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first and most prominent object which claims attention, is the representation of the Sun's circumference, with its deep radiations, bounding the upper margin of the map. It is apparent, however, that this segment is hardly one-sixth of the whole circumference of which it is a part. Were the map sufficiently large to admit the entire orb of the Sun, even upon so diminutive a scale as there represented, we should then see the Sun and Planets in their just proportions-the diameter of the former being 112 times the diameter of the Earth.
It was intended, originally, to represent the Earth upon a scale of one inch in diameter and the other bodies in that proportion; but it was found that it would increase the map to four times its size; and hence it became necessary to assume a scale of half an inch for the Earth's diameter, which makes that of the Sun 56 inches, and the other bodies, as represented upon the map.
The relative position of the Planets' orbits is also represented, on a scale as large as the sheet would permit. Their relative distances from the Sun as a center, and from each other, are there shown correctly. But had we wished to enlarge the dimensions of these orbits, so that they would exactly correspond with the scale to which we have drawn the planets, the map must have been nearly two miles in length. "Hence," says Sir John Herschel," the idea that we can convey correct notions on this subject, by drawing circles on paper, is out of the question."
To illustrate this-Let us suppose ourselves standing on an extended plane, or field of ice, and that a globe 4 feet 8 inches in diameter is placed in the center of the plane, to represent the Sun. Having cut out of the map the dark circles representing the planets, we may proceed to arrange them in their respective orbits about the Sun, as follows:
First, we should take Mercury, about the size of a small currant, and place it on the circumference of a circle 194 feet from the Sun; this circle would represent the orbit of Mercury, in the proper ratio of its magnitude. Next, we should take Venus, about the size of a rather small cherry, and place it on a circle 362 feet from the Sun, to represent the orbit of Venus. Then would come the Earth, about the size of a cherry, revolving in an orbit 500 feet from the Sun. After the Earth we should place Mars, about the size of a cranberry, on a circle 762 feet from the Sun. Neglecting the Asteroids, some of which would not be larger than a pin's head, we should place Jupiter, hardly equal to a moderate-sized melon, on a circle at the distance of half a mile (2601 feet) from the Sun; Saturn, somewhat less, on a circle nearly a mile (4768 feet) from the Sun; Herschel, about the size of a peach, on the circumference of a circle nearly 2 miles (9591 feet) from the Sun; and last of all Neptune, a little larger than Herschel, and on à circle of nearly 8 miles (15,366 feet) from the Sun.
To imitate the motions of the planets in the above-mentioned orbits, Mercury must describe its own diameter in 41 seconds; Venus, in 4 minutes 14 seconds; the Earth, in 7 minutes; Mars, in 4 minutes 48 seconds; Jupiter, in 2 hours 56 minutes; Saturn, in 8 hours 13 minutes; Herschel, in 12 hours 16 minutes; and Neptune, in 23 hours 25 min. Many other interesting subjects are embraced in Map I.; but they are either explained on the map, or in the following chapters, to which they respectively relate.
THE SUN-HIS DISTANCE, MAGNITUDE, &o.
327. THE Sun is a vast globe, in the center of the solar system, dispensing light and heat to all the planets, and governing all their motions. It is the great parent of vegetable life, giving warmth to the seasons, and color to the landscape. Its rays are the cause of various phenomena on the surface of the earth and in the atmosphere. By their agency, all winds are pro
of Map I.? Its scale? Remark of Dr. Herschel? does he furnish? 327. Sucject of Chapter II.?
What illustrations of the Solar Systems
duced, and the waters of the sea are made to circulate in vapor through the air, and irrigate the land, producing springs and rivers.
328. The Sun is by far the largest of the heavenly bodies whose dimensions have been definitely ascertained. Its diameter is about 886,000 miles. Consequently, it contains a volume of matter equal to fourteen hundred thousand globes of the size of the Earth. Of a body so vast in its dimensions, the human mind, with all its efforts, can form no adequate conception
THE SUN AND THE MOON'S ORBIT.
