Observatory, is shown in table 15-4. The SolarEphemeris is issued (on request) each year by majorengineering instrument makers.Find the sun’s declination as follows:1. Accepting the observation as having been madeat the meridian, record the local apparent time as 12“.2. Add the longitudinal equivalent time to obtainGreenwich apparent time (GAT).3. Add or subtract the equation of time (true solartime minus local civil time) from GAT to obtain GMT(Greenwich mean time). The equation of time is givenin the Solar Ephemeris or Nautical Almanac for theinstant of Oh (midnight) daily at Greenwich for the wholeyear.4. Correct the apparent declination for the date forthe elapsed GMT from O“.5. In case the local standard time of the observationis recorded, find the GMT at once by simply adding thetime zone difference. Then, after all the necessarycorrections are made, substitute the value to one of theformulas enumerated above, analyzing carefully to seewhich formula is appropriate.EXAMPLE: Suppose that on 28 May 1985 in theNorthern Hemisphere, you obtained a correctedmeridian altitude (h) of the sun ofat longitude86°08’W.The sun bears south of the observer. Thecomputation to get the corrected declination is asfollows:Local apparent timeLongitudinalequivalent timeGreenwich apparentt i m eEquation of timeGreenwich mean time(GMT)Declination at(table 15-4)NOTE: 23.94”= Difference for 1 hour (table 15-4)Correction for elapsed7’03.7”Corrected declination21°36’56.4”From the computation shown above, you see thatthe declination is positive, so it is a north declination.The transit was pointed south, so this is a case in whichthe body observed was between the zenith and theequator. This is, then, a Case I situation, in which thelatitude equals declination plus zenith distanceor 22°22’54”). Therefore, the latitudeis equal toSUN OBSERVATIONS FOR AZIMUTHSun observations, as compared with starobservations, provide the surveyor with a moreconvenient and economical method for determining anaccurate astronomic azimuth. A sun observation can beeasily incorporated into a regular work schedule. Itrequires little additional field time, and when reasonablecare is exercised and proper equipment is used, anaccuracy within 10 seconds can be obtained.Two methods are used for determining an azimuthby observing the sun: the altitude method and thehour angle method. The basic difference betweenthese two is that the altitude method requires anaccurate vertical angle and approximate time,whereas the hour angle requires a very accurate timebut no vertical angle.In the past, the altitude method has been morepopular primarily because of the difficulty ofobtaining accurate time in the field. The develop-ment of time signals and accurate timepieces,particularly digital watches with split time featuresand time modules for calculators, has eliminatedthis obstacle to the extent that the hour anglemethod is now preferred. The hour angle method ismore accurate, faster, requires less training forproficiency, has fewer restrictions on time of dayand geographic location, has more versatility(total-station instruments may be used), and isapplicable to Polaris and other stars.To apply the hour angle method, you measurethe horizontal angle from a line to the sun. Then,knowing the accurate time of observation and yourposition (latitude and longitude), you can computethe azimuth of the sun. This azimuth is thencombined with the horizontal angle to yield theazimuth of the line.To compute the azimuth of the sun, use thefollowing equation:15-17
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