The Astronomy of Milton's 'Paradise Lost', Thomas Nathaniel Orchard [best selling autobiographies .TXT] 📗

lunar orbit, by which the mean longitude of the Moon is increased or diminished. Elliptic inequality, parallactic inequality, the variation, and secular acceleration, are other perturbations of the lunar motion, which depend directly or indirectly on the attractive influence of the Sun and the motion of the Earth in her orbit.

As the plane of the Moon’s orbit is inclined at an angle of rather more than 5° to the ecliptic, it follows that the orb, in its journey round the Earth, intersects this great circle at two points called the ‘Nodes.’ When crossing the ecliptic from south to north the Moon is in its ascending node, and when crossing from north to south in its descending node. In December the Moon reaches the most northern point of its course, and in June the southernmost. Consequently we have during the winter nights the greatest amount of moonlight, and in summer the least. In the evenings the moonlight is least in March and greatest in September, when we have what is called the Harvest Moon.

The telescopic appearance of the Moon is very interesting and beautiful, especially if the orb is observed when waxing and waning. As no aqueous vapour or cloud obscures the lunar surface, all its details can be perceived with great clearness and distinctness. Indeed, the topography of the Moon is better known than that of the Earth, for the whole of its surface has been mapped and delineated with great accuracy and precision. The Moon is in no sense a duplicate of its primary, and no analogy exists between the Earth and her satellite. Evidence is wanting of the existence of an atmosphere surrounding the Moon; no clouds or exhalations can be perceived, and no water is believed to exist on the lunar surface. Consequently there are no oceans, seas, rivers, or lakes; no fertile plains or forest-clad mountains, such as are found upon the Earth. Indeed, all the conditions essential for the support and maintenance of organic life by which we are surrounded appear to be nonexistent on the Moon. Our satellite has no seasons; its axial rotation is so slow that one lunar day is equal in length to fourteen of our days; this period of sunshine is succeeded by a night of similar duration. The alternation of such lengthened days and nights subjects the lunar surface to great extremes of heat and cold.

When viewed with a telescope, the surface of the Moon is perceived to consist of lofty mountain chains with rugged peaks, numerous extinct volcanoes called crater mountains, hills, clefts, chasms, valleys, and level plains—a region of desolation, presenting to our gaze the shattered and upturned fragments of the Moon’s crust, convulsed by forces of a volcanic nature which have long since expended their energies and died out. The mountain ranges on the Moon resemble those of the Earth, but they have a more rugged outline, and their peaks are more precipitous, some of them rising to a height of 20,000 feet. They are called the Lunar Alps, Apennines, and Cordilleras, and embrace every variety of hill, cliff, mound, and ridge of comparatively low elevation. The plains are large level areas, which are situated on various parts of the lunar surface; they are of a darker hue than the mountainous regions by which they are surrounded, and were at one time believed to be seas. They are analogous to the prairies, steppes, and deserts of the Earth.

Valleys.—Some of these are of spacious dimensions; others are narrow, and contract into gorges and chasms. Clefts or rills are long cracks or fissures of considerable depth, which extend sometimes for hundreds of miles across the various strata of which the Moon’s crust is composed.

The characteristic features of the Moon’s surface are the crater mountains: they are very numerous on certain portions of the lunar disc, and give the Moon the freckled appearance which it presents in the telescope, and which Galileo likened to the eyes in the feathers of a peacock’s tail. They are believed to be of volcanic origin, and have been classified as follows: ‘Walled plains, mountain rings, ring plains, crater plains, craters, craterlets, and crater cones.’ Upwards of 13,000 of these mountains have been enumerated, and 1,000 are known to have a diameter exceeding nine miles. Walled plains consist of circular areas which have a width varying from 150 miles to a few hundred yards. They are enclosed by rocky ramparts, whilst the centre is occupied by an elevated peak. The depth of these formations, which are often far below the level of the Moon’s surface, ranges from 10,000 to 20,000 feet. Mountain rings, ring plains, and crater plains resemble those already described, but are on a smaller scale; the floors of the larger ones are frequently occupied by craters and craterlets. The latter exist in large numbers, and some portions of the Moon’s surface appear honeycombed with them, the smaller craters resting on the sides of larger ones and occupying the bottoms of the more extensive areas. There is no kind of formation on the Earth’s surface that can be compared with these crater mountains, which indicate that the Moon was at one time a fiery globe convulsed by internal forces which found an outlet in the numerous volcanoes scattered over her surface.

