Celestial Navigation in the GPS Age by Karl John

Author:Karl, John [Karl, John]
Language: eng
Format: epub
Publisher: Paradise Cay Publications, Inc.
Published: 2012-07-25T00:00:00+00:00

Index Error

After adjusting the horizon mirror to be perpendicular to the frame, with the horizon and its reflected image in coincidence, we would like the sextant's arc to read exactly zero. This really never happens; instead, there's an index error. So carefully set the micrometer drum to zero. Then by adjusting the index correction screw on the horizon mirror, bring the two images into coincidence. This can affect the previous perpendicularity adjustment, so repeat that one. After readjusting these interacting adjustments a couple of times, quit when the index error is small—say, under a few minutes of arc. But the last adjustment you make should be the perpendicularity adjustment. Then just accept the remaining index error and apply it to your observation before any other corrections. This correction is listed on our sight reduction forms because we expect that it is always needed. A decent quality sextant will hold these adjustments for quite some time, but nonetheless, you should check the index error before each series of sights.

Another index correction method is to look at the sun and its reflected image. The best sensitivity is obtained not by aligning the sun's disk with itself, but by bringing one limb of the sun into coincidence with its other limb. Then do the reverse. A little thought, particularly with the aid of a sketch, shows that the sum of these two readings should be four times the sun's semidiameter, whereas the difference is twice the index error. I like this idea because of the sensitivity of aligning the edges of the sun's disk, the increased accuracy by dividing by two to get the index error, and the double check on the sextant by comparing the sun's semidiameter with the almanac's report for that date. (Some older sextants do not have a horizon shade sufficiently dark to look at the sun.)

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