunate, because until her time of rotation is known we cannot know much about her physical condition. Her rotation, we know, determines the length of her day and night, or whether, indeed, she has any. The time of it has been calculated to be anywhere from a little less than one of our days to two hundred and twenty-five, the latter being also the time of her revolution about the sun. Astronomers of equal reputation have come to exactly opposite results in their investigations. To one, the spectroscope has indicated the short day and night; to another it has shown no day and night, but a planet with one face forever toward the sun, like Mercury. What appeared to be stable surface markings have been observed, but have indicated under the eyes of different observers both the short day and no day at all. The disc has been measured during a transit, and shows so little flattening as to indicate a slow rotation and the long day. On the other hand, the best authorities think it unlikely that at the distance of Venus the sun could so retard the planet’s rotation as to make it coincide with its time of revolution. Thus the question is still an open one.

The truth may be that, owing to the density of her atmosphere, the surface of Venus has never been seen at all, and that the apparently stable markings are but clouds more or less lacking in stability. The difficulty of observing Venus will probably make it impossible to determine this point by visual observation. It may some day be settled beyond a doubt by the spectroscope. In some way it will surely be settled. Astronomers have too often made possible what seemed to be impossible for us to doubt that some one will find a way to discover this secret of Venus. With them a failure to prove a conclusion does not mean to abandon the subject, but to try some other means of getting at the truth.

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The sun viewed from Venus would appear considerably larger than it does to us. Its apparent diameter to us is a little more than thirty-two minutes, while on Venus it would be something more than thirty-eight minutes; that is, it would appear about one-fifth larger on Venus than it does to us. This is enough to make a material difference between the two planets in the amount of heat and light they receive. Venus receives nearly twice (1.9) as much heat and light from the sun as we receive, but less than one-third as much as Mercury. If she had no atmospheric protection, there is no question but that she would have a climate disastrously warm for a race of beings constituted as we are. The normal temperature of an unprotected body at the distance of Venus is about 158° Fahrenheit (70° Centigrade).

If Venus is finally proved to have no alternations of day and night, she is still better off than Mercury, who has practically no atmosphere to protect him from the intense heat of the sun. How much protection she has depends altogether on the extent of her atmosphere. It is probably not enough to make the hot side comfortable from our point of view; and Venus, being undoubtedly a solid body with no internal heat, the cold side must be cold beyond anything we have any conception of. But there may be a very considerable part on each side that, owing to the refraction of light by the atmosphere, is more or less well lighted, and is also more or less protected by this same beneficent atmosphere from deadly extremes of heat and cold. In this situation there would undoubtedly be lively currents of air from the heated side to the cooler; but even these may in some way carry with them some tempering effects on the climate, as we know such currents do here on the earth.

If it should prove that the length of the day and night on Venus is something near that of the earth’s (and this seems not unlikely), she would then be indeed more like a twin sister to us. Being next to each other in our distances from the sun, and of nearly the same size, differing but little in density, mass, volume, and force of gravity, with her greater normal heat probably reduced by her heavier atmosphere to a temperature producing climatic conditions not very unlike ours, and with not very different alternations of day and night, we might well be considered more nearly related than any of the other members of the solar family.

The seasons, however, on Venus and the earth would not have much resemblance to each other. The axis of the earth is inclined to the ecliptic nearly twenty-three and one-half degrees, so that we receive the sun’s rays with varying degrees of obliquity during our yearly journeying around it, which is the cause of our agreeable change of seasons. Venus travels with her axis so slightly inclined to her orbit (a little more than three degrees) that each particular parallel of latitude receives practically the same amount of sunlight every day in the year, though at different parallels the sun’s rays strike with varying degrees of obliquity. However delightful or disagreeable the climate may be, there are no changes of seasons to speak of, and one could find variety only by going from place to place on the planet. She receives no compensation for this monotony by alternately receding from and approaching the sun as Mercury does, or by librations, such as he has. Her orbit being, as we have seen, so nearly circular as to permit of only small variations in her distance from the sun, and her axis so nearly perpendicular to her orbit, it follows that she has nothing to mark the year; and, whether she turns on her axis many times or only once during a revolution, life on Venus would be very monotonous to any one accustomed to our delightful variety of climate and seasons. Still, there is nothing in this monotony to prevent Venus from being a fairly comfortable habitation in some parts for such beings as inhabit the earth. The only real obstacle to habitability on Venus would be her lack of rotation and all that it involves.

