Astronomy for Amateurs

A.—Mercury, Venus, the Earth, Mars And now we are in the Solar System, at the center, or, better, at the focus of which burns the immense and dazzling orb. We have appreciated the grandeur and potency of the solar globe, whose rays spread out in active waves that bear a fecundating illumination to the worlds that gravitate round him; we have appreciated the distance that separates the Sun from the Earth, the third of the planets retained within his domain, or at least I trust that the comparisons of the times required by certain moving objects to traverse this distance have enabled us to conceive it. We said that the four planets nearest to the Sun are Mercury, at a distance of 57 million kilometers (35,000,000 miles); Venus, at 108 million (67,000,000 miles); the Earth, at 149 million (93,000,000 miles); and Mars at 226 million (140,000,000 miles). Let us begin our planetary journey with these four stations. [Pg 114] MERCURY A little above the Sun one sometimes sees, now in the West, in the lingering shimmer of the twilight, now in the East, when the tender roseate dawn announces the advent of a clear day, a small star of the first magnitude which remains but a very short time above the horizon, and then plunges back into the flaming sun. This is Mercury, the agile and active messenger of Olympus, the god of eloquence, of medicine, of commerce, and of thieves. One only sees him furtively, from time to time, at the periods of his greatest elongations, either after the setting or before the rising of the radiant orb, when he presents the aspect of a somewhat reddish star. This planet, like the others, shines only by the reflection of the Sun whose illumination he receives, and as he is in close juxtaposition with it, his light is bright enough, though his volume is inconsiderable. He is smaller than the Earth. His revolution round the Sun being accomplished in about three months, he passes rapidly, in a month and a half, from one side to the other of the orb of day, and is alternately a morning and an evening star. The ancients originally regarded it as two separate planets; but with attentive observation,[Pg 115] they soon perceived its identity. In our somewhat foggy climates, it can only be discovered once or twice a year, and then only by looking for it according to the indications given in the astronomic almanacs. Fig. 32.—Orbits of the four Planets nearest to the Sun. Mercury courses round the Sun at a distance of 57,000,000 kilometers (35,000,000 miles), and accomplishes his revolution in 87 days, 23 hours, 15 minutes; i.e., 2 months, 27 days, 23 hours, or a little less than three of our months. If the conditions of life are the[Pg 116] same there as here, the existence of the Mercurians must be four times as short as our own. A youth of twenty, awaking to the promise of the life he is just beginning in this world, is an octogenarian in Mercury. There the fair sex would indeed be justified in bewailing the transitory nature of life, and might regret the years that pass too quickly away. Perhaps, however, they are more philosophic than with us. Fig. 33.—Orbits of the four Planets farthest from the Sun. [Pg 117] The orbit of Mercury, which of course is within that of the Earth, is not circular, but elliptical, and very eccentric, so elongated that at certain times of the year this planet is extremely remote from the solar focus, and receives only half as much heat and light as at the opposite period; and, in consequence, his distance from the Earth varies considerably. Fig. 34.—Mercury near quadrature. This globe exhibits phases, discovered in the seventeenth century by Galileo, which recall those of the Moon. They are due to the motions of the planet round the Sun, and are invisible to the unaided eye, but with even a small instrument, one can follow the gradations and study Mercury under every aspect. Sometimes, again, he passes exactly in front of the Sun, and his disk is projected like a black point upon the luminous surface of the flaming orb. This occurred, notably, on May 10, 1891, and November 10, 1894; and the phenomenon[Pg 118] will recur on November 12, 1907, and November 6, 1914. Mercury is the least of all the worlds in our system (with the exception of the cosmic fragments that circulate between the orbit of Mars and that of Jupiter). His volume equals only 5/100 that of the Earth. His diameter, in comparison with that of our planet, is in the ratio of 373 to 1,000 (a little more than 1⁄3) and measures 4,750 kilometers (2,946 miles). His density is the highest of all the worlds in the great solar family, and exceeds that of our Earth by about 1⁄3; but weight there is less by almost 1⁄2. Mercury is enveloped in a very dense, thick atmosphere, which doubtless sensibly tempers the solar heat, for the Sun exhibits to the Mercurians a luminous disk about seven times more extensive than that with which we are familiar on the Earth, and when Mercury is at perihelion (that is, nearest to the Sun), his inhabitants receive ten times more light and heat than we obtain at midsummer. In all probability, it would be impossible for us to set foot on this planet without being shattered by a sunstroke. Yet we may well imagine that Nature's fecundity can have engendered beings there of an organization different from our own, adapted to an existence in the proximity of fire. What magnificent landscapes may there be[Pg 119] adorned with the luxuriant vegetation that develops rapidly under an ardent and generous sun? Fig. 35.