1 article for "Deflection of the Vertical"

**Deflection of the Vertical**[Astro*Index]

The angle between the *geocentric zenith direction*, determined by a line passing through the center of the earth and the observer, and the *geodetic zenith direction*, determined by a plumb line passing through the observer. The distinction is necessary because the earth is not exactly spherical and a plumb line does not necessarily pass through the center of the earth. While the earth is not a sphere, neither is it a spheroid, due to gravitational peculiarities; it is termed a geoid, defined as the shape where, at all points, local gravity is normal. The difference between a geoid and a standard elliptical spheroid is so small that for the purposes of definition a standard spheroid is used and geodetic zenith is the same as astronomical zenith, determined by a plumb line through the observer. The difference between the Earth's greater and lesser radii is 13 miles (3963 miles at the equator; 3950 miles at the poles).

See also:

♦ Geocentric ♦ Geodetic

Astro*Index Copyright © 1997 Michael Erlewine