Since we are not sure that we can see the surface of Venus, we cannot say what that surface is. Nevertheless, there is some reason to suspect that we would find there mountains of vast height. Certain irregularities have been observed at times, of a kind to indicate mountains covered with snow, extending beyond the clouds. They have been estimated to be many miles higher than any mountains we have on earth, their height depending somewhat upon the temperament of the observer. But inasmuch as these same high mountains have sometimes been thought to be only masses of clouds, it seems hardly safe to pronounce definitely upon them.

TRANSITS

On rare occasions, when Venus is in inferior conjunction, she makes a transit, and can then be seen as a black dot moving over the bright face of the sun. Transits can occur only when the earth and the planet are near the point where their orbits cross each other. The earth is at this point every year on June 7th and December 7th; but the orbit of Venus is such that she is there on the proper dates only four times in a period of two hundred and forty-three years. In every two hundred and forty-three years four transits take place. They occur in pairs, eight years apart, and in the same month. If a pair occur in June, it will be one hundred and five and one-half years after the last one of the pair until we have the first of the December pair of transits. After that it will be one hundred and twenty-one and a half years until we have the first of another pair of June transits.

The first transit of Venus that was scientifically observed was in December, 1639. It was the last of a December pair, there having been a transit eight years before, in December, 1631. One hundred and twenty-one and a half years later, in 1761, a June transit occurred, and in 1769 another one took place in June. Then there were no more for one hundred and five and one-half years, when we had a December pair in 1874 and 1882. The next ones will be in June, 2004 and 2012.

Great importance was attached to those transits that occurred in 1874 and 1882, because they were expected to be useful in determining with greater exactness the distance of the sun. Extensive preparations were made for scientific observation of them; but the results were not satisfactory, largely because the atmosphere of Venus prevented her from showing a sharp outline at the moment of entering upon and of leaving the face of the sun. The main scientific value of a transit of Venus now is in the opportunity it may offer to investigate the nature of her atmosphere. Even though that interesting question may have been practically settled before another transit takes place, it will be important to know to what degree the phenomena observed at the next transit confirm the decision.

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On account of the surpassing brilliancy of Venus, the brightest of all the heavenly bodies after the sun and moon, she was to the ancients the most important of all the stars and planets. She was the supreme evening and morning star. As evening star she was known as Hesperus, or Vesper; as a morning star she was called Phosphorus, or Lucifer, and under all these names she is frequently mentioned in Greek and Latin and kindred literatures.

The symbol of Venus is ♀, a figure which is nothing more than the conventionalized form of a looking-glass, an article that is often pictured in the hands of the goddess for whom our beautiful planet was named. In her general aspect she is as placidly splendid and charming as ever a goddess could be, and it is not strange that the happy ears that could hear such strains should find her, as they did, singing a rich contralto in the music of the spheres. Jupiter and Saturn, under this mythological apportionment, sang bass, Mars took care of the tenor strains, and the high soprano was carried by our little dwarf Mercury.

XII

MARS

The planet that varies most in the beauty of its aspect is Mars. It is as much as fifty times brighter when it is nearest to us than it is at its greatest distance from us. At its brightest it is many times more brilliant than any of the first-magnitude stars; but when it leaves our neighborhood and goes far off into space in its journey around the sun, its glory is so dimmed that it becomes not brighter than an ordinary second-magnitude star, such as the pole-star, and less brilliant than the brightest stars in the Big Dipper.

These extreme changes of brightness are due not so much to any great distance that Mars goes from us in comparison with other planets as to its coming so very near to us at times. It is, after all, a small body, and no great distance, as heavenly distances go, is required to make it show so. But the eccentricity of its orbit brings it sometimes very near us, and its near approaches are at a time when we can see its entire disc, and not a mere crescent, such as we see when Venus is nearest to us. Mars does not come quite so near to us as Venus comes, but when he is in the best position to be seen he is much nearer than she is when in her best position. For we have seen that Venus is brightest before she reaches her nearest position to us, while Mars is brightest when he is at his nearest to us. When Venus is at greatest elongation she is three times farther away than Mars is at his nearest.

HOW TO IDENTIFY MARS