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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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