Part 24

PLATE III.--TOTAL ECLIPSE OF THE SUN.

Observed July 29, 1878, at Creston, Wyoming Territory.

A telescopic view of the sun's corona or extreme outer atmosphere and of the solar flames or prominences during a total eclipse. At the moment of observation the dark disk of the moon, while still hiding the sun's main body, had passed far enough eastward to allow the rosy pink chromospheric prominences to be seen on its western border. On all sides of the sun's hidden disk, the corona shows its pale greenish light extending in halo-like rays and streamers, and two very remarkable wings stretch eastward and westward very nearly in the plane of the ecliptic and in the direction of the positions of Mercury and Venus respectively at the time of observation. The full extent of these wings could not be shown in the Plate without reducing its scale materially, since the westerly wing extended no less than twelve times the sun's diameter, and the easterly wing nearly as far, or over ten million miles. A circlet of bright light immediately bordering the moon's disk is the so-called inner corona, next to which the wings and streamers arc brightest, thence shading off imperceptibly into the twilight sky of the eclipse. Other noteworthy peculiarities of the corona, as observed during this eclipse, are the varying angles at which the radiating streamers are seen to project, the comparatively dark intervals between them, and the curved, wisp-like projections seen upon the wings. An especially noticeable gap appears where the most westerly of the upward streamers abruptly cuts off the view of the long wing. The largest and brightest of the curving streamers on the westerly wing coincides with the highest flame-like protuberance. To some observers of this eclipse the upward and downward streamers seemed pointed at their outer extremities and less regular in form.

Approximate scale, 135,000 miles--1 inch.

PLATE IV.--AURORA BOREALIS.

As observed March 1, 1872, at 9h. 25m. P. M.

The view presents the rare spectacle of an aurora spanning the sky from east to west in concentric arches. The Polar Star is nearly central in the background, the constellation of the Great Bear on the right and Cassiopeia's chair on the left. The large star at some distance above the horizon on the right is Arcturus. The almost black inner segment of the aurora resting upon the horizon, has its summit in the magnetic meridian, which was in this case a little west of north, its arc being indented by the bases of the ascending streamers. Both streamers and arches were, when observed, tremulous with upward pulsations and there was also a wave-like movement of the streamers from west to east. The prevailing color of this aurora is a pale whitish green and the complementary red appears especially at the west end of the auroral arch. The summits of the streamers are from four hundred to live hundred miles above the earth and the aurora is therefore a phenomenon of the terrestrial atmosphere rather than of astronomical observation proper.

PLATE V.--THE ZODIACAL LIGHT.

Observed February 20, 1876.

An observation of the cone of light whose axis lies along the Zodiac, whence it derives its name. It is drawn as seen in the west, with its base in the constellation Pisces, and its apex near the familiar group of the Pleiades in the constellation Taurus. The first bright star above the horizon in the base of the cone is the planet Venus and at some distance above is the reddish disk of Mars, the two being in rare companionship as evening stars. Above the constellation Pisces, two bright stars of Aries lie just outside the cone at the right. The nearest bright star above these at the right is Beta, the leading star of the constellation Triangula. Further at the right the three prominent stars nearly in a line are, in ascending order, Delta, Beta and Gamma of the constellation Andromeda. Above these at the left, the brightest star of a quadrangular group of four is the remarkable variable star Algol (Beta) of the constellation Perseus, which changes from the second to the fourth magnitude in a period of less than three days. At the left and a little above the Pleiades is the ruddy star Aldebaran, one of the Hyades and chief star in the constellation Taurus. These are the principal stars visible in this portion of the sky at the time of the observation. Their relative positions are represented as seen in the sky and not by the common method of star-atlases, which allows for the change from a spherical to a plane surface. Their magnitude in the order of brightness is indicated only approximately.

PLATE VI.--MARE HUMORUM.

From a study made in 1875.

