Page 367 equipment and instruments 4518 



may be made while viewing any distant object, terrestrial or celestial, thi'ough the 

 sextant. 



After the index error has been corrected, the second adjustment — the perpendicu- 

 larity of the horizon glass— should again be verified, since its adjustment is often dis- 

 turbed during the last operation. 



The adjustment for index error is not essential since its amount may be determined 

 and applied as a correction to all angles observed. But in hydrographic surveying this 

 would be a nuisance. For Imiited use, when very small angles are involved as in sex- 

 tometer distances, readings may be taken both on and off the arc and the two meaned. 

 The result will be free of any index error. 



For astronomic observations and the measurement of other sextant angles where 

 utmost precision is required, the same adjustments are necessary. In addition, the 

 exact index error should be determined at each set of observations for application as a 

 correction to the observed angles. 



The amount of the index error may be determined by one of the following three 

 methods: 



(a) With the sextant held vertically and pointed at the sea horizon bring the direct and reflected 

 images into coincidence and read the setting of the index arm. Repeat several times, bringing the 

 reflected horizon down to coincidence and up to coincidence alternately, taking a mean of the results. 

 If the zero of the vernier is to the right of the zero of the arc, or ojf the arc, the correction is positive 

 and should be added to measured angles. If it is to the left, or on the arc, the correction is negative 

 and should be subtracted from the measured angles. 



(6) Substitute a star at night for the sea horizon, point the sextant at the star and bring the direct 

 and reflected images into coincidence and read the setting of the index arm. Repeat several times 

 and use a mean of the results as above. 



(c) Measure the apparent diameter of the sun with the sextant held vertically, bringing the 

 upper limb of the reflected image to touch the lower limb of the direct image. Read the angle. Then 

 bring the lower limb of the reflected image to touch the upper limb of the direct image and read the 

 setting off the arc. Half of the difference of the two readings is the index correction, positive or nega- 

 tive as the larger of the two values is off or on the arc, respectively. For example, if the diameter 

 measures 33' 50" on the arc and 32' 40" off the arc, the index correction would be yi (33' 50" — 

 32' 40") =35" (minus). Several such observations should be taken and the mean used. The 

 accuracy of the result may be verified by comparing the sun's semidiameter for the date of observation 

 as taken from the Nautical Almanac with one-quarter of the sum of the two readings irrespective of 

 sign. 



The adjustments of all sextants used in hydrographic surveying should be verified, 

 and readjustments made if necessary, each morning before the start of the day's work. 

 Thoy must be verified each night at the close of the day's work. The fact that this has 

 been done at both times and the amount of the index correction found at the end of the 

 day, if any, must be entered in the Sounding Record. 



4518. Sextant Calibration 



The error due to eccentricity and inaccurate graduation is a constant for any one 

 angle. These are the errors that are determined by test at the United States Naval 

 Observatory. A sextant for which these values are not available can be tested in the 

 field by comparing angular values determined by triangulation or measured with a 

 theodolite, with sextant measurements. In an area where there are numerous triangu- 

 lation stations the sextant to be calibrated may be placed on a leveled planetable at 

 one of the triangulation stations and the angles between other surrounding triangulation 

 stations carefullv measared. 



