Chapter 18

Leibnitz Mountains. On the S. limb extending W. from S. lat. 80 deg. beyond the Pole on to the Fourth Quadrant. Perhaps the loftiest range on the limb. Madler's measures give more than 27,000 feet as the height of one peak, and there are several others nearly as high.

FOURTH QUADRANT.

The Altai Mountains. A fine conspicuous serpentine range, extending from the E. side of Piccolomini in a north-easterly direction to the region between Tacitus and Catherina, a length of about 275 miles. The loftiest peak is over 13,000 feet. The average height of the southern portion is about 6000 feet. The region lying on the S.E. of this range is a vast tableland, devoid of prominent objects, rising gradually towards the mountains, which shelve rapidly down to an equally barren expanse on the N.W.

The Pyrenees. These mountains, on the E. of Guttemberg, border the western side of the Mare Nectaris. Their loftiest peak, rising nearly to 12,000 feet, is on the S.E. of Guttemberg.

LIST OF THE PRINCIPAL RAY-SYSTEMS, LIGHT-SURROUNDED CRATERS, AND LIGHT- SPOTS.

[In this list, which does claim to be exhaustive, most of the objects noted by Schmidt are incorporated.]

FIRST QUADRANT.

Autolycus. Encircled by a delicate nimbus, throwing out four or five prominent rays extending towards Archimedes. Seen best under evening illumination.

Aristillus. The centre of a noteworthy system of delicate rays extending W. towards the Caucasus; and on the S. disappearing among the rays of Autolycus. They are traceable on the Mare Nubium near Kirch.

Theaetetus. A very brilliant group of little hills E. of this formation.

Eudoxus A. A light-surrounded crater W. of Eudoxus, with distinct long streaks, one of which extends to the S. wall of Aristoteles.

Aristoteles A. A light-surrounded crater in the Mare Frigoris, N.E. of Aristoteles.

Aratus. A very conspicuously brilliant crater in the Apennines, with a smaller light-surrounded crater W. of it.

Sulpicius Gallus. A light spot near.

Manilius. Surrounded by a light halo and streaks.

Taquet. Has a prominent nimbus, and indications of very delicate streaks.

Plinius A. Is surrounded by a well-marked halo.

Posidonius gamma. Among the hills E. of this formation a light spot resembling Linne, according to Schmidt. He first saw it in 1867, when it had a delicate black spot in the centre. Dr. Vogel observed and drew it in 1871 with the great refractor at Bothkamp. These observations were confirmed by Schmidt in 1875 with the 14-feet refractor at Berlin.

Littrow. A very bright light-spot with streaks, on the site of a little crater and well-known cleft E. of this ring-plain.

Romer. A light-surrounded mountain on the E.

Macrobius. Two light-surrounded craters on the E. of this formation, the more northerly being the brighter.

Cleomedes A. (On the floor.) Surrounded by a nimbus and rays. Large crater, A, on the E. has also a nimbus and rays.

Agrippa. Exhibits faint rays.

Godin. Exhibits faint rays.

Proclus. A well-known ray-centre, some of the rays prominent on part of the Mare Crisium.

Taruntius. Has a very faint nimbus, with rays, on a dark surface.

Dionysius. A brilliant crater with a prominent, bright, excentrically placed nimbus on a dark surface, on which distinct rays are displayed.

Hypatia B. A very small bright crater on a dark surface: surrounded by a faint nimbus.

Apollonius. Among the hills S. of this, there is a small bright streak system.

Eimmart. There is a large white spot N.W. of this.

Geminus is associated with a system of very delicate rays.

Menelaus. A brilliant object. It is traversed by a long ray from Tycho.

SECOND QUADRANT.

Anaxagoras. The centre of an important ray-system.

Timocharis is surrounded by a pale irregular nimbus and faint rays, most prominently developed on the W. side of the formation.

