# ANTITWILIGHT

The pink or purplish zone of illumination bordering the shadow of the earth in the dark part of the sky opposite the sun after sunset or before sunrise. Also called ANTI CREPUSCULAR ARCH.

## Related Terms

### 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

### 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 sphere-formed by the intersection of the celestial sphere and a plane perpendicular to the zenith-nadir line is called sensible horizon if the plane is through any point, such as the eye of an observer; geoidal horizon if through any sea-level 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

### 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

### DIATOMACEOUS EARTH FILTRATION

Is a process in which a filter cake or precoat of diatomaceous earth is used as a filter medium.

### INSULAR SHELF

A zone around an island that extends from the low water line to a depth at which there is usually a marked increase of slope towards oceanic depths.

### ISOSTASY

A supposed equality existing in vertical sections of the earth, whereby the weight of any column from the surface of the earth to a constant depth is approximately the same as that of any other column of equal area, the equilibrium being maintained by plastic flow of material from one part of the earth to another

### GROUND

Zero voltage, or any point connected to the earth or 'ground'.

### LINE OF SIGHT

The straight line between two points, which does not follow the curvature of the earth

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