Perturbations of the orbits of artificial earth satellites due to the attractions of the sun and the moon. The most important effects are secular variations in the mean anomaly, in the right ascension of the ascending node, and in the argument of perigee.
Related Terms 
HALFTIDE LEVEL
A tidal datum midway between mean high water and mean low water. Mean sea level may coincide with halftide level, but seldom does; the variation is generally about 3 centimeters and rarely exceeds 6 centimeters. Also called MEAN TIDE LEVEL.

HELIOCENTRIC PARALLAX
The difference in the apparent direction or positions of a celestial body outside the solar system, as observed from the earth and sun. Also called STELLAR PARALLAX, ANNUAL PARALLAX

ANTIFRICTION BEARING
A rolling contact type bearing in which the rotating or moving member is supported or guided by means of ball or roller elements. Does not mean without friction.

HORIZON
The great circle of the celestial sphere midway between the zenith and nadir, or a line resembling or approximating such a circle. The line where earth and sky appear to meet, and the projection of this line upon the celestial sphere, is called the visible or apparent horizon. A line resembling the visible horizon but above or below it is called a false horizon. The circle of the celestial sphereformed by the intersection of the celestial sphere and a plane perpendicular to the zenithnadir line is called sensible horizon if the plane is through any point, such as the eye of an observer; geoidal horizon if through any sealevel point; and celestial or rational horizon if through the center of the earth. The geometrical horizon was originally considered identi cal with the celestial horizon, but the expression is now more commonly used to refer to the intersection of the celestial

HIGH WATER MARK
A line or mark left upon tide flats, beach, or alongshore objects indicating the elevation of the intrusion of high water. It should not be confused with the MEAN HIGH WATER LINE or MEAN HIGHER HIGH WATER LINE

NATIONAL TIDAL DATUM EPOCH
The specific 19year cycle adopted by the National Ocean Survey as the official time segment over which tide observations are taken and reduced to obtain mean values(e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of apparent periodic and apparent secular trends in sea level. The present National Tidal Datum Epoch is 1960 through 1978.

HORIZONTAL GEODETIC DATUM
The basis for computations of horizontal control surveys in which the curvature of the earth is considered It consists of the astronomical and geodetic latitude and the astronom ical and geodetic longitude of an initial point (origin); an azimuth of a line from this point; the parameters (radius and flattening) of the reference ellipsoid; and the geoidal separation at the origin. A change in any of these quantities affects every point on the datum. For this reason, while positions within a system are directly and accurately relatable, those points from different datums must be transformed to a common datum for consistency. The horizontal geodetic datum may extend over a continent or be limited to a small area.

INFERIOR CONJUNCTION
The conjunction of an inferior planet and the sun when the planet is between the earth and the sun

HYDROSPHERE
The water portion of the earth as distinguished from the solid part, called the LITHOSPHERE, and from the gaseous outer envelope, called the ATMOSPHERE

INEQUALITY
A systematic departure from the mean value of a tidal quantity. inert
