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Altitude as indicated by a sextant or similar instrument, before corrections are applied.

Related Terms

HEIGHT OF EYE CORRECTION

The correction to sextant altitude due to dip of the horizon. Also called DIP CORRECTION

INDEX PRISM

A sextant prism which can be rotated to any angle corresponding to altitudes between established limits. It is the bubble or pendulum sextant counterpart of the INDEX MIRROR of a marine sextant.

INDEX MIRROR

The mirror attached to the index arm of a marine sextant

INDEX ARM

A slender bar carrying an index; particularly the bar which pivots at the center of curvature of the arc of a marine sextant and carries the index and the vernier or micrometer.

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.

IRRADIATION CORRECTION

A correction due to irradiation, particularly that sextant altitude correction caused by the apparent enlargement of the bright surface of a celestial body against the darker background of the sky.

SHIFT TIDES

Sighting the positions of the sun and moon using a sextant and using a nautical almanac to determine the location and phase of the moon and calculating the relative effect of the tides on the navigation of the ship

LIMB

1. The graduated curved part of an instrument for measuring angles, such as the part of a marine sextant carrying the altitude scale, or ARC. 2. The circular outer edge of a celestial body, usually referred to with the designation upper or lower

AIR TEMPERATURE CORRECTION

A correction due to nonstandard air temperature, particularly the sextant altitude correction due to changes in refraction caused by difference between the actual temperature and the standard temperature used in the computation of the refraction table. The Nautical Almanac refraction table is based upo

ALTITUDE INTERCEPT

The difference in minutes of arc between the computed and the observed altitude (corrected sextant altitude), or between precomputed and sextant altitudes. It is labeled T (toward) or A (away) as the observed (or sextant) altitude is greater or smaller than the computed (or precomputed) altitude

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