Copernicus. Next to Tycho, the most extended ray-centre on the visible surface. Some distance on the E., in E. long. 25 deg., N. lat. 11 deg., lies a very small but conspicuous system, and in E. long. 22 deg., N. lat. 8 deg. a bright light spot among little hills.

Gambart A. A bright crater with large nimbus and rays.

Landsberg A. A light-surrounded crater on a dark surface, with companions, referred to under the Third Quadrant.

Encke. There is a light-surrounded crater S. of this.

Kepler. A noted ray-centre. It is surrounded by an extensive halo, especially well developed on the E., across the Mare Procellarum.

Bessarion. Two bright craters: the more northerly is prominently light-surrounded, while its companion is less conspicuously so.

Aristarchus.--The most conspicuous bright centre on the moon, the origin of a complicated ray-system.

Delisle. S. of this formation there is a tolerably bright spot on the site of some hills.

Timaeus. A ray-centre.

Euler. Feeble halo with streaks.

Galileo. Between this and Reiner is a curious bright formation with short rays, referred to in the Catalogue, under Reiner.

Cavalerius. A light streak originating in the W. wall, and extending on to the Oceanus Procellarum.

Olbers. A considerable ray-system, but seldom distinctly visible.

Lichtenberg. Faintly light-surrounded.

THIRD QUADRANT.

Tycho. The largest and best known system on the visible surface.

Zuchius. A remarkable ray-system, but one which is only well seen when libration is favourable.

Bailly. N. of the centre of this great enclosure are two very distinct radiating streaks.

Schickard. Four conspicuous light spots, probably craters, on the S.E.

Byrgius A. A brilliant ray-centre, most of the rays trending eastward from a nimbus.

Hainzel. There are several bright spots E. of this formation.

Mersenius. Two or three light-rays originate from a point on the W. rampart.

Mersenius C. A light-surrounded crater with short rays.

Grimaldi. There are three bright spots on the W. wall.

Damoiseau. A light-surrounded crater W. of Damoiseau, E. long. 58 deg., S. lat. 6 deg.

Flamsteed C. A light-surrounded crater on a dark surface.

Lubieniezky A. Crater with halo on a dark surface.

Lubieniezky F. Crater with halo on a dark surface.

Lubieniezky G. Crater with halo on a dark surface.

Birt _a_. A light-surrounded crater.

Landsberg. E. of Landsberg, four light-surrounded craters, forming with Landsberg A (in the Second Quadrant) an interesting group.

Lohrmann A. A light-surrounded crater, with a light area a few miles N. of it. S. lat. 1 deg., E. long. 61 deg.

Euclides. Has a conspicuous nimbus with traces of rays, a typical example.

Guerike. There is a crater, with nimbus, W. of this, in E. long. 12 deg., S. lat. 11 deg. 5 min.

Parry. A very brilliant light-spot in the S. wall.

Parry A. Surrounded by a bright nimbus.

Alpetragius B. A conspicuous light-surrounded crater, one of the most remarkable on the moon.

Alpetragius _d_ (E. long. 11 deg., S. lat. 13 deg. 8 min.). A bright spot, seen by Madler as a crater, but which, as Schmidt found in 1868, no longer answers to this description.

Mosting C. A light-surrounded crater.

Lalande. Has a large nimbus and distinct rays.

Hell. A large ill-defined spot in E. long. 4 deg., S. lat. 33 deg. This is most probably the site of the white cloud seen by Cassini.

Mercator. There is a brilliant crater and light area under E. wall.

FOURTH QUADRANT.

Stevinus _a_. A crater E. of Stevinus; it is a centre of wide extending rays.

Furnerius A. Prominently light-surrounded, with bright streaks, radiating for a long distance N. and S.

Messier A. The well-known "Comet" rays, extending E. of this.

Langrenus. Has a large but very pale ray-system. It is best seen under a low evening sun. Three long streaks radiate towards the E. from the foot of the _glacis_ of the S.E. wall.

Censorinus. A very brilliant crater with faint rays.

Theophilus. The central mountain is faintly light-surrounded.

