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Timing Celestial Observations, Continued - 14221_166
Timing Celestial Observations Background The  importance  of  obtaining  the  exact  GMT  (UTC)  of  every  celestial observation  was  mentioned  earlier.  Obviously,  it  would  be  impractical  if every time you took a sight on the bridge wing, you had to dash into the charthouse  and  look  at  the  chronometer.  Every  observation,  consequently, is timed the instant it is made, either by a stopwatch or by a comparing watch. Techniques There  are  several  methods  available  for  timing  observations.  In  this  text we  will  cover  the  preferred  method  only.  The  preferred  method  consists of  one  person  taking  observations  and  another  person  marking  the  exact time  of  the  observation.  The  person  marking  the  time  will  need  to  use  a comparing watch set to GMT from a time tick or set to chronometer time. The  stopwatch  can  be  started  exactly  on  some  convenient  minute  or  hour of  the  chronometer.  If  its  rate  is  known  to  be  small,  there  is  no  necessity for  working  out  any  chronometer  minus  watch  (C-W)  computation, provided  the  interval  during  which  observations  are  taken  is  short.  For  a single  observation,  the  stopwatch  can  be  stopped  (or,  reversing  the procedure,  the  watch  may  be  started)  when  the  sight  is  taken,  but  seldom is  only  one  observation  made.  For  this  reason,  the  stopwatch  must  usually be read like any other watch. A comparing watch can be set to the chronometer time and can be used to keep time if its rate is also small.    Some  navigators,  though,  prefer  to keep  their  watches  on  zone  time;  hence,  observation  time  must  be computed.  It  doesn’t  matter  whether  computation  is  made  before  or  after the  observation.  It  is  essential  to  have  the  interval  as  short  as  possible between time of sight and time of computation.    Otherwise,  enough  time may elapse for the watch to gain or lose a sufficient amount to cause an error.  For  better  accuracy  and  to  avoid  careless  errors,  it’s  a  good  idea  to make C-W computations both before and after a round of sights. 5-19

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