October 8, 190S] 



NA TURE 



SURVEYING FOR ARCH/EOLOGISTS.' 

 V. 



Why the Mcasuretnent of Altitude is Necessary. 



IT is now time to enter more into detail on a point 

 to wliich reference has already been made, as it 

 is one of great importance to all British archaeologists. 



571 



But before we consider them, I must refer to 

 another matter. 



The light from sun or star when it enters the 

 earth's atmosphere is refracted or bent out of its 

 course, and the more slantingly it enters the atmo- 

 sphere, as happens near rising and setting, the greater 

 the refraction. In consequence of this the sun or a 



AITITUDES 



SUNRBS SUMMER SOLSTICE l.t,tuds59'n 



^4'»tH 



N sr 



Fig. 19. — The conditions of ■'siinris; " at the summer soUtice in lat. 5(^' N. 



as Britain lies in a mid-latitude. If a star or the sun 

 did not rise or set every day in Britain as happens 

 at the poles, or rose and set vertically, as happens at 

 the equator, the height of the horizon would not 

 come into play. 



As a matter of 

 fact, however, in 

 Britain some celes- 

 tial bodies do rise 

 and set, and not 

 vertically ; their 

 paths, as we have 

 seen, are inclined 

 to the horizon, and 

 therefore the azi- 

 muth of the rising 

 or setting place de- 

 pends upon the 

 height of the hori- 

 zon, and I may 

 add that the zenith 

 distance must be 

 less than 90° if the 

 horizon is raised 

 by hills. 



In order to con- 

 sider this matter 

 more closely, I 

 give in the accom- 

 panying figures the 

 actual facts of the 

 sunrise on the N.E. 

 horizon at the 

 longest day of the 

 year in two British 



star appears higher in the heavens than it really is, 

 and therefore appears to rise earlier and set later. 



In Fig. 19 we see, diagrammatically, the effect of 

 hills and refraction on the azimuth of the summer 

 solstice sunrise in lat. 59° N. The long dotted line 



SeaLevel 



I905AD. 20 



leSOac. 



Fig. 20. — Showing azimuths in lat. N. 50" for 



16S0 



latitudes, Stenness, lat. 59° N., and Cornwall, lat. 50°. 

 They will illustrate the effect of latitude upon azimuth 

 as well as the change of azimuth in presence of hills 

 which now specially concerns us. 



^ Continued from p. 544. 

 NO. 2032, VOL. 7S] 



the summer solstice sunrise, with different heights of hills for 1905 A.D. and 

 B.C. (From " Stonehenge," p. 290.) 



shows the slanting direction of the sun's path in 

 relation to the horizon. The double circle indicates the 

 position of the sun's centre, at the sea horizon and 

 neglecting refraction. The azimuth, as shown by the 

 scale at the bottom of the diagram, is N. 39° 16' E. 



