Figure 15-14.—Sun-observation example field notes.hair becomes tangent to the sun’s image. Averaging theof the line (ZL) computed for each. A general equationdirect and reverse angles results in an angle to the centerof the sun.Since the sun travels on a curved path, averaging theangles introduces a systematic error-the magnitudebeing a function of time between pointings. This errorcan be eliminated by computing an azimuth for eachpointing and averaging the azimuths.An alternate procedure is to take both direct andreverse pointings on the same edge (usually the trailingedge). A correction, dH, is calculated from the sun’ssemidiameter and is applied to the average horizontalangle. The semidiameter of the sun is tabulated in theephemeris. The correction dH (a function of the sun’saltitude), should be computed by using the followingformula (both observations, direct and reverse, shouldbe made within 4 minutes):When you are pointing the left edge (left whenfacing the sun), add dH to an angle right. When pointingthe right edge, subtract the left edge, which is always thetrailing edge at latitudes greater than 23.5°N; the leftedge is always the leading edge at latitudes greater than23.5°S. The number of sets of data varies, depending onaccuracy requirements. For most applications, aminimum of three sets should be taken and an azimuthfor ZL isZL can be normalized to between 0° and 360° byadding or subtracting 360°.After azimuths of the line have been computed, theyare compared and, if found to be within an acceptablelimit, averaged.In working through the following example, refer tothe field notes shown in figure 15-14. These notesillustrate the standard procedure of incrementinghorizontal circles and micrometer settings for adirectional theodolite.EXAMPLE: Determine the true azimuth of a line(line AB) on the ground from a celestial observation.1. Set the transit at station A and train it on B.2. Adjust the horizontal circle at zero and lock thelower motion.3. Train the telescope on the sun and record thetime at the instant the vertical cross hair is aligned onthe edge of the sun.4. Read and record the horizontal angle.5. Invert the telescope and take another reading.6. Repeat Steps 3 and 4 until all the necessary sunshots are completed.7. Proceed with the computation15-20