304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



J ANGLE BETWEEN L f 



LOCAL VERTICAL 

 AND LINE OF SIGHT 



TO STAR 



/ 



/ ^ '"' : LINE OF SIGHT . 



' ^..::::::::::::;r TO HORIZON > 



ALTITUDE OF STAR 



SEXTANT-ANGLE READING (AFTER 

 VARIOUS CORRECTIONS ARE AP- 

 PLIED) GIVES ALTITUDE OF STAR 

 WITH RESPECT TO HORIZON PLANE. 

 ANGLE BETWEEN LOCAL VERTICAL 

 AND LINE OF SIGHT TO STAR IS 90 

 DEGREES MINUS THE CORRECTED 

 SEXTANT-ANGLE READING. 



Figure 2. — Use of the sextant in celestial navigation. 



which angles to the local vertical are observed at the terrestrial point 

 to be located, by means of instruments like the sextant (fig. 2) . Angle 

 measurements of this kind are useful in navigation only if the posi- 

 tions that the observed stars occupy on the celestial sphere are known 

 and if the instantaneous orientation of a selected meridian fixed to 

 the earth is measured with respect to the celestial sphere. 



Data on celestial-sphere points are available from the body of 

 knowledge developed in descriptive astronomy and are recorded for 

 the purposes of navigation in star tables and almanacs. The meridian 

 selected as the reference for navigation is arbitrarily taken as the one 

 passing through Greenwich. At any instant, the hour angle of this 

 meridian may be determined with the aid of a chronometer reading, 

 which provides knowledge of the time elapsed since the Greenwich 

 meridian last occupied a reference orientation with respect to the 



