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Sextant Sight

The process of measuring the sextant height of a celestial body and recording the watch time of the measurement to the second. Once the object and horizon are both in view (achieved in sun sights by just hunting around the arc, or in star sights by precomputation or by inverting the sextant), the sight is completed by adjusting the micrometer until the object just skims the horizon while rocking the sextant.

Log readings should be recorded before and after each sight session, to identify the sight or to use later in a running fix.

Whenever possible, sextant sights should be of objects higher than 15° and lower than 75°. Lower sights are less accurate due to refraction uncertainties, and higher sights require special sight reduction techniques. Within these limits, sun sights can be taken any time of day, but star sights are typically limited to the period between civil and nautical twilight when it is dark enough to see the stars, yet not too dark to see the horizon. Checking the index correction is an important part of any sight session. Sun sights require no preparation, but star or planet sights should be precomputed before the sight session. See Index Correction, High-Altitude Sights, Precomputation, Rocking the Sextant, and Inverting the Sextant.

See: sextant sight, standard time, time zone, universal time, watch error, watch rate, watch time, zone description, and zone time.

See also: Time Keeping in Navigation and Weather.



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