A view of one of the lunar plains, or so-called seas (Maria), with an encircling mountainous wall consisting of volcano-like craters in various stages of subsidence and dislocation. The sun-light coming from the west casts strong shadows from all the elevations eastward, and is just rising on the terminator, where the rugged structure of the Moon's surface is best seen. The lighter portions are the more elevated mountainous tracts and crater summits. The detailed description of this Plate given in the body of the Manual is repeated here for convenience of reference: The Mare Humorum, or sea of moisture, as it is called, is one of the smaller gray lunar plains. Its diameter, which is very nearly the same in all directions, is about 270 miles, the total area of this plain being about 50,000 square miles. It is one of the most distinct plains of the Moon, and is easily seen with the naked eye on the left-hand side of the disk. The floor of the plain is, like that of the other gray plains, traversed by several systems of very extended but low hills and ridges, while small craters are disseminated upon its surface. The color of this formation is of a dusky greenish gray along the border, while in the interior it is of a lighter shade, and is of brownish olivaceous tint. This plain, which is surrounded by high clefts and rifts, well illustrates the phenomena of dislocation and subsidence. The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000 feet in height, is seen in the upper left-hand corner of the gray plain. Close to Vitello at the east is the large broken ring-plain Lee, and farther east, and a little below, is a similarly broken crater called Doppelmayer. Both of these open craters have mountainous masses and peaks on their door, which is on a level with that of the Mare Humorum. A little below, and to the left of these objects, is dimly seen a deeply imbedded oval crater, whose walls barely rise above the level of the plain. On the right-hand side of the great plain is a long fault, with a system of fracture running along its border. On this right-hand side may be seen a part of the line of the terminator, which separates the light from the darkness. Towards the lower right-hand corner is the great ring-plain Gassendi, 55 miles in diameter, with its system of fractures and its central mountains, which rise from 3,000 to 4,000 feet above its floor. This crater slopes towards the plain, showing the subsidence to which it has been submitted. While the northern portion of the wall of this crater rises to 10,000 feet, that on the plain is only 500 feet high, and is even wholly demolished at one place where the floor of the crater is in direct communication with the plain. In the lower part of the sea, and a little to the west of the middle line, is found the crater Agatharchides, which shows below its north wall the marks of rills impressed by a flood of lava, which once issued from the side of the crater. On the left-hand side of the plain is seen the half-demolished crater Hippalus resembling a large bay, which has its interior strewn with peaks and mountains. On this same side can be seen one of the most important systems of clefts and fractures visible on the Moon, these clefts varying in length from 150 to 200 miles.

Approximate scale, 15 miles--1 inch.

PLATE VII.--PARTIAL ECLIPSE OF THE MOON.

Observed October 24, 1874.

A view of the Moon partially obscured by the Earth's shadow, whose outline gives ocular proof of the earth's rotundity of form. The shadowed part of the Moon's surface is rendered visible by the diffused sun-light refracted upon it from the earth's atmosphere. Its reddish brown color is due to the absorption, by vapors present near the earth's surface, of a considerable part of this dim light. On both the obscured and illuminated tracts the configurations of the Moon's surface are seen as with the naked eye. The craters appear as distinct patches of lighter color, and the noticeably darker areas are the depressed plains or Maria. The large crater Tycho, at the lower part of the disk, is the most prominent of these objects, with its extensive system of radiating streaks. The largest crater above is Copernicus, at the left of which is Kepler and still above are Aristarchus and Herodotus appearing as if blended in one. Above and at the left of the great crater Tycho, the first dark tract is the Mare Humorum of Plate VI., seen in its natural position, with the crater Gassendi at its northern (upper) extremity and Vitello on its southern (lower) border. The advancing border of the shadow appears, as always, noticeably darker than the remainder, an effect probably of contrast. The illuminated segment of the Moon's disk has its usual appearance, the lighter portions being the more elevated mountainous surfaces, and the dark spaces the floors of extensive plains.

Approximate scale, 140 miles--1 inch.

