Timing Celestial Observations, Continued
The C-W computation is watch time (WT) to the half-second subtracted
from chronometer time (CT). If WT is greater, 12 hours must be added
to CT. The C-W is never greater than 12 hours because both watch and
chronometer are graduated only to 12. Now that you know the value of
C-W, it is necessary only to add this value to the WT of any observation
to find the correct CT, then apply chronometer error (CE), and you have
the GMT (UTC) of the observation.
To work an example, assume that you have a chronometer whose error
(CE) is -7m 4s; in other words, it is 7m 4s behind GMT (UTC). Your
watch is set to ZT and reads 5h 26m 42s when the chronometer reads 10h
19m 00s. First, find the C-W. Its WT subtracted from CT.
You step out on the bridge with our sextant and watch, and sight on Sirius
at WT 5h 34m 21s, date 15 October, longitude 101°34.2'E. What should
be the GMT (UTC) of this sight? Applying the formula
CT = WT + C-W, we find:
Now, lets consider the date 15 October at 101°34.2'E. Is it the same day
at Greenwich? Lets see. The ZT is 5h 34m 21s. The ZD is -7. Subtract
ZD from ZT to get GMT (UTC). You cant subtract 7 from 5, but 5h on
15 October is the same as 29th on 14 October, and 7 from 29 is 22.
Therefore, 1 Oh 33m 43s is not a.m. on 15 October, but p.m. on 14
October. From this computation, it follows that GMT (UTC) is 22h 33m
43s on 14 October.
In problems like these, you must check the date carefully every time to
avoid a 12-hour error such as the one we encountered just now.