—The Earth viewed from Mercury. Observations of Mercury are taken under great difficulties, just because of the immediate proximity of[Pg 120] the solar furnace; yet some have detected patches that might be seas. In any case, these observations are contradictory and uncertain. Up to the present it has been impossible to determine the duration of the rotation. Some astronomers even think that the Sun's close proximity must have produced strong tides, that would, as it were, have immobilized the globe of Mercury, just as the Earth has immobilized the Moon, forcing it perpetually to present the same side to the Sun. From the point of view of habitation, this situation would be somewhat peculiar; perpetual day upon the illumined half, perpetual night upon the other hemisphere, and a fairly large zone of twilight between the two. Such a condition would indeed be different from the succession of terrestrial days and nights. As seen from Mercury, the Earth we inhabit would shine out in the starry sky[9] as a magnificent orb of first[Pg 121] magnitude, with the Moon alongside, a faithful little companion. They should form a fine double star, the Earth being a brilliant orb of first magnitude, and the Moon of third, a charming couple, and admired doubtless as an enchanted and privileged abode. It is at midnight during the oppositions of the Earth with the Sun that our planet is the most beautiful and brilliant, as is Jupiter for ourselves. The constellations are the same, viewed from Mercury or from the Earth. But is this little solar planet inhabited? We do not yet know. We can only reply: why not? VENUS When the sunset atmosphere is crimson with the glorious rays of the King of Orbs, and all Nature assumes the brooding veil of twilight, the most indifferent eyes are often attracted and captivated by the presence of a star that is almost dazzling, and illuminates with its white and limpid light the heavens darkened by the disappearance of the God of Day. Hail, Venus, Queen of the Heavens! the "Shepherd's Star," gentle mother of the loves, goddess of beauty, eternally adored and cherished, sung and immortalized upon Earth, by poets and artists. Her splendid brilliancy attracted notice from earliest antiquity, and we find her,[Pg 122] radiant and charming, in the works of the ancients, who erected altars to her and adorned their poetry with her grace and beauty. Homer calls her Callisto the Beautiful; Cicero names her Vesper, the evening star, and Lucifer, the star of the morning—for it was with this divinity as with Mercury. For a long while she was regarded as two separate planets, and it was only when it came to be observed that the evening and the morning star were always in periodic succession, that the identity of the orb was recognized. Her radiant splendor created her mythological personality, just as the agility of Mercury created that of the messenger of the gods. We do not see her aerial chariot in the Heavens drawn by a flight of doves with white and fluttering wings, but we follow the lustrous orb led on through space by solar attraction. And in the beautiful evenings when she is at her greatest distance from our Sun, the whole world admires this white and dazzling Venus reigning as sovereign over our twilight[10] for hours after sunset, and in addition to the savants who are practically occupied with astronomy, millions of eyes are raised to this celestial splendor, and for a moment millions of human beings feel some curiosity about the mysteries of the Infinite. The brutalities of daily life would fain petrify our dreams, but thought is not yet stifled to the point of checking all aspirations after eternal truth, and when we gaze at the starry sky it is hard not to ask ourselves the nature of those other worlds, and the place occupied by our own planet in the vast concert of sidereal harmony. [Pg 123] Fig. 36.—The Evening Star. [Pg 124] Even through a small telescope, Venus offers remarkable phases. Fig. 37.—Successive phases of Venus. Fig. 37 gives some notion of the succession of these, and of the planet's variations in magnitude during its journey round the Sun. Imagine it to be rotating in a year of 224 days, 16 hours, 49 minutes, 8 seconds at a distance of 108 million kilometers (67,000,000 miles), the Earth being at 149 million kilometers (93,000,000 miles). Like Mercury, at certain periods it passes between the Sun and ourselves, and as its illuminated hemisphere is of course turned toward the orb of day, we at those times perceive only a sharp and very luminous crescent. At[Pg 125] such periods Venus is entirely, so to say, against the Sun, and presents to us her greatest apparent dimension (Fig. 38). Sometimes, again, like Mercury, she passes immediately in front of the Sun, forming a perfectly round black spot; this happened on December 8, 1874, and December 6, 1882; and will recur on June 7, 2004, and June 5, 2012. These transits have been utilized in celestial geometry in measuring the distance of the Sun. You will readily divine that the distance of Venus varies considerably according to her position in relation to the Earth: when she is between the Sun and ourselves she is nearest to our world; but it is just at those times that we see least of her surface, because she exhibits to us only a slender crescent. Terrestrial astronomers are accordingly very badly placed for the study of her physical constitution. The best observations can be made when she is situated to right or left of the Sun, and shows us about half her illuminated disk—during the day for choice, because at night there is too much irradiation from her dazzling light. These phases were discovered by Galileo, in 1610. His observations were among the first that confirmed the veracity of the system of Copernicus, affording an evident example of the movement of the planets round the sun. They are often visible to the unaided eye with good sight, either at dusk, or through light clouds. [Pg 126] Fig. 38.—Venus at greatest brilliancy. Venus, surrounded by a highly dense and rarefied atmosphere, which increases the difficulties of observing her surface, might be called the twin sister of the Earth, so similar are the dimensions of the two worlds. But, strange as it may seem to the many admirers, who are ready to hail in her an abode of joy and happiness, it is most