PLATE VIII.--THE PLANET MARS.

Observed September 3, 1877, at 11h. 55m. P. M.

A view of the southern hemisphere of Mars, when in the most favorable position for observation, and when exceptionally free from the clouds, which very frequently hide its surface configurations. Since, of all the planets, Mars is most like the earth, Plate VIII. may give a fair idea of the appearance of our globe to a supposed observer on Mars. The dark gray and black markings, are regarded as tracts of water, or of some liquid with similar powers of absorbing light; and, for the same reason, the lighter portions, of a prevailing reddish tint, are supposed to be bodies of land, while the bright white portions are variously due to clouds, to polar snow or ice, and the bright rim of white along the limb, to the depth of the atmosphere through which the limb is seen. The chief permanent features of the planet's surface have been named in honor of various astronomers.

The large dark tract on the left is De La Rue Ocean, the isolated oval spot near the centre is Terby Sea, and on the right is the western end of Maraldi Sea, with strongly indented border. Directly north of (below) De La Rue Ocean, is Maedler Continent; above it stretches Jacob Land; and surrounding Terby Sea is Secchi Continent. Extending into the centre of De La Rue Ocean is a curious double peninsula, called, in consequence of the dimness of former observations, Hall Island. The sharply defined, white-crested northern borders of De La Rue Ocean and Maraldi Sea may indicate the existence there of lofty coast ranges, more or less constantly covered with opaque clouds strongly reflecting light. The white spot in the centre of Maedler Continent, of a temporary nature, has a similar explanation. The intervals of olivaceous gray on Secchi Continent and elsewhere may perhaps be ascribed to the flooding and drying up of marshes and lowlands, as these markings have been observed to vary somewhat in connection with the change of seasons on Mars. The greenish tints observed along the planet's limb, alike on the darker and lighter surfaces, are probably due to an optical effect, the green being complementary to the prevailing red of the disk. The brilliant oval white spot near the southern (upper) pole of the planet is a so-called polar spot, in all probability consisting of a material similar to snow or ice and here observed in the midst of a dark open sea.

Approximate scale, 300 miles--1 inch.

PLATE IX.--THE PLANET JUPITER.

Observed November 1, 1880, at 9h. 30m. P. M.

This planet is perpetually wrapped in dense clouds which hide its inner globe from view. The drawing shows Jupiter's outer clouded surface with its usual series of alternate light and dark belts, the disk as a whole appearing brighter in the centre than near the limb. The darker gray and black markings indicate in general the lower cloud-levels; that is, partial breaks or rifts in the cloudy envelope, whose prevailing depth apparently exceeds four thousand miles. While the deepest depression in the cloudy envelope is within the limits of the Great Red Spot, the vision may not even here penetrate very deeply. Two of Jupiter's four moons present bright disks near the planet's western limb, and cast their shadows far eastward on the disk, that of the "second satellite" falling upon the Red Spot. On the Red Spot are seen in addition two small black spots, no explanation of which can yet be offered. The broad white ring of clouds bordering the Red Spot appeared in constant motion. The central, or equatorial belt, shows brilliant cloudy masses of both the cumulus and stratus types, and the underlying gray and black cloudy surfaces are pervaded with the pinkish color characteristic of this belt. The dark circular spots on the wide white belt next north showed in their mode of formation striking resemblances to sun-spots. They afterward coalesced into a continuous pink belt. The diffusion of pinkish color over the three northernmost dark bands, as here observed, is unusual. About either pole is seen the uniform gray segment or polar cap. The equatorial diameter is noticeably longer than the polar diameter, a consequence of the planet's extraordinary swiftness of rotation. To the same cause may also be due chiefly the distribution of the cloudy belts parallel to the planet's equator, though the analogy of the terrestrial trade-winds fails to explain all the observed phenomena.

Approximate scale, 5,500 miles--1 inch.

PLATE X.--THE PLANET SATURN.