equatorial tides— Tides occurring semimonthly as the result of the Moon being over the 

 Equator. At these times the tendency of the Moon to produce a diurnal inequality in the 

 tide is at a minimum. 



equilibrium argument— The theoretical phase of a constituent of the equilibrium tide. It is 

 usually represented by the expression (V + u), in which V is a uniformly changing 

 angular quantity involving multiples of the hour angle of tlie mean Sun, the mean 

 longitudes of the Moon and Sun, and the mean longitude of the lunar or solar perigee; 

 the u is a slowly changing angle depending on the longitude of the Moon's node. When 

 pertaining to an initial instant of time, such as the beginning of a series of observations, it 

 is expressed by (V^ + u). 



equinoxes— The two points in the celestial sphere where the celestial Equator intersects the 

 ecliptic; also, the times when the Sun crosses the Equator at these points. The vernal 

 equinox is the point where the Sun crosses the Equator from south to north, about 

 March 21. Celestial longitude is reckoned eastward from the vernal equinox. The 

 autumnal equinox is the point where the Sun crosses the Equator from north to soutli, 

 about September 23. 



equipotential surface (geodesy)- A surface having the same potential of gravity at every 

 point (no work is done when a body is moved about in such a surface). When bodies of 

 the same mass are moved from one equipotential surface to another, the same amount of 

 energy is developed or expended, accordingly, as the bodies are lowered or raised, and 

 regardless of the route followed. In lifting a unit mass from sea level to any other 

 specified equipotential surface, the amount of work required is the same at the Equator 

 as at the poles: at the Equator the height of any equipotential surface above sea level is 

 greater and the force of gravity at the surface is less than for tlie same equipotential 

 surface at the poles. An equipotential surface is a level surface. A particular equipotential 

 surface is identified by its dynamic number. 



error of closure— See misclosure. 



error of closure, angles— The amount by which the actual sum of a series of angles fails to 

 equal the theoretically exact value of that sum. See error of closure of horizon and error 

 of closure of triangle. 



error of closure in azimuth— The amount by which two values of the azimuth of a line, 

 derived by different surveys or along different routes, fail to be exactly equal to each 

 other. Generally, one value is derived by computations carried through the survey 

 (triangulation or traverse); the other is an adjusted or fixed value determined by an 

 earlier or a more precise survey, or by independent astronomical observations. 



error of closure in leveling— The amount by which two values of tlie elevation of the same 

 bench mark, derived by different sur\'eys or through different survey routes or by 

 independent observations, fail to be exactly equal to each other. The closure may be 

 developed in a line of leveling which starts and ends on different bench marks whose 

 elevations are held fixed; or it may start and close on the same bench mark. 



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