Definition
Determined by setting the instrument to zero and viewing the horizon or a star; any step in the horizon line, or separation of the two star images, is the index error. If the reading is on the arc it is subtracted, and if off the arc it is added, remembered as if it is on, take it off. Sun measurement of on-and-off arc diameters provides a check, since the two values should sum to about four times the semi-diameter.
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