 | Sir J. Butler Williams - 1846 - 368 páginas
...from the direction of the spiritlevel or plummet, they are consequently measured by reference to a tangent to the surface of the earth at the point of observation. Let the points A and B represent two remote objects, and C the centre of the earth. With the radius... | |
 | 1900 - 870 páginas
...assumed to lie in the surface of the celestial sphere. The sensible horizon is the circle in which a plane, tangent to the surface of the earth at the point of observation, cuts the surface of the celestial sphere. The real horizon is the great circle in which a plane through... | |
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It is found to be always exactly perpendicular to the plane of the horizon; or, to a plane tangent to the surface of the earth at the point of observation. 