Crepuscular rays extending downward toward the horizon


Related Terms

HORIZONTAL SCANNING

In radar scanning, rotating the antenna in azimuth around the horizon or in a sector. Also known as searching lighting.

HORIZON SYSTEM OF COORDINATES

A set of celestial coordinates based on the celestial horizon as the primary great circle; usually altitude and azimuth or azimuth angle.

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

HORIZON MIRROR

The mirror part of the horizon glass. The expression is sometimes used somewhat loosely to refer to the horizon glass.

HORIZON GLASS

The glass of a marine sextant, attached to the frame, through which the horizon is observed. The half of this glass nearer the frame is silvered to form the HORIZON MIRROR for reflecting the image of a celestial body; the other half is clear.

INDEX ERROR

The error in the reading of an instrument equal to the difference between the zero of the scale and the zero of the index. In a marine sextant it is due primarily to lack of parallelism of the index mirror and the horizon glass at zero reading.

ARC OF VISIBILITY

The portion of the horizon over which a lighted aid to navigation is visible from seaward.

ANTARCTIC WHITEOUT

The obliteration of contrast between surface features in the Antarctic when a covering of snow obscuring all landmarks is accompanied by an overcast sky, resulting in an absence of shadows and an unrelieved expanse of white, the earth and sky blending so that the horizon is not distinguishable. A similar occurrence in the Arctic is called ARCTIC WHITEOUT.

ARCTIC WHITEOUT

The obliteration of contrast between surface features in the Arctic when a covering of snow obscuring all landmarks is accompanied by an overcast sky, resulting in an absence of shadows and an unrelieved expanse of white, the earth and sky blending so that the horizon is not distinguishable. A similar occurrence in the Antarctic is called ANTARCTIC WHITEOUT.

NAUTICAL TWILIGHT

The time of incomplete darkness which begins (morn- ing) or ends (evening) when the center of the sun is 12° below the celestial horizon. The times of nautical twilight are tabulated in the Nautical Almanac; at the times given the horizon is generally not visible and it is too dark for marine sextant observations.

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