203.000 240.000 240.000 203.000
Were the Sun a hollow sphere, perforated with a thousand openings to admit the twinkling of the luminous atmosphere around it-and were a globe as large as the Earth placed at its center, with a satellite as large as our Moon, and at the same distance from it as she is from the earth, there would be present to the eye of a spectator on the interior globe, a universe as splendid as that which now appears to the uninstructed eye -a universe as large and extensive as the whole creation was conceived to be in the infancy of astronomy.
The mean distance of the Moon from the Earth is 240,000 miles, consequently the average diameter of her orbit is 480,000 miles; and yet, were the Sun to take the place of the Earth, he would fill the whole orbit of the Moon, and extend 200,000 miles beyond it in every direction! To pass from side to side through his center, at railroad speed (30 miles an hour), would require nearly three and a half years,
and to traverse his vast circumference nearly eleven years.
Here let the student refer to Map I., where the Relative Magnitudes of the Sun and Planets are exhibited. Let him compare the segment of the Sun's circumference, as there represented, with the entire circumference of the Earth. They are both drawn upon the same scale. The segment of the Sun's circumference, since it is almost a straight line, must be a very small part of what the whole circumference would be, were it repre sented entire. Let the student understand this diagram, and he will be in some measure able to conceive how like a mere point the Earth is, compared with the Sun, and to form in his mind some image of the vast magnitude of the latter.
329. The next thing which fills the mind with wonder, is the distance at which so great a body must be placed, to occupy, apparently, so small a space in the firmament. The Sun's mean distance from the Earth is twelve thousand times the Earth's diameter, or a little more than 95,000,000 of miles. We may derive some faint conception of such a distance, by considering that the swiftest steamboats, which ply our waters at the rate of 200 miles a day, would not traverse it in thirteen hundred years; and, that a cannon ball, flying night and day, at the rate of 16 miles a minute, would not reach it in eleven years.
330. The Sun, when viewed through a telescope, presents the appearance of an enormous globe of fire, frequently in a state of violent agitation or ebullition; dark spots of irregular form,
328. His magnitude? Diameter? Compared with the Earth? What illustration given? What reference to the Map? 329. Distance of the Sun? What illustration given? 230. How does the Sun appear through a telescope? Describe these spots?
rarely visible to the naked eye, frequently pass over his disc, from east to west, in the period of nearly fourteen days.
SPOTS ON THE SUN.
These spots are usually surrounded by a penumbra, or less deeply shaded border, and that, by a margin of light more brilliant that that of the Sun. A spot when first seen on the eastern edge of the Sun, appears like a line which progressively extends in breadth, and increases its apparent velocity, till it reaches the middle, when it begins to contract, and to move less rapidly, till it ultimately disappears at the western edge. In some rare instances, the same spots re-appear on the east side, and are permanent for two or three revolutions. But, as a general thing, the spots on the Sun are neither permanent nor uniform. Sometimes several small ones unite into a large one; and, again, a large one separates into numerous small ones. Some continue several days, weeks, and even months, together; while others appear and disappear, in the course of a few hours. Those spots that are formed gradually, are, for the most part, as gradually dissolved; whilst those that are suddenly formed, generally vanish as quickly.
331. It is the general opinion, that spots on the Sun were first discovered by Galileo, in the beginning of the year 1611; though Scheiner, Harriot, and Fabricius, observed them about the same time. During a period of 18 years from this time, the Sun was never found entirely clear of spots, excepting a few days in December, 1624: at other times, there were frequently seen twenty or thirty at a time, and in 1625, upwards of fifty were seen at once. From 1650 to 1670, scarcely any spots were to be seen; and, from 1676 to 1684, the orb of the Sun presented an unspotted disc. Since the beginning of the eighteenth century, scarcely a year has passed, in which spots have not been visible, and frequently in great numbers. In 1799, Dr Herschel observed one nearly 30,000 miles in breadth.
A single second of angular measure, on the Sun's disc, as seen from the Earth, corresponds to 462 miles; and a circle of this diameter (containing therefore nearly 220,000 square miles) is the least space which can be distinctly discerned on the Sun as a visible area, even by the most powerful glasses. Spots have been observed, however, whose linear diameter has been more than 44,000 miles; and, if some records are to be trusted, of even still greater extent.
