Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/872

802 Herschel imagined that he even observed volcanoes in activity ; but it is now known that he mistook for volcanic light the effects of earth-light reflected from those parts of the moon s surface which have the highest reflective capa city. It has been urged that, as the brightness of the moon is sensibly equal at all times, she cannot be surrounded by an atmosphere similar to that of our earth. Moreover, if a lunar atmosphere existed, its influence would be percep tible in the occultations of the planets, or fixed stars, by the moon. The effects of refraction as exerted by an atmosphere resembling the earth s would cause a star to remain visible for a considerable time after it had really passed behind Hie moon s globe. In fact, it is easily shown that an atmosphere like our earth s would prevent any star from being con cealed at all. It would render visible a star really placed directly behind the mcon s centre ; for the actual refractive effect required for this purpose would correspond to a deviation of the rays of light by about 1 6, or 8 in entering, and 8 in passing out of the lunar atmosphere. Now the earth s atmosphere produces a refractive effect of 34 on an orb in the horizon, or more than four times as much as would be required in a lunar atmosphere in order to render visible a star directly beyond the moon s centre. In point of fact, stars are occulted by the moon the instant they reach her edge ; or if at any time there is apparent delay, this is such only as may be fairly attributed to the effect of irradiation. In recent years the surface of the moon has been studied with increased care.

Lohrmann, a land surveyor of Dresden, planned the construction of a lunar chart on a large scale in twenty-five sections, and in 1824 the first four sections were published. He was unable, however, to complete this arduous task, as failing sight compelled him to desist. He published in 1838 a good general chart of the moon, 15 J inches in diameter. {{ti|1em|Amongst contributions to this department of astronomical research must be mentioned the well-executed map by Messrs Beer &amp;lt;fc Madler (see their work Der Mond}. It is the result of several years careful study and micrometrical measurement of the surface of the moon, and every point discovered by the telescope has been laid down with great precision. In Der Mond these astronomers give the measurements of 919 spots, and 1095 determinations of the height of lunar mountains.}} Schmidt of Athens has completed the most important contribution yet made to selenographic research. Passing over his earlier and, in his own estimation, unsatisfactory processes of charting, commenced in 1839, we may note that, according to his present plan, the complete chart has a diameter of 6 feet. It is constructed, however, in 25 sections (after Lohrmann s plan), and these are now com pleted. But the work still awaits publication, as the expenses of engraving must be very great, and are beyond Schmidt s means.

Photography has been applied successfully to the work of picturing our satellite, though it will probably be long before photographic charts can be produced comparable with the work of Schmidt, or even of Beer and Madler, so far as details are concerned. The moon was first photographed by Dr J. W. Draper of New York, in 1840. Bond photographed ,the moon successfully in 1850, using the telescope of the Cambridge (U.S.) Observatory. Between the years 1850 and 1857, the moon was photographed by Secchi in Rome, Bcrtch and Arnauld in France, Phillips, Crookes, Hartnup, De la Rue, Fry, Huggins, Dancer, Baxehdell, and others in England. Later, De la Rue produced lunar photographs of remark able accuracy and beauty. But the very best pictures of the inoon yet obtained are by Puitherfurd of New York, using a refractor of 11^ inches aperture, and 13 feet focal length, corrected for the chemical rays.

A series of interesting researches has been made into the light-rfeflecting capacity, or, in point of fact, the whiteness of our satellite, by Zollner of Leipsic. Bouguer had assigned to cne moon a brightness equal to 300 1 000 th part of the sun s. Wollaston made the proportion much smaller, determining it at &0l 1 070 th only. Zollner employed two different methods of research. In one he determined the moon s illumination by tests of her surface brightness ; in the other he obtained point-like images of the sun and moon for comparison with corresponding images of candle-flames. The results, lying between those obtained by Bouguer and Wollaston, are also in close agreement inter se. According to the former method, the light of the full moon is 6l8 1 000 th part of the sun s light ; according to the other, the propor tion is as one to 619,000. But the most interesting results obtained by Zollner are those relating to the varying brightness of the moon at her different phases. He found that the defect of light when the moon is not full is much greater than it would be if the moon were a smooth but unpolished globe. From the agreement between the observed results and an empiric formula, intended to correspond for phases within 70 of the full moon with the case of an orb grooved meridionally so that the slopes of the grooves sides are inclined 52 to the surface, Zollner infers that the average slope of the lunar elevations amounts to about 52. The following table indicates the degree of approximation:—

Arc from Moon s place to point opposite tli- Sun. Theoretical Brightness. Full Moon s as 100. Observed Brightness. ZBllner. Moon regarded as smooth. By Zollner s formula. + 1 99-98 98-60 98-60 5 99-63 9279 87-20 8 99-06 88-41 92-19 11 98-24 84-04 88-76 -13 97-57 81-21 82-60 + 19 94-93 72-29 68-41 24 92-13 65-15 71-38 27 90-18 61-00 57-90 -27 90-18 61-00 63-47 + 28 89-50 59-60 56-15 -28 89-50 59 60 57-00 + 23 85-82 52-90 48-60 -39 80-87 45-00 41-70 + 40 80-04 43-70 47-10 41 77-78 42-50 43-95 -42 76 27 41-40 38-00 + 46 74-61 36-7C 36-10 -52 68-87 27-63 29-11 + 58 62-91 24-30 27-10 -62 58-89 20-60 20-40 -69 51-82 15-20 14-60

—Eclipses of the Sun and Moon.

Strictly speaking, the term eclipse extends to all cases in Eclipse which one celestial body is wholly or partially obscured by another, either by direct intervention or by the interception of light illuminating the former, so that the term includes transits of inferior planets, the concealment of stars by the moon or by a planet, or the concealment of a satellite by its primary. But the term eclipsr is for convenience limited to the three following orders of phenomena : (1.) The total or partial concealment of the sun by the moon, called a solar eclipse. (2.) The obscuration of the moon by the shadow of the earth, called a lunar eclipse. (3.) The con cealment of a satellite of a planet by the shadow of the primary, called eclipses of satellites, and distinguished from occultations in which the disk of the primary hides the &quot;satellite from view.