326 



♦ KNOWLEDGE ♦ 



fJusE 1, 1883. 



THE MOON IN A TIIRKE-INCH 

 TELESCOPE. 



Bv A Fia.i.ow OF THE Royal Astronomical Society. 



ADVANCING sunlight is now l)ringinf» into view a 

 highly complicated mass of walled plains and craters 

 in the south-western quadrant of the moon; and with some 

 of the more notable among them we will begin our work 

 to-night. Maurolycus (3.">8 in our map) is a splendid object 

 about the time of the moon's first quarter. The great 

 complexity of the wall will attract the attention of the 

 observer. A few crater pits may be detected with the 

 instrument we are employing on the walls, as well as on 

 the floor of Maurolycus, and there are numerous hills on 

 the latter also visible under favourable illumination. 

 Another splendid oliject in this neighbourhood is Stofler 

 (354), but the inside of this is very much more level and 

 undisturbed than that of its neighbour. The system of 

 briglit streaks radiating from Tycho (previously referred to 

 on p. :222),passesover this region, with the curious result that 

 the bold and most conspicuous formations of which we are 

 speaking to all intents and purposes disappear at full moon 

 altogether ! In Walter (iOO), Regiomontanus (201), and 

 Purbach (202), we have an instance of three crater plains 

 in connection with each other, and lying, approximately, 

 north and south of each other, one example of which we 

 have already seen in Theophilus, Cyrillus, and Catherine, 

 and another of which we are immediately to examine in 

 Arzachel, Alphonsus, and Ptolemy. North-east of Purbach 

 lies Thebit (20.3), a crater well worth examining, as atten- 

 tive inspection will show that another crater has burst the 

 original wall, and has itself in turn been intruded on by a 

 more minute one still. Here, again, we are able to trace the 

 chronological sequence of the successive eruptions. A 

 strange formation, known as "Straight wall," but looking 

 (when the moon is eight or nine days old) like a stag's 

 horn on the top of an alpenstock, will be noted not far to 

 the east of Thebit. And now we arrive at that truly 

 superb triple system, Arzachel (204), Alphonsus (207), and 

 Ptolemy (208). 



Arzachel, Alphonsus and Ptolemy. 



Arzachel is about G')\ miles i^i diameter, with terraced 

 •walls, diversified by clefts and craters. Alphonsus is 83 

 miles across, and has very complicated walls. The 

 northern one opens by clefts and valleys into Ptolemy — an 

 enormous walled plain of 1 1 5 miles in diameter. The con- 

 spicuous crater in its floor will at once strike the eye. In 



Alphonsus, the chief object in the interior is a mountain; 

 while in Arzachel both a mountain and a crater will be 

 noted by the observer. In our sketch above, Alpetragius 

 (20.'>) will lie seen to have its interior wholly immersed in 

 shadow. This beautiful crater is about 27 miles across, and 

 is so comparatively deep as to be only free from shadow for 

 less than a week during the entire lunation. Herschel (212) 

 is a fine ring plain, 24 miles in diameter, with a central 

 mountain. llh:eticus (104) is noticeable as lying actually 

 on the lunar equator. Godin (103) and Agrippa (102), 

 two ring plains of 23 and 27 miles in diameter respec- 

 tively, arc fine objects when seen near the termi- 

 nator. The observer should carefully examine that 

 curious object Hyginus (93) and its neighbourhood 

 about the time of the first quarter, employing for this 

 purpose as high a power as his telescope will bear 

 (say 160). He will note the curious rill running right 

 through the crater, the snail-shaped or spiral mountain 

 just to the north of it, a brilliant ridge to the west of this 

 again, and so on. Hereabouts it is that the alleged dis- 

 covery of the depression known as "Hyginus N." was 

 made. The incipient observer with a 3-inch telescope 

 must not, however, blame either himself or his instrument 

 should he fail to distinguish this mysterious object. 

 Manilius (9.5) is a fine object under proper illumination. 

 Its diameter is 25i miles. The Sea of Serenity, at which 

 we now arrive (H) contains numerous objects to reward 

 the observer. Among them is the curious one, Linn6 (74), 

 which, save when almost on the terminator, presents 

 the appearance of a minute whitish cloud, or little 

 smudge of light. It may be found on a line drawn 

 from Pliny (61) through Bessel (73). The two splendid 



.^ ^J 



C«i 



Eiidoxus and Aristotle. Autolycus, Aristillus, and Archimedes. 



craters, Eudoxus (77) and Aristotle (7S) here drawn 

 present a grand spectacle when near the boundary of light 

 and darkness, either with a waxing or a waning moon. Our 

 own sketch above was made when the sun was rather too 

 high above their horizon. Under suitable illumination 

 Aristotle will be seen to be surrounded by radiating chains 

 of hills. Cassini (81) is a curious object about the time of 

 the moon's first quarter. Its diameter is about 36 miles, 

 and it contains a ring plain, some nine miles across, withm 

 it. The edge of the ring of Cassini must be consideraWy 

 serrated or cut into peaks and spire.s. With Archimedes 

 (120), Aristillus (83), and Autolycus (84), we shall con- 

 clude our work to-night. The examination of the region in 

 which they are situated may well afl!brd us an entire even- 

 ing's occupation on a future occasion. 



Archimedes is a comparatively shallow ring plain of 

 50 miles in diameter. The inside, with our instrumental 

 means, will appear quite smooth ; but powerful telescopes 

 show minute craterlets and spots in it. When, however, 

 this great plain is fully illuminated, a three-inch telescope 

 will show that the floor is striped or streaked with alter- 



