HEIGHT MEASURED. 155 the line EOM, drawn from E to the centre of the moon, makes a right angle with the sun-ray SOL, which touches the terminator at O, and strikes the top of the mountain at L. 289. Now an observer at E, sees the top of the mountain in the direction of the line EL, and with the proper instrument he can easily ascertain the magnitude of the angle LEO; which is the angular distance between the summit of the mountain and the terminator. Having obtained this value, and knowing the apparent diameter of the moon, and its length in miles; the height of the mountain (CL) can be ascertained by means of a property of the right angled triangle LOM; viz., that in every right angled triangle the square of the hypothenuse1 is equal to the sum of the squares of the other two sides. 290. The calculation is made as follows. Let us suppose that the angle LEO is equal to one twelfth part of the apparent diameter of the moon (31' 20";) then will the line LO be very nearly equal to one twelfth part of the moon's diameter measured in miles; viz., 90 miles. Now the square of LM equals the square of LO (90 × 90,) added to the square of OM (1080x1080 ;) that is to 1,174,500. The square root of this quantity, or 1083.74 is therefore, the length of the line LM in miles. LM is then 1083.74 miles long; but it consists of two parts, to wit, the height of the mountain LC, and the radius of the moon CM. Now the length of the latter is 1080 miles, subtracting then 1080 miles (CM) from 1083.74 miles (ML,) the remainder 3.74 miles (LC,) is the height of the mountain; nearly three miles and three quarters. 291. It is not necessary that the moon should be in quadrature in order to determine, by this method, the height of the lunar mountains, but this phase has been selected because the calculations are shorter and less intricate, than when the moon is in other positions in her orbit. A distinguished German astronomer, Schroeter, has 1. The hypothenuse of a triangle is the side opposite the right angle. Show how the calculation is made? pursued a different method from the one just given. He estimated the altitudes of the moon's mountains, by the length of the shadows cast upon its surface. 292. NAMES AND HEIGHTS OF THE LUNAR MOUNTAINS. The method now universally adopted, by the most distinguished astronomers, to designate remarkable regions in the moon, is to assign to these localities the names of men renowned for their attainments in science and literature; as for instance, Newton, Tycho, Kepler, Herschel. 293. The surface of the moon is more rugged than that of the earth; for though the former is much smaller than the latter, yet its mountains nearly equal in altitude the highest of our own. 294. Prof. Mädler of Prussia, who has studied the physical condition of the moon with more success than any living astronomer, has constructed, in connection with Prof. Beer, another Prussian astronomer of high reputation, large lunar maps; in which the most remarkable spots and regions of the moon are laid down with great exactness. Their magnitudes have also been ascertained, and their forms delineated with the utmost precision. 295. The heights of no less than 1095 lunar mountains have been determined by these astronomers, and out of twenty measured by Mädler, three tower to an altitude of more than 20,000 feet, while the rest exceed the height of 16,000 feet, or about three miles. The names of a few of the loftiest mountains are as follows: Feet. Newton, 23,800 Curtius, 22,200 Feet. 20,800 Casatus, 296. The highest lunar mountain, as we perceive, reaches an altitude of nearly 24,000 feet, or about four miles and a half; which is nearly the height of the loftiest mountains of our globe. If our mountains were as How did Schroeter estimate the heights of the lunar mountains? What method has been adopted in order to designate the remarkable regions in the moon? What is said respecting the surface of the moon? State what has been done by Prof's. Mädler and Beer? How many lunar heights have been determined by these astronomers? What is said respecting the heights of twenty, measured by Mädler? Give the names and altitudes of the four highest? much higher than the lunar mountains as the earth is larger than the moon, the Himmalehs and Andes would soar to altitude of 16 miles, above the level of the ocean. 297. LUNAR CRATERS. The moon is not only distinguished for lofty mountains, but also, as we have stated, for singularly formed cavities and craters which are depressed far below the general surface. They are of various sizes, and are scattered all over the disk of the moon; being however most numerous in the southwestern part. In form they are nearly all circular, and are shaped like a bowl; and from the level bottom of most of the larger a conical hill usually rises at the centre. 298. Oftentimes the circular walls of these craters are entirely below the general surface of the moon, but they are usually elevated somewhat above the surface, forming a ring mountain; whose height on the outside is frequently not more than one-third or one-half of its altitude on the inside; measuring from the bottom of the crater to the top of the mountain. Twelve craters according to Schroeter are more than two miles deep, and to some of these a depth of over four miles is assigned by the same observer. 299, That these appearances, which are regarded as cavities are such in reality, is evident from the fact, that the side nearest the sun is in shadow, while the side most remote is illumined by his beams. Just as the eastern side of a well is in shadow in the morning, when the sun shines, while the western side at the top is bright with the solar rays. 300. One of the finest instances of a ring mountain with its enclosed crater is the spot called Tycho. The breadth of the crater is nearly fifty miles, the height of the mountain on the inside is about 17,000 feet, and on the outside it is not less than 12,000; the bottom of the crater, is therefore 5,000 feet below the general surface of the moon. If the mountains of our globe were as much higher than the lunar mountains as the earth is larger than the moon, how high would the Andes and Himmalehs soar? What is said respecting the lunar craters? Of their sizes and forms? What is said in regard to the circular walls of these craters? How deep are these craters according to Schroeter, State the proofs that these spots are really cavities. Describe Tycho ? From the centre of the enclosed area a beautiful mountain rises to the height of almost one mile. 301. By the aid of a powerful telescope, Tycho is seen as it is delineated in Fig. 57. The ranges of the FIG. 57. ring mountain are here beheld on the right hand of the figure, with their summits bathed in light, while their sides opposite to the sun, rest in the deepest shade. On the left hand, nearest to the sun, the solar rays, streaming over the encircling mountain walls of the crater, leave half of it in darkness; the heavy shadow of the central mountain projecting far into the illumined portion. 302. Many of the craters are of great dimensions, the largest being nearly 150 miles in diameter. The diame Explain the cut. What is said respecting the magnitude of these craters? LUNAR VOLCANOES. 159 ters of the six broadest as inferred from the observations of Prof. Mädler, are as follows: the same astronomer: 2 were between 1 and 2 miles wide 123+ 4 แ 3 3 66 แ แ 12 แ 9 แ 66 "10 And 36 were above 10 miles across. 303. LUNAR VOLCANOES. The existence of active volcanoes has been announced more than once by astronomers. In 1787, Sir William Herschel, gave notice to the world that he had observed three lunar volcanoes in actual operation, two of which were either just ready to break out, or were nearly extinct; while the third was in a state of eruption. The burning part of the latter was estimated to be three miles in extent, while the adjacent regions were illumined with the glare of its fires. Since this period the attention of many astronomers has been directed to this subject, and their investigations have led to the conclusion that the remarkable appearances, which were regarded as indicating the existence of volcanoes, can be satisfactorily attributed to other causes, and the opinion is now prevalent among astronomers, that active lunar volcanoes do not now exist. 304. The aspects of the moon however, indicate that it has been the theatre of intense volcanic action, and the ring mountains or craters strikingly reveal this fact. "In some of the principal craters," says Sir John Herschel, Give the diameters of the six broadest, according to Mädler's measurements? State what is said of the diameters of 148 craters measured by the same astronomer? What was the belief of Sir William Herschel in respect to the existence of active lunar volcanoes? Have these remarkable appearances been regarded as active volcanoes by later astronomers? What is now the prevalent opinion among astronomers? Are there any indications in the aspects of the moon that active volcanoes once existed? |