ASTRONOMY. 



completed a synodic revolution. The synodic 

 period or lunation, then, is the period between 

 two new moons, or two conjunctions of the sun 

 and moon ; it is the lunar month, and its mean 

 length is 29 days, 12 hours, 44 minutes. The 

 sidereal month is 27 days, 7 hours, 43 minutes. 



The moon, besides revolving round the earth, 

 also turns on its own axis, and, by a remarkable 

 coincidence, the rotation on the axis is completed 

 in exactly the same time as the revolution in the 

 orbit ; which is probably the case with all other 

 secondary planets. In consequence of this coinci- 

 dence, the same side of the moon is always turned 

 towards the earth, as is evident from her surface 

 presenting constantly the same easily recognised 

 marks in the same positions. 



Phases of the Moon. The light of the sun, 

 falling upon the moon, is partly absorbed into its 

 body ; but a small portion is reflected or thrown 

 back, and becomes what we call moonlight. The 

 illuminated part, from which we derive moonlight, 

 is at all times increasing or diminishing to our 

 eyes, as the moon proceeds in her revolution round 

 our globe. When the satellite is on the opposite 



Phases oi the Moon : 



A, B, C, D, F, G, H, K, appearances presented by the moon to an 

 observer situated at the pole of her orbit ; A', B', C', D', F", G', 

 H', K', ' phases ' of the moon at the end of each eighth part of 

 her course ; S, position of sun ; E, position of earth. 



side of the earth from the sun, or in opposition, 

 we, being nearly between the two, see the whole 

 of the illuminated surface, which we accordingly 

 term full-moon. As the moon advances in her 

 course, the luminous side is gradually averted 

 from us, and the moon is said to wane. At length, 

 when the satellite has got between the earth and 

 the sun, or into conjunction, the luminous side is 

 entirely lost sight of; the moon is then said to 

 change. Proceeding in her revolution, she soon 

 turns a bright edge towards us, which we call the 

 new-moon. This gradually increases in breadth, till 

 she is one quarter of her circuit from the sun, or 

 in quadrature, when half the disk is illuminated, 

 and it is then said to be half-moon. The luminary, 

 when on the increase from new to half, is termed 

 crescent (increasing) ; when between half and full, 

 it is gibbous (hump-backed). 



In the early days of the new-moon, we usually 

 see the dark part of the body faintly illuminated, 

 an appearance termed the old-moon in the new- 

 vioon's arms. This faint illumination is produced 

 by the reflection of the sun's light from the earth, 

 or what the inhabitants of the moon, if there were 

 any, might call earth-light. 



It is estimated that it would take 547,513 full- 

 moons to give as much light as the sun ; which is 

 twice as many as would find room in the whole 



The Moon, 

 photographed* 



half-sky. The moon's rays were till recently 

 believed to be without heat ; but by concentrating 

 them in a lens of three feet diameter, the Ita- 

 lian philosopher Melloni obtained a sensible ele- 

 vation of temperature. 



The physical condition of the moon is in many 

 respects remarkable. As a powerful telescope 

 shews an object as if it were 

 a thousand times nearer 

 than it is, we can view the 

 moon as if it were only 

 240 miles off, and thus the 

 geography of its surface 

 is pretty accurately known. 

 The darker patches, which 

 used to be considered seas, 

 are found to be smooth 

 planes, and have all the 

 appearance of having once 

 been sea - bottoms. The 

 brighter parts are mountainous. 



' The mountains of the moon are not mountains, 

 in the common acceptation of the term ; they are 

 circular pits, hollowed out into the lunar substance, 

 and surrounded by a ring-shaped elevated border, 

 more or less abrupt and broken. There are some 

 that are not more than 300 or 400 yards across \ 

 others exceed 100 miles.' 



These circular mountains are denominated, 

 according to their magnitudes, Walled Plains, 

 Ring-mountains, Craters, and Holes. The most 

 remarkable of the ring-mountains is that called 

 Tycho, after the illustrious Danish astronomer. 

 The enclosure is a circle forty-seven miles in diam- 

 eter, and the inner side of the ridge is as steep 

 as a wall, and 16,000 feet high, while the height 

 above the surrounding surface outside is only 

 12,000 feet. On the floor of the enclosed hollow 

 stand a few isolated hills, one of them nearly a 

 mile in height. 



The heights of the lunar mountains are measured 

 by means of the shadows they cast during the 

 phases. More than a thousand have been thus 

 determined, several of which reach a height of 

 23,000 feet. One peak named Dorfel is 26,691 

 feet high. Considering that the moon's diameter 

 is little more than one-fourth that of the earth, the 

 lunar mountains are thus on a much grander scale 

 than the terrestrial. The mountains of the moon 

 have in many respects a volcanic character ; but 

 no trace of an active volcano has yet been dis- 

 covered. In addition to these ring-shaped hollows, 

 there are long trenches with raised sides, called 

 rilles, and bright rays, proceeding from a centre, 

 like cracks, the nature of which is a mystery. 



The moon is ascertained to be without any 



atmosphere ; nor is there the least appearance of 



liquid of any kind, although the surface would 



seem to have been at one time partly covered with 



water. The direct rays of the sun will thus shine 



for fourteen days with a fierceness far beyond 



anything experienced on the earth ; but there can 



be no accumulation of heat, and on the unillu- 



, minated side the cold must be for other fourteen 



! days more rigorous than on the summits of our 



: loftiest mountains. The present cold and dead 



appearance of the moon's surface, compared with 



what it must once have been, is accounted for by 



supposing that the original heat of the moon, 



* Copied by permission of Messrs Smith. Berk, and Beck, from 

 Warren De la Rue's photograph 



