National Geographic : 1931 May
Straight line, horizontal at airplane, to true or sensiblehorizon s___ - - -A / /ne otsghthorizontaiatairplanse curved byatmosphericrefractione above sea le' ----- -- - , tvnn 0f a rp 1 ''' - airplane Ground/evel e1ao 53 mles from -pl - eVilla Mercedes *i n - ,,-,,. sea meve - -* . : 30,000 a A DIAGRAM ILLUSTRATING THE CURVATURE OF THE EARTH, AS SHOWN IN THEPHOTOGRAPH ON PAGE 634 The snow-clad mountains in the illustration on the preceding page form a background that brings into view the surface of a stratum of haze which obscures the lower portions of the snow (see illustration, page 632). The top of the stratum of haze, at an elevation of 6,700 feet above sea level, conforms closely to the sea-level surface of the earth and does not share the irregularities of the ground, 4,000 feet below. The haze forms a visible horizon 153 miles from the camera, and 70 miles of this visible horizon appear laterally in the photo graph. Although this horizon is only i/36oth of the circum ference of the earth, its curvature can be plainly seen. The sensible horizon, as distinguished from the visible horizon, is the line where a plane surface, level at the point of observa tion, meets the sky. The white line ruled across the sky in the photograph indicates the position of the sensible horizon. Al though the highest of the mountain peaks in the photograph are at a greater elevation than the camera, they appear below the sensible horizon, due wholly tothe curve ofthe earth's surface. Due to refraction, thepath of aray oflight through the air is not straight, but curved downward byamaximum of about i/7th the amount of theearth's curvature, depending chiefly on the density of the air. The effect ofthis refraction istomake the earth seem a littleless curved than itis,and tomake distant objects appear alittle higher. This fact has tobetaken into account in trigonometric leveling for determining eleva tions. At the distance of Mount Aconcagua, 287 miles, the earth curves 54,900 feet from astraight horizontal line, but only 47,100 feet from a curved line of sight that ishorizontal tothe observer. The short white line under Mount Aconcagua shows the posi tion of sea level, abovewhich the mountain rises toaheight variously computed at from 22,800 to23,100 feet, with 23,098 as the figure most frequently adopted.