758 HANDBOOK OF PHOTOGRAPHY 



Photography oj the Moon. — While the moon is a very much smaller body than the 

 earth, its proximitj- to our planet renders it a disk of approximately the same diameter 

 as the sun in the sky. The solar camera forms, therefore, a useful piece of apparatus 

 for lunar photography, the only difference in operation being in the length of the 

 exposure. Under similar conditions of aperture, an exposure on the moon should be 

 in the neighborhood of 3^ sec. as compared with Hooo sec. on the sun itself. The large 

 amount of detail on the lunar surface, including mountain ranges and craters, together 

 with the ever constant change in the illumination as the sun rises and sets over the 

 principal lunar features, presents a fascination to the amateur photographer who has a 

 telescope that may be adapted to the purpose. 



If two exposures on the moon are made with the moon at the same phase, but 

 separated approximately one month apart in time, it will be found in general that the 

 features in the second photograph occupy slightly different positions in regard to the 

 lunar disk than in the first photograph taken. This is due to the fact that the lunar 

 axis is tipped slightlj'' toward the earth and also on account of the fact that, while the 

 moon rotates uniformly on its axis in a period of one month, thereby always presenting 

 its same face to the earth, its slightly eccentric orbit causes it to travel at a nonuniform 

 motion . The apparent displacements of certain lunar features, therefore, with respect 

 to the disk for the reasons mentioned, are called "librations." If the two photographs 

 taken at intervals of one month at corresponding phases of the moon are mounted 

 together and viewed with a stereoscope, a beautiful stereoscopic effect results from 

 these librations, the moon presenting in the stereoscope a very real spherical form. 

 For one who is interested in lunar photography detailed maps of the lunar surface have 

 been published and may be obtained from book sellers and scientific supply houses. 



Photography of Comets. — The appearance of a comet in the sky affords unusual 

 opportunity for the celestial photographer. Great detail may be obtained from 

 cometary photographs that cannot be observed by the naked eye. The principal trick 

 in photographing a comet is to have an auxiliary guiding telescope attached to the 

 photographic telescope so that the head or nucleus of the cometary object may be set 

 on the cross wires of the guiding eyepiece and kept in position by means of the slow 

 motions provided through the entire exposure. This is necessarj^ since a comet is 

 a relativelj^ swift-moving object among the stars. 



The driving clock of the telescope can be expected to follow only the general diurnal 

 motion of the sky as the stars move from east to west. As the observer must make the 

 photographic plate register constantly with the comet itself, star images on the plate 

 after development will be found to take the form of streaks or trails rather than round 

 images. The length of the streak or trail will obA'ioush* increase with the length of the 

 exposure, and the general direction of these star trails will represent the motion of the 

 comet among the stars while the object is being photographed. The exposure time 

 will depend entirely upon the brightness of the object and the amount of detail desired. 

 It may extend from a few minutes to several hours. The development of the first 

 trial plate will give an index as to the requisite exposure time to bring out features that 

 may be desired. 



Photography of Meteors. — The photograph}^ of meteors or shooting stars requires 

 cameras of relativelj^ short focus. Since the appearance of a meteor is in a large 

 sense accidental and there is no way of determining with anj^ accuracy the precise 

 point in the skj^ at which meteors appear, an astronomer frequently employs a battery 

 of cameras of wide-angle lenses pointed in different directions, so that a large area of 

 the sky can be covered. On the occasion of anticipated meteoric showers, such as the 

 Perseids which occur during the month of August or the Leoneids which appear in 

 the middle of November, the cameras may be pointed in the general direction of the 

 so-called "meteor radiant," the constellation of Perseus for the August meteoric shower 



