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THE LUNAR CRATER COPERNICUS.

Enlarged from a Negative made with the Great Telescope of the Lick Observatory, August 25, 1890.

The

of lunar craters is six degrees to seven degrees, while the average interior slope is thirty-five degrees; that is, they are really very gently sloped mounds with a steep-sided pit in the midst. terraces of the walls deserve careful attention; and, if the cut is examined with a common hand-magnifier, they can be seen a little better.

The floor of the crater is by no means smooth; and from it rise two groups of central peaks, the highest of which is some twentyfour hundred feet. Like all the central peaks of lunar craters, they are much lower than the bounding walls.

Copernicus is surrounded by a mass of mountains, hills and ridges of highly complex structure, and by a marvelous system of brilliant streaks radiating from the crater as a center, and extending in some cases for four hundred miles, or even more, till they meet similar streaks from other craters,-from Kepler, Aristarchus, etc., which are shown on the cuts following.

This famous lunar mountain has been drawn and described many times. The very best drawings show a number of minor features which are much too small to be represented in the engraving; but no drawing has ever given anything like the true plastic effect, and even the very best drawings fail to show details which are evident on the original photograph.

To make such a drawing at the telescope, the observer must begin by sketching in the forms and shadows accurately, correcting here and adding there, until after one or two nights a skeleton for his finished picture is obtained. By this time the shadows have so changed that most of the work must be put by for a month, until the same phase of illumination recurs.

The next opportunity must be devoted to more corrections and additions, and so on, lunation after lunation, until finally the best possible result is attained. For instance, SCHMIDT's first recorded observation of Copernicus was in 1842 and his last in 1873. And even this best possible result will be highly unsatisfactory. If it is a map it will lack plastic effect; if it is a picture the minor topographical features will necessarily be more or less neglected. It is here that photography becomes of priceless advantage. The preparations for the photograph must be made with the greatest care; the picture must be taken when the atmosphere is steady, clear and transparent; when there is no wind to shake the telescope. But when the right opportunity occurs an exposure of a few tenths of a second is sufficient; and a perma

nent autographic record of things as they are is obtained. The negative can then be treated in many ways and many differing copies obtained, each one true in itself, but each one bringing out some one point with especial clearness. In the first place it can be enlarged so as to bring out the minor features. It can be "en-smalled" so as to sacrifice the minor details, while the grander relations are made more prominent. Each of these results can again be copied in various manners.

A certain exposure given to the copy will produce the best general plastic effect, and it is such copies that are desired by the artist and the general reader. But every single feature on the original has an illumination and a distinctness of its own. If we double or treble, etc., the first exposure, or if we halve it or take a third or even a tenth part of it in making our copies, each of the results will show some special feature or region or relation in a new and in a true light.

In this way the negatives of the Lick Observatory have brought out quite new features,―ruined craters fifty miles in diameter, long streaks and ridges, not suspected or even not perceptible in ordinary visual observation. The key of this method is that the contrasts can be artificially (photographically) increased.

If the reader will look at Figure 1 once more, he can probably follow the following identifications. The numbers in parenthesis are the diameters of the craters in miles.

The prominent crater about three-fourths of an inch south of Copernicus (56), is Reinhold (31), and the next marked feature in the same direction is the crater Landsberg (28). Between Reinhold and Copernicus are two small deep crater-pits close together, A and A'. These are, by the way, precisely south of the center of Copernicus.

The crater half an inch north of Copernicus is Gay-Lussac (15); and the mountains in which the latter is situated are the lunar Carpathians, whose peaks vary from twenty-five hundred to seven thousand feet in altitude. Towards the northwest, about an inch in the picture, is the ring-crater Eratosthenes (37). From Eratosthenes two spurs of mountains extend, one southwards to the white outlines of the ring-crater Stadius (43), the other northwestwards. The region bordering these two spurs and lying towards the southeast is the Sinus Estuum. The eastern wall of Eratosthenes is 7,450 feet above the outer surface, and 15,800 feet (the height of Mt. Blanc) above the interior of the crater.

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Made with the Great Telescope of the Lick Observatory, August 31, 1890, at 14h. 27m.

Moon's Age, 16 days 18 hours.

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