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Observatory,  is  shown  in  table  15-4.  The   Solar Ephemeris is issued (on request) each year by major engineering  instrument  makers. Find the sun’s declination as follows: 1. Accepting the observation as having been made at the meridian, record the local apparent time as  12“. 2. Add the longitudinal equivalent time to obtain Greenwich apparent time (GAT). 3. Add or subtract the equation of time (true solar time minus local civil time) from GAT to obtain GMT (Greenwich mean time). The equation of time is given in the Solar Ephemeris  or Nautical Almanac for the instant of Oh (midnight) daily at Greenwich for the whole year. 4. Correct the apparent declination for the date for the elapsed GMT from  O“. 5. In case the local standard time of the observation is recorded, find the GMT at once by simply adding the time  zone  difference.  Then,  after  all  the  necessary corrections are made, substitute the value to one of the formulas enumerated above, analyzing carefully to see which  formula  is  appropriate. EXAMPLE:  Suppose that on 28 May 1985 in the Northern   Hemisphere,   you   obtained   a   corrected meridian  altitude  (h) of the sun of                                 at longitude 86°08’W.  The  sun  bears  south  of  the  observer.  The computation  to  get  the  corrected  declination  is  as follows: Local  apparent  time Longitudinal equivalent time Greenwich   apparent t i m e Equation of time Greenwich mean time (GMT) Declination at (table 15-4) NOTE: 23.94” = Difference for 1 hour (table 15-4) Correction  for  elapsed 7’03.7” Corrected  declination 21°36’56.4” From  the  computation  shown  above,  you  see  that the declination is positive, so it is a north declination. The transit was pointed south, so this is a case in which the body observed was between the zenith and the equator. This is, then, a Case I situation, in which the latitude   equals   declination   plus   zenith   distance or 22°22’54”). Therefore, the latitude is equal to SUN OBSERVATIONS FOR AZIMUTH Sun   observations,   as   compared   with   star observations,   provide   the   surveyor   with   a   more convenient and economical method for determining an accurate  astronomic  azimuth.  A  sun  observation  can  be easily incorporated into a regular work schedule. It requires little additional field time, and when reasonable care is exercised and proper equipment is used, an accuracy within 10 seconds can be obtained. Two methods are used for determining an azimuth by observing the sun: the altitude method and the hour angle method.  The basic difference between these  two  is  that  the  altitude  method  requires  an accurate   vertical   angle   and   approximate   time, whereas  the  hour  angle  requires  a  very  accurate  time but  no  vertical  angle. In the past, the altitude method has been more popular   primarily   because   of   the   difficulty   of obtaining  accurate  time  in  the  field.  The  develop- ment  of  time  signals  and  accurate  timepieces, particularly digital watches with split time features and  time  modules  for  calculators,  has  eliminated this   obstacle   to   the   extent   that   the   hour   angle method is now preferred. The hour angle method is more   accurate,   faster,   requires   less   training   for proficiency,  has  fewer  restrictions  on  time  of  day and   geographic   location,   has   more   versatility (total-station   instruments   may   be   used),   and   is applicable to Polaris and other stars. To  apply  the  hour  angle  method,  you  measure the  horizontal  angle  from  a  line  to  the  sun.  Then, knowing the accurate time of observation and your position  (latitude  and  longitude),  you  can  compute the  azimuth  of  the  sun.  This  azimuth  is  then combined  with  the  horizontal  angle  to  yield  the azimuth of the line. To  compute  the  azimuth  of  the  sun,  use  the following  equation: 15-17

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