DR. DICK, in a letter to the author, says: "I have for many years examined the solar spots with considerable minuteness, and have several times seen spots which were not less than the one twenty-fifth part of the Sun's diameter, which would make them about 22,192 miles in diameter, yet they were visible neither to the naked eye, nor through an opera glass magnifying about three times. And, therefore, if any spots have been visible to the naked eye-which we must believe, unless we refuse respectable testimonythey could not have been much less than 50,000 miles in diameter."
331. Who first saw them? When? How was it for the next 18 years? How in 1625? From 1650 to 1670? From 1676 to 1684? How since the beginning of the eighteenth century? Dr. Herschel's measurements? Dr. Dick's remarks and conclusion?
332. The apparent direction of these spots over the Sun's disc is continually varying. Sometimes they seem to move across it in straight lines, at others in curve lines. Sometimes the spots seem to move upward, as they cross from east to west, while at other times they incline downward, while the curve lines are sometimes convex towards one pole of the Sun, and sometimes towards the other.
333. All these phenomena are owing to the fact that the axis of the Sun is inclined to the ecliptic, so that viewing him from different points in the Earth's orbit, the apparent direction of the spots must necessarily vary. The following diagrams may serve to illustrate :
Let E F represent the plane of the ecliptic. In March, the spots describe a curve, which is convex to the south, as shown at A. In June, they cross the Sun's disc in nearly straight lines, but incline upward. In September, they curve again, though in the opposite direction; and in December, pass over in straight lines, inclining downward. The figures B and D show the inclination of the Sun's axis.
The following diagram will serve still further to illustrate the cause of the change of direction of the solar spots.
SOLAR SPOTS OBSERVED FROM DIFFERENT POINTS.
Let the student imagine himself stationed upon the earth at A, in March, looking upon the sun in the center, whose north or upper pole is now inclined toward him. The spots will then curve downward. Three months afterward-viz., in June-the earth will be
332. In what general direction do these spots move? What variations? 888. What is the cause of these varying phenomena ?
at B; when the sun's axis will incline to the left, and the spots seem to pass upward to the right. In three months longer, the observer will be at C, when the north pole of the sun will incline from him, and the spots seem to curve upward; and in three months longer, he will be at D, when the axis of the sun will incline to the right, and the spots seem to incline downward.
334. From the regularity with which these spots revolve, it is concluded, with good reason, that they adhere to the surface of the Sun and revolve with it. They are all found within 30° of his equator, or within a zone 60 in width.
335. The apparent revolution of a spot, from any particular point of the Sun's disc, to the same point again, is accomplished in 27 days, 7 hours, 26 minutes, and 24 seconds; but during that time, the spot has, in fact, gone through one revolution, together with an arc, equal to that described by the Earth in her orbit in the same time; which reducest he time of the Sun's actual rotation on his axis, to 25 days, 9 hours, and 36 minutes.
SIDEREAL AND SYNODIC REVOLUTIONS OF THE SUN.
Let S represent the sun, and A the earth in her orbit. When she is at A, a spot is seen upon the disc of the sun at B. The sun revolves in the direction of the arrows, and in 25 days 10 hours the spot comes round to B again, or opposite the star E. This is a sidereal revolution.
During these 25 days 8 hours, the earth has passed on in her orbit some 25°, or nearly, to C, which will require nearly two days for the spot at B to get directly toward the earth, as shown at D. This last is a synodic revolution. It consists of one complete revolution of the sun upon his axis, and about 27° over.
336. The part of the Sun's disc not occupied by spots, is far from being uniformly bright. Its ground is finely mottled with an appearance of minute dark dots, or pores, which, attentively watched for several days in succession, are found to be in a constant state of change.
What the physical organization of the Sun may be, is a question which astronomy, in its present state, cannot solve. It seems, however, to be surrounded by an ocean of inexhaustible flame, with dark spots of enormous size, now and then floating upon its surface. From these phenomena, Sir W. Herschel supposed the Sun to be a solid, dark body, surrounded by a vast
334. Are these spots supposed to adhere to the body of the Sun? On what part of the Sun are they found? 835. What is their time of apparent revolution? The actual time? How arrived at? 336. What said of the part of the Sun about his poles? Of his physical organization? What does it seem to be? How did Sir W. Herschel regard it?