From what is the word Planet derived? What are primary planets? How many primary planets are there? What are secondary planets? How many secondary planets have been discovered? Which of the primaries have satellites? How many have each? SECTION IV. Of Mercury. (Mercury is the planet nearest the sun, So it is still considered after the most accurate modern discoveries. It shines by a very brilliant and white light but the short period in which it can be viewed, and the position of its body seen through the mists of the horizon, have prevented important discoveries being made on its surface. Of all the planets Mercury is the most swift in its motion. On this account the name was given to it by the ancients' after the nimble messenger of the gods,' It was 'represented by the figure of a youth with wings at his head and feet; whence is derived, the character by which it is commonly represented.' So great is the velocity of this planet, that it performs more than two revolutions to one of Venus, and, commencing at a conjunction, would pass the earth three times before it would complete a period, the synodic revolution of Mercury, as seen by us, being 115 d. 21 h. 3 m. 34 s. The mean diameter of the sun as seen from Mercury is 1° 22'. His mean distance from the sun is to that of the earth about as 4 to 10.3. The intensity of the light and heat of the sun at Mercury must be about as 6.6 to 1 at the earth, being inversely as the squares of the distances. The heat of the sun at Mercury was found by Sir Isaac Newton sufficient to make water boil. Hence beings, constituted like the inhabitants of this earth, cannot endure the climate of Mercury, if Sir Isaac was right, and the degree of heat be in proportion to the proximity of the planet to the sun. But, from what has been before considered, the circumstances of caloric and atmosphere may be so diversified; they may be so rare at the surface of Mercury, as to render the climate of this planet not only tolerable, but salubrious, a comfortable abode for animal life. This however we know, that with infinite ease the Deity could form constitutions suited to any situation or climate, destined by him for the creatures of his care. The surface of Mercury, contains nearly thirty-two millions of square miles. It may therefore sustain a population far more numerous than the present inhabitants of the earth.) According to Dr. Herschel, Mercury is equally luminous in every part of his body, having neither dark spots, nor uneven edge, but a disk well defined in every part. Mr. Schroeter; on the contrary, pretends to have discovered in this planet not dark spots only, but mountains. On the authority of the latter observer, rests the discovery of a revolution of Mercury on his axis./ ELEMENTS OF MERCURY. Diameter, 3180 miles. Mean diameter as seen from the sun, 16′′. Place of ascending node, 16° 19′ 10′′ Taurus. * 12′ 10′′. Retrograde motion of the nodes in 100 years 11′ 22′′. Place of aphelion, 8s 11° 49′ 54′′. Motion of the aphelion in longitude for 100 years, 1° 33′ 45′′. Diurnal rotation according to Schroeter, 24 h. 5 m. 28 s. In the elements of the planets, when there is a variation by time, the computation is to the 1st of January, 1831. Mean distance from the sun, 37,000,000 miles. How is Mercury situated? With what light does it shine? Why was it called Mercury? What is the velocity of Mercury compared with that of Venus and the earth? How much greater is the intensity of the sun's light and heat at Mercury than at the earth? What did Sir Isaac Newton find the heat of the sun at Mercury to be? Can Mercury be habitable? What population may Mercury sustain? How did the disk of Mercury appear to Dr. Herschel ? How did it appear to Schroeter? SECTION V. Of Venus. Venus is to us the most brilliant luminary seen in the nocturnal heavens. She appears west of the sun from her inferior to her superior conjunction, and, rising before him, is called Phosphor, Lucipher, or the evening star. Appearing east of the sun from her superior to her inferior conjunction, she sets after him, and is called Hesperus, Vesper, or the evening star. She is in rotation east or west of the sun about 292 days; but obscured by his light when near that luminary, she is not visible quite so long. It is said, that before the time of Pythagoras, the morning and evening stars were supposed to be different, and that he first discovered them to be the same. The apparent motion of Venus round the sun is retarded by the motion of the earth in its orbit, both be ing in the same direction. Her real revolution is performed in 224 d. 16 h. 49 m. 15 s. ; her apparent or synodic, in 583 d. 22 h. 7 m. 20 s. She appears, therefore, east or west of the sun longer than the whole time of a revolution in her orbit. Venus is denoted by the character ?, which is supposed to be a rude representation of a female figure with a trailing or flowing robe.' Venus must at times present to the inhabitants of Mercury a view far more brilliant than can be enjoyed by us. When at her least distance, she turns to them her whole illuminated side. But when she is nearest to us, her dark side is towards the earth. This planet is distant from the earth when nearest about 27,000,000 miles; when most remote, about 163,000,000 miles. Were the whole of its enlightened surface turned towards the earth, when it is nearest, it would exhibit a light and brilliancy twentyfive times greater than it generally does, and appear like a small brilliant moon.' Dick's Christian Philosopher. The bright side of Venus is turned nearly or quite towards us, at her superior conjunction; but she is then invisible, being near the sun, or hidden behind his body. When visible, and the illuminated part nearly round before or after that conjunction, she appears small on account of her great distance. Venus shines with a light extremely pleasant. Her silver brightness far surpasses that of the moon, and is unequalled by any of the heavenly luminaries, except, sometimes by Jupiter, or by Sirius, the most brilliant of the starry train.' Venus may occasionally be seen in the day time by the naked eye. The obstruction of her morning and evening light frequently causes shadows, well defined like those of a new moon. The distance of Venus from the sun is to that of the earth from the sun about as 7 to 10. The light and heat derived from the solar rays are to the inhabitants of Venus nearly double to those enjoyed by the inhabitants of the earth. Dr. Herschel observed spots on Venus. To him she appeared much brighter round her limb, than at the intervening line between the enlightened and dark part of her disk. From this he concluded, that Venus like the earth had an atmosphere, and that it was more luminous than the body of the planet. The height of this atmosphere according to the computation of some, is about fifty miles. Such computation, however, ought to be received with great allowance for uncertainty. The surface of the planet being enveloped in her atmosphere may be the reason, that so few spots have been seen on her disk. Plate iii. fig. 1 represents the spots on Venus observed by Bianchini. On the nineteenth of June, 1780, Dr. Herschel observed the spots, as represented in fig. 2, where a d c is a spot of darkish blue color, and c e b a brighter spot. They meet in an angle at the point c, about one third of the diameter of Venus from the cusp a. Fig. 3 represents the appearance of Venus with her blunt horn and rugged edge. 'Mr Schroeter,' says Dr. Brewster, 'seems to have been very successful in his observations upon Venus; but the results, which he has obtained, are more different than could have been wished from the observations of Dr. Herschel. He discovered several mountains in this planet, and found, that like those of the moon they were always highest in the southern hemisphere; their perpendicular heights being nearly, as the diameters of their respective planets. From the eleventh of December, 1789, to the eleventh of January, 1790, the southern hemisphere of Venus appeared much blunted with an enlightened mountain, in the dark hemisphere, nearly 22 miles high.' He states the result of four mountains measured by him: 66 First, 22.05 miles, The bluntness and sharpness alternately apparent in the horns of Venus arise, he supposes, from the shadows of high mountains. From the changes, which appear in her dark spots, and, as inferred by Mr. Schroeter, from the illumination of her cusps when she is near her inferior conjunc-. tion, the atmosphere of Venus is considered very dense. |