An ellipsoid of revolution, the shorter axis of which is the axis of revolution. An ellipsoid of revolution, the longer axis of which is the axis of revolution, is called a PROLATE SPHEROID. The earth is approximately an oblate spheroid.
Related Terms 
GYRO ERROR
The error in the reading of the gyrocompass, expressed in degrees east or west to indicate the direction in which the axis of the compass is offset from true north.

GYROSCOPE
A rapidly rotating mass free to move about one or both axes perpendicular to the axis of rotation and to each other. It is characterized by GYROSCOPIC INERTIA and PRECESSION. Usually shortened to GYRO. The term also refers colloquially to the GYROCOMPASS.

GYROSCOPIC DRIFT
The horizontal rotation of the spin axis of a gyroscope about the vertical axis

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

HORIZONTAL EARTH RATE
The rate at which the spin axis of a gyroscope must be tilted about the horizontal axi

AXIALLOAD BEARING
A bearing in which the load acts in the direction of the axis of rotation.

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

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

INPUT AXIS
The axis of applied torque of a gyroscope
