1DNAB MOUNTAINS. ] 



ASTRONOMY. 



939 



Julius Coesar, Plato, Endyraion, and many others of the 

 same class. The central masses of mountains are, in 

 general, very steep towards the summit, but are sur- 

 rounded by low scattered mountains at the base. Several 

 of the walled concavities are deficient in the same man- 

 ner as the walled plains, a greater portion of the outer 

 wall being broken down, and several gaps apparent in 

 the circumference. The pits of the walled concavities are 

 deeper than the surrounding country, the exterior height 

 of the wall being generally one-half or one-third of the 

 interior. But there is no proportion between the diameter 

 and depth of the annular mountains, the smaller being 

 sometimes deeper than the larger. 



The walled mountains form but a small portion of the 

 circular formations on the moon. Far more numerous 

 than those, are the craters, by which name are designated 

 the smaller concavities which present little or no ap- 

 pearance of a wall, and some of which are so minute that 

 they can scarcely be perceived with the best telescopes. 

 In some parts they are so numerous, that the lunar sur- 

 face presents the appearance of a spongy mass covered 

 with innumerable minute pores. Some of these craters, 

 small as they appear, are furnished, like those of the 

 former class, with a miniature peak in the centre. These 

 are found at all portions of the moon, and are very 

 plentiful in the interior of the walled concavities and 

 plains, in the rocky chains and seas, and the level sur- 

 faces. When the inoon is full, they appear as bright 

 speaks. They require a favourable time to be seen at 

 all properly, for, unless the sun is at a favourable alti- 

 tude, they can scarcely be perceived. 



In viewing the moon when at full, several radiating 

 bright stream" will bo noticed proceeding from many of 

 the mountains, which pass to considerable distances from 

 their apparent centres. None of the mountains, how- 

 ever, are surrounded by those bright radiating streaks 

 to the same extent as Tycho, and many of them proceed 

 to a distance of COO miles. It is scarcely possible to 

 imagine that these could be streams of lava, which flowed 

 from the central eruption ; and it is more plausibly con- 

 jectured that they must have flowed upwards through 

 fissures, produced by the central explosion of the votaino. 

 In either case, however, it may serve to give an idea of 

 the tremendous force of action required in order to cause 

 these appearances. 



HEIGHT OF THE LUNAR MOUNTAINS. The height of 

 the lunar mountains is found either by observing the dis- 

 tance of the illumined summit of a mountain from the 

 generally illuminated portion of its surface ; or, other- 

 wise, by the length of the shadow which it casts upon the 

 plane. Both these methods will be easily understood 

 from the following considerations. In determining their 

 height by the former method, we find the distance of the 

 illumined summit o (Fig. 61) from the moon's illumined 



Fig. 51. Fig. 52. 



edge c o, and we perceive that it is a tangent to the circle 

 Knowing the two sides o c' and o a' of the right- 



angled triangles, we calculate o a' ,J oc'*-{-a'c' 2 , which 

 is the distance of the summit of the mountain from 

 the centre of tho moon, and hence, subtracting the 

 radius o c', the remainder gives the height of the 

 mountain above the surface. By this method we find its 

 height in seconds of arcs, and this is nothing else than 

 if we observe it resting on the limb of the moon. Its 

 height may easily be reduced to feet. In general this 

 method will give too small a value of the height, as the 

 sun may have shone on the mountain for too great 

 a length of time previously. It was by this means 

 that Hevelius and Galileo ascertained the heights of 

 many of the lunar mountains, some of which appeared 

 to be as high as five English miles. More recent ob- 

 servations have shown them to be very high, though 

 not of such an altitude as this. 



This method, however, is not applicable unless the 

 moon is in quadrature, or half-full. The second method 

 has been more commonly made use of on that account. 

 As the mountain a ( Fig. 52) casts the shadow a c, the 

 length of this is measured by means of the micrometer. 

 Describing the circle p, q, r, s, the solar ray directed to- 

 wards b c will cut the circle in c'. The distance c'd' =cd 

 being measured, and the radius o c' known, and the two 

 sides of the right-angled triangle o c d, the angle odd is 

 known, and consequently the angle o cf a'. In the tri- 

 angle o c' a' we thus know the sides o c, and c' a, and the 

 included angle, and consequently the side o a'. Subtract- 

 ing the radius o c', the height of the mountain casting the 

 shadow a c' is determined. 



Many observers have perceived bright spots on the dark 

 portion of the lunar disc. Sir W. Herschel describes three 

 which he once saw upon it, two of which appeared almost 

 extinct, but the third was more luminous, and resembled 

 a piece of burning charcoal covered with white ashes. 

 A similar appearance was once noticed with the naked eye 

 by two observers at Norwich and London. When the po- 

 sitions of those spots have been examined, it has been 

 found that they are almost always situated at the 

 brightest, and consequently the most reflective, por- 

 tions of the lunar surface. When the disc of the 

 moon is illumined by earth-light, as is the case when 

 slightly crescent, or, as it is commonly called, the 

 "old moon in the new moon's arms" (Fig. 53), those 



Fig. 53. 



bright parts, which return most sun-light, also reflect the 

 earth's light stronger than any others. The mountains 

 Kepler, Tycho, and Copernicus are thus frequently visible 

 in this manner, as also the bright mountainous districts. 

 But, above all, Aristarchus is most commonly visible at 

 those times. This is the brightest part of the moon, and 

 was thought by Hevelius to be a burning volcano of sul- 

 phur and saltpetre. 



In the following engraving, the cut B represents the 

 earth when opposite the sun in the tropic of Capricorn, 

 on the 21st of December, the longest night in the year, 

 and moon full in the tropic of Cancer, the horizontal 

 line being our horizon, showing that the diurnal arc ot 

 the sun is small, while the moon has a very large one to 

 traverse. The contrary of this takes place in the sum- 

 mer solstice. This explains why the moon in summer 

 seems to describe a very small, and in winter a very large 



