176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



on the moon. But astronomers do not agree as to the validity of any 

 single change, and the bulk of the available evidence goes to show 

 that there has been no appreciable change on the moon's surface 

 within the past century. 



It seemed, therefore, to the moon committee that a lunar map 

 should be prepared which is free from the personal equation and not 

 dependent on the skill of the observer to depict correctly what he 

 sees on the surface of the moon. The positions of approximately 

 4,000 points on the moon's surface have been accurately measured by 

 Saunders, Franz, and others and expressed in terms of selenographic 

 longitude and latitude. With the aid of these data on position it is 

 possible to ascertain the amount and direction of libration in each 

 photograph of the moon. If each photograph could be transformed 

 so that its plane coincides with the plane of mean libration, namely, 

 the plane on which all lunar maps are projected, the transformed 

 photograph would form part of a lunar map and at the same time be 

 free from the personal equation of the one who makes the map. To 

 prepare a photographic map of the moon it is necessary to trans- 

 form photographs taken with the aid of the 100-inch telescope at 

 JNIount Wilson so that the plane of projection is the same for all 

 photographs. A map is a projection on a definite plane ; the type of 

 projection and the plane of projection must be quite definite if the 

 map is to be satisfactory. For the transformation of the photo- 

 graphs a special moon house has been built at Mount Wilson. It is 

 a specially insulated structure with double walls, corrugated sheet 

 iron on the outside and paper on the inside with ventilation between 

 the walls so that they quickly respond to temperature changes out- 

 side. The floor is covered with a layer of sawdust 6 inches thick to 

 prevent radiation from the ground. As a result, the temperature 

 distribution within the 150-foot building is remarkably uniform and 

 seeing conditions are good so long as the temperature outside is not 

 changing rapidly and there is no appreciable wind. 



To transform a given moon photograph taken at the Cassegrain 

 focus of the 100-inch telescope (focal length 135 feet), the moon 

 positive, 15 inches in diameter, is mounted in front of a powerful 

 beam of light reflected by an Army searchlight mirror 3 feet in 

 diameter; the light passes through the positive to a parabolic sil- 

 vered mirror of 67.5 feet focal length and 135 feet distant and 

 thence back to a carefully turned globe of bronze, 15 inches in diame- 

 ter and coated with magnesia powder. This coating furnishes a 

 white diffuse reflecting surface. The image of the moon formed on 

 it is in all respects similar to the moon in the relations of the sur- 

 face features one to the other; in other words, it is a miniature 

 moon which can be photographed from any direction. For this 