Madler. This ring-plain and the neighbourhood on the N. and N.W., include many bright areas and curious streaks.

Almanon. About midway between this and Argelander is a very brilliant little crater.

Beaumont. Between this and Cyrillus stand three considerable craters with nimbi.

Cyrillus A. A prominent light-surrounded crater.

Alfraganus. A light-surrounded crater with rays.

POSITION OF THE LUNAR TERMINATOR

Though the position of the Lunar Terminator is given for mean midnight throughout the year in that very useful publication the Companion to the Observatory, it is frequently important in examining or comparing former drawings and observations to ascertain its position at the times when they were made. For this purpose the subjoined tables (which first appeared in the Selenographical Journal) will be found useful, as they give for any day between A.D. 1780 and A.D. 1900 the selenographical longitude of the point where the terminator crosses the moon's equator, which it does very nearly at right angles.

[Tables and examples]

LUNAR ELEMENTS

Moon's mean apparent diameter - 31 min. 8 sec.

Moon's maximum apparent diameter - 33 min. 33.20 sec.

Moon's minimum apparent diameter - 29 min. 23.65 sec.

Moon's diameter, in miles - 2163 miles.

Volume (earth's = 1) - 1/49.20 or 0.02033.

Mass (earth's = 1) - 1/81.40 or 0.0128.

Density (earth's = 1) - 0.60419, or 3.444 the density of water (water being unity).

Surface area, about 14,600,000 square miles (earth's surface area, 196,870,000 miles)

Earth's surface area = 1, moon's - About 2/27 or 0.07407.

Action of gravity at surface - 0.16489 or 1/6.065 of the earth's.

Surface of moon never seen - 0.4100.

Surface of moon seen at one time or another - 0.5900.

Synodical revolution, or interval from new moon to new moon (commonly called a lunation) - 29 d. 12 h. 44 m. 2.684 s. - 29.5305887 days.

Sidereal revolution, or time taken in passing from one star to the same star again - 27 d. 7 h. 43 m. 11.545 s. - 27.3216614 days.

Tropical revolution, or time taken in passing from "the first point of Aries" to the same point again - 27 d. 7 h. 43 m. 4.68 s. - 27.321582 days.

Anomalistic revolution, or time taken in passing from perigee to perigee - 27 d. 13 h. 18 m. 37.44 s. - 27.55460 days.

Nodical revolution, or time taken in passing from rising node to rising node - 27 d. 5h. 5m. 35.81 s. - 27.21222 days.

Distance (mean) in terms of the equatorial radius of the earth - 60.27.

Distance in miles (mean) - 238,840 miles.

Distance, maximum - 252,972 miles.

Distance, minimum - 221,614 miles.

Mean excentricity of moon's orbit - 0.05490807.

Inclination of moon's orbit to the ecliptic (mean) - 5 deg. 8 min. 39.96 sec.

Inclination of moon's axis to the ecliptic - 87 deg. 27 min. 51 sec.

Inclination of moon's equator to the ecliptic - 1 deg. 32 min. 9 sec.

Maximum libration in latitude - 6 deg. 44 min.

Maximum libration in longitude - 7 deg. 45 min.

Maximum total libration from earth's centre - 10 deg. 16 min.

Maximum diurnal libration - 1 deg. 1 min. 28.8 sec.

Angle subtended by one degree of selenographical latitude and longitude at the centre of the moon's disc, when at its mean distance - 16.566 sec.

Length of a degree under these conditions - 18.871 miles.

Selenographical arc at the centre of the moon's surface, subtending an angle of one second of arc - 3 min. 37.31 sec.

Miles at the centre of the moon's disc, subtending an angle of one second of arc - 1.139

[It must be remembered that this value is _increased_, in departing from the centre, in the proportion of the secants of the angular distance from the centre.]

Period of similar phase - 59 d. 1h. 28m. = 2 lunations.

Or, more accurately - 442 d. 23 h. = 15 lunations.