NAVIGATION — CANOES TO SPACESHIPS — DRAPER 303 



except for a few special cases where boats might be moved by known 

 wind and water currents. This complete dependence on currents or on 

 visible terrestrial objects was relieved many centuries ago by the dis- 

 covery that under proper conditions heavenly bodies could be used to 

 assist travel over the surface of the earth. In effect, the stars were 

 found to act as points in a knowable space located at a great distance 

 from the earth (pi. 1). The sun, the moon, and the planets did not 

 appear to be fixed in this space, but followed paths that reduced their 

 usefulness for the purposes of guidance. On clear nights, the star 

 Polaris showed the direction of north and provided information on 

 latitude by its angle above the horizon. Other stars with known posi- 

 tions in the pattern of the celestial sphere were also used, but celestial 

 navigation remained an incomplete art for many centuries. The prin- 

 cipal reason for this imperfection was the earth's rotation, which 

 made it impossible to determine the angular position of the earth with 

 respect to the stars. Without good information on this position, esti- 

 mates of longitude necessarily remained of low quality. 



CELESTIAL NAVIGATION 



The key problem in longitude measurements was that of finding 

 the rotational angle of the earth with respect to a reference position 

 having known relationships to points fixed on the celestial sphere. 

 Astronomical knowledge recorded in star tables and almanacs easily 

 gave angles between lines of sight to celestial objects and the vertical 

 at any terrestrial point, if the earth could be assumed to remain in a 

 particular position. In practice, the earth never fulfilled this assump- 

 tion, but continuously moved with respect to any possible reference 

 position. Because the angular velocity of the earth among the stars 

 was and is effectively constant and well known, an accurate means for 

 indicating sidereal time (time based on rotation of the earth referred 

 to the celestial sphere) would have made it possible to find longitude 

 by fixing the angle of the earth from a selected reference position. 



The basic problem of timekeeping for navigation was first solved 

 during the 18th century by John Harrison, who received a prize from 

 the British Admiralty for his achievement. The 19th-century de- 

 velopments of instruments and other devices that accompanied and 

 followed Harrison's work on the marine chronometer — improved sex- 

 tants, logarithmic multiplication, almanacs, and other aids — brought 

 the art of celestial navigation by visual observations very close to the 

 high level that it has today. 



Celestial navigation is basically the art of using the celestial sphere 

 as a reference space in which visible stars provide geometrical points 

 for relating positions in terrestrial space to a system of coordinates 

 outside the earth. These star lines of sight are the directions from 



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