RECENT PROGRESS ACCOMPLISHED BY AID OF PHOTOG- 

 RAPHY IN THE STUDY OF THE LUNAR SURFACE. 1 



By MM. Loewy and Puisettx. 



(With three plates.) 



I. — RESUME OF THE PRINCIPAL QUESTIONS WHICH HAVE BEEN 

 SUGGESTED BY LUNAR OBSERVATIONS. 



Next to the sun, which furnishes us with heat and light, the moon, 

 of the remaining celestial bodies, excites the greatest curiosity and 

 suggests the greatest number of problems to investigators. The suc- 

 cession of its phases has furnished a unit of time, intermediate between 

 that of the day and the year, which has been universally adopted. 

 The eclipses which our satellite undergoes or occasions when it aligns 

 itself with the earth and the sun have excited even the most undevel- 

 oped imaginations since antiquity. They still serve chronologists in 

 verifying historical dates. Representing a source of important progress 

 in physical astronomy, they give rise, several times in a century, to 

 reunions and to the combined efforts of many astronomers. The rapid- 

 ity of the apparent motion of the moon among the stars makes it very 

 valuable for the determination of geographical coordinates ; its ephein- 

 eris is an indispensable manual to travelers and to mariners. The 

 researches of mathematicians have shown that the attraction of our 

 satellite is the principal cause of the tides, and of precession and nuta- 

 tion, that is, changes in the direction of the earth's axis in space. The 

 perturbations which the moon undergoes in its orbit teach us much in 

 regard to the interior structure of our globe, of its ellipticity and of the 

 fundamental constants of astronomy, such as the mass of the earth and 

 the solar parallax. Its influence on the meteorological phenomena, 

 strongly fixed in popular opinion, is still the object of persevering and 

 impartial researches. Finally, by its relative proximity, the moon pre- 

 sents itself as an obliging stepping-stone when we wish to extend our 

 investigations beyond the limits of the globe which has been assigned 

 to us as our abode. Deprived of the vaporous envelopes which appear 

 to obscure the surfaces of Venus, Mars, and Jupiter, it reveals even in 

 the smallest telescopes numerous sharp and persistent details. No 



1 Translated from Armuaire du Bureau des Longitudes, Paris, 1898. 



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