The most remarkable of these volcanic mountains have been named after distinguished men. (1) Copernicus is one of the most imposing; its crater is 56 miles in diameter, and situated at its centre is a mountain with six peaks 2,400 feet in height. The ring by which it is surrounded rises 11,000 feet above the floor of the crater, and consists of terraces believed to have been created by the partial congelation and periodic subsidence of a lake of molten lava which occupied the enclosed area.

(2) Tycho is one of the most magnificent and perfect of lunar volcanoes, and is also remarkable as being a centre from which, when the Moon is full, there radiates a number of bright streaks which extend across the lunar surface, over mountain and valley, through ring and crater, for many hundreds of miles. Their nature is unknown, and nothing resembling them is found on the Earth. Tycho has a diameter of 50 miles and a depth of 17,000 feet. The peak which rises from the floor of the crater attains a height of 6,000 feet, and the rampart consists of a series of terraces which give variety to the appearance of the inner wall. The surface of the Moon round Tycho is honeycombed with small volcanoes.

(3) Clavius is one of the most extensive of the walled plains; it has a diameter of 142 miles and an area of 16,500 square miles. The rocky annulus which surrounds it is very lofty and precipitous, and at one point reaches a height of 17,300 feet. Upwards of 90 craters have been counted within this space, one of the peaks attaining to an elevation of 24,000 feet above the level floor of the plain. It is believed that the lowest depths of this wild and precipitous region are never penetrated by sunlight, they are so overshadowed by towering crag and fell which intercept the solar rays; and, as there is no atmosphere to cause reflection, they are consequently enveloped in perpetual darkness.

(4) Plato has a diameter of about 60 miles and an area of 2,700 square miles; its central peak rises to a height of 7,300 feet. It has an irregular rampart which is broken up into terraces averaging about 4,000 feet high; three cones, each with an elevation of from 7,000 to 9,000 feet, rest on its western border.

(5) Theophilus is the deepest of the visible craters on the Moon. It has a diameter of 64 miles, and the inner edge of the ring rises from the level floor to a height ranging from 14,000 to 18,000 feet. A group of mountains occupies the centre of the area, the highest peak of which reaches an elevation of 5,200 feet. Cyrillus and Catharina, two adjacent craters, are each about 16,000 feet deep and connected by a wide valley.

(6) Aristarchus is the brightest spot on the Moon, and appears almost dazzling in the telescope. The crater has a diameter of 42 miles, the centre of which is occupied by a steep mountain. The rampart on the western side rises to a height of 7,500 feet, on the east it becomes a plateau which connects it with a smaller crater called Herodotus. Bright streaks radiate from Aristarchus when there is full moon, and extend for a considerable distance over the surface of the orb.

Though the face of the Moon has been carefully scanned for two centuries and a half, and selenographers have mapped and delineated her features with the utmost accuracy and precision, yet no perceptible change of a reliable character has been perceived to occur on any part of the orb. The surface of the hemisphere directed towards the Earth appears to be an alternation of desert plains, craggy wildernesses, and extinct volcanoes—a region of desolation unoccupied by any living thing, and ‘upon which the light of life has never dawned.’ Owing to the absence of an atmosphere, there is neither diffuse daylight nor twilight on the Moon. Every portion of the lunar surface not exposed to the Sun’s rays is shrouded in darkness, and black shadows can be observed fringing prominences of silvery whiteness. If the Moon were enveloped in an atmosphere similar to that which surrounds the Earth, the reflection and diffusion of light among the minute particles of watery vapour which permeate it would give rise to a gradual transition from light to darkness; the lunar surface would be visible when not illumined by the direct rays of the Sun, and before sunrise and after sunset, dawn and twilight would occur as upon the Earth. But upon the Moon there is no dawn, and the darkness of night envelops the orb until the appearance of the edge of the Sun’s disc above the horizon, then his dazzling rays illumine the summits and loftiest peaks of the lunar mountains whilst yet their sides and bases are wrapped in deep gloom. Since the pace of the Sun across the lunar heavens is 28 times slower than it is with us, there is continuous sunshine on the Moon for 304 hours, and this long day—equal to about a fortnight of our time—is succeeded by a night of similar duration. As there is no atmosphere overhead to diffuse or reflect the light, the Sun shines in a pitch-black sky, and at lunar noonday the planets and constellations can be seen displaying a brilliancy of greater intensity than can be perceived on Earth during the darkest night. Every portion of the Moon’s surface is bleak, bare, and untouched by any softening influences. No gentle gale ever sweeps down her valleys or disturbs the dead calm that hangs over this world; no cloud ever tempers the fierce glare of the Sun that pours down his unmitigated rays from a sky of inky blackness; no refreshing shower ever falls upon her arid mountains and plains; no sound ever breaks the profound stillness that reigns over this realm of solitude and desolation.