. The book of photography; practical, theoretical and applied. Fig. 801.—Gregorys ReflectiKg Telescope. vertical plane which passes through thesuns centre at mid-day. Any horizontalrotation from this position is termedAzimuth, and any vertical rotation, Alti-tude. But it is evident that measure-ments taken on this plan are of restricteduse, only applying to the observerslocality, since both the horizon and thesuns direction at its highest elevationvary for different places on the the apparent motion of the heavenly. Fig. 802.—Cassegrains Eeflecting Telescope. bodies alters both in alt
. The book of photography; practical, theoretical and applied. Fig. 801.—Gregorys ReflectiKg Telescope. vertical plane which passes through thesuns centre at mid-day. Any horizontalrotation from this position is termedAzimuth, and any vertical rotation, Alti-tude. But it is evident that measure-ments taken on this plan are of restricteduse, only applying to the observerslocality, since both the horizon and thesuns direction at its highest elevationvary for different places on the the apparent motion of the heavenly. Fig. 802.—Cassegrains Eeflecting Telescope. bodies alters both in altitude and azimuth,any mechanical arrangement for keepinga star continually in view in the telescopicfield would be unnecessarily complicated. The Celestial Sphere. In the second method, all measurementsare referred to the centre of the earth,and thus the observers position on theearths surface does not interfere withsubsequent comparison of the observa- 598 THE BOOK OF PHOTOGRAPHY. tions made. This system of measurementdepends on the small magnitude of theearths i-adius when compared with thedistances of the bodies that are beingobserved. As a first illustration, considerthe directions in which a couple of menstanding side by side must look to see thesame object at a distance of five there is an angle (^oo of ^degree approximately) between their linesof sight, but the two directions are for all would take a year in travelling, the starsdistance is termed a light-year. As far asposition in the sky is
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