122 



♦ KNOWLEDGE ♦ 



[Aug. 24, 1883. 



THE MOON IN A THREE-INCH 

 TELESCOPE. 



By a Fellow of the Eoyal Astronomical Society. 



THE light of the rising sun continues to creep over the 

 lunar disc, and we are rapidly approaching her eastern 

 limb ; in other words, she is now entering that phase 

 denominated full in the almanacs, when the whole of her 

 surface which is turned towards the earth is simultaneously 

 visible. For reasons before stated, howes-er (Tol. II. 

 p. 222), this is the very worst aspect under which details 

 can be examined, or even identified, and we shall, therefore, 

 describe the leading formations which still remain to be 

 spoken of, as they appear when tolerably near the termi- 

 nator. And here we may note that ring-plains and moun- 

 tains situated on, or very near, the actual visible limb of 

 the moon, are seen in a much more natural manner by the 

 terrestrial observer than those more centrally placed on her 

 disc ; since they are, of course, looked at much more side- 

 ways ; like our own mountain ranges as we view them 

 from the surface of the earth. The student will be 

 struck with this if he wiU go carefully round the eastern 

 (and especially the north-eastern) limb of the moon 

 within a day or two of her beinjj full. It is time, however, 

 that we began our examination of such individual objects. 

 as offer points of peculiar interest. Beginning from the 

 south, we shall be struck with the Dorfel mountains (246 in 

 our Map, Vol. II., p. 223), seen in profile on the actual 

 limb of the moon. The three most conspicuous peaks of 

 this tremendous range are believed to exceed 26,000 feet 

 in height. The highest mountain in the world. 

 Mount Everest, in the Himalayas, is 29,000 feet in 

 altitude, but did this bear the same proportion to the 

 earth's diameter that the Dorfel mountains do to the 

 moon's, then would it be 106,079 feet, or more than twenty 

 miles in perpendicular height. In this neighbourhood 

 Phocylides (242) may be looked at as a considerable walled 

 plain, with a flat interior. "We, however, mention it here 

 chiefly as a guide to that curious object, Wargentin (24.3), 

 which looks like an extremely truncated column, some 

 .54 miles in diameter. Webb aptly compares this to " a 

 large, thin cheese." When the moon is eleven or twelve 

 days old, Schickard (239), an enormous walled plain, will 

 repay scrutiny. From north to south this measures some 

 134 miles, and is nearly as broad, though, of course, it is 

 considerably foreshortened as we view it. The interior 

 is very nearly level, but a three-inch telescope will show 

 the diversity of shade which characterises it. Mersenius 

 (231) is a fine ring plain more than 41 miles across, 

 and contains various small hills, craterlets, ifcc, quite 

 beyond the power of our instrument. What will strike 

 the young observer is the aspect of its floor, which is 

 convex, like a watch-glass. Just as Fracastorius (372) 

 appears as a bay liounding the southern extremity of 

 the Sea of Nectar (Vol. II., p. 294), so does Letronne (224), 

 formed by the mountains extending from Gassendi, appear 

 at one extremity of the Sea of Storms (Q. ) The huge dark 

 plain Grimaldi (272) is nearly 148 miles long by 129 broad, 

 and would have ranked as a "sea " had it been situated near 

 the centre of the moon, instead of close to her Kmb. 

 Grimaldi is even darker than Plato, and, as we have pre- 

 viously remarked (Vol. II., p. 26.5,) may often be seen on 

 the dark limb of the moon when illuminated by earthshine. 

 Riccioli (273) is another enormous walled plain, and is very 

 nearly as dark in parts as Grimaldi itself. Just to the 

 south-east of these two last-named formations lie the Lunar 

 Cordilleras (274,) and the D'Alembert Mountains (275), 

 What is probably a portion of this latter chain re-appears 

 as the Rook Mountains (276). Rather further south along 



the limb, when the moon is nearly thirteen days old, the 

 series of ring plains, Lohrmann (153), Hevelius (154), and 

 Cavalerius (155), otier an interesting spectacle. Hevelius 

 has a convex interior, but by no means so regular as that of 

 Mersenius or does the convexity fill the enclosed area in 

 the same way. Leaving the moon's limb now for the Ocean 

 of Storms we arrive at the most brilliant spot on the whole 

 surface, Aristarchus (148), of which we have spoken 

 before (Vol. II., p. 265), as conspicuous on the dark limb 

 when the moon is young. We had a curious ill ustration 

 of the extreme brightness of this formation on the occasion 

 in which the subjoined drawing was made (the night of 

 the 15th instant). 



Aristarchus and Herodotus. Jloon's age 12E6 days. 



Huge, black, cumulus clouds were driving at intervals 

 across the sky, and several times when the moon was abso- 

 lutely blotted out from view in the field of the telescope, 

 Aristarchus continued to shine like a small ill-defined planet. 

 It is dLfiicvdt or impossible to reproduce this extraordinary 

 lustre in a wood-engraving ; it is actually unpleasant to the 

 eye even in a three-inch telescope. The diameter of Aris- 

 tarchus is twenty-eight miles, and its walls are terraced — 

 albeit the terracing is seen with considerable difficulty, 

 owing to the glare. It has a concave interior with a 

 central mountain — if possible even more brilliant than the 

 internal walls themselves. Its eastern wall extends into 

 a table-land by which it is connected with Herodotus (149). 

 This last-named formation is less than twenty-four miles 

 across, and is very notably darker than Aristarchus. The 

 chief object of interest in connection with Herodotus is the 

 curious serpentine valley or cleft which originates in it, 

 and which was well seen when our sketch was made. 

 Schmidt asserts that this is 1,663 feet deep in places. It 

 enters Herodotus at a point concealed by shadow at the 

 epoch of our drawing. 



With this will terminate our description of the moon's 

 surface. As we began by saying, we are not writing a 

 treatise on Selenography, and our object has merely been to 

 invite the attention of the beginner to certain typical 

 Lunar formations, which can be observed with the instru- 

 ment which has been employed for the purpose of these 

 papers. Our map will in itself supply the student with 

 ample work for a considerable period, inasmuch as it will 

 enable him to identify four hundred of the principal forma- 

 tions on the face of the moon. The possessor of a tele- 

 scope whom we may have succeeded in interesting in the 

 study of Lunar detail will probably procure Xeison's great 

 book on " The ]Moon," a work containing more detailed 

 information with reference to Selenography proper than 

 any one extant in the English language. 



One of the series of descriptive papers of double stars 

 observable with our instrumental means was crowded out 

 at the time by Editorial exigences. It will now appear, 

 and be followed by others on the Sun in a three-inch 

 telescope, and the daylight planets. Mercury and Venus, 

 as viewed with equivalent optical power. In completing 

 these, we shall hope to have furnished the amateur with a 



