Underway - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 129

Some Fine-Tuning
As recreational practitioners of celestial navigation, we could stop here and already have a
robust and pragmatic method for producing reliable fixes. But while we're at it, we may as
well push the idea a little further.
One thing to keep in mind when applying the above procedure is that our position changes
over the course of the sights, effectively producing a running fix over a short distance. In
most cases, we will not be concerned about the small error this introduces. For example, if
our first and our last sights were taken twenty minutes apart at a boat speed of six knots,
the resulting intercept error for a star that was observed 60° forward or aft of the beam
would be 1.7 nautical miles at most - unlikely to bother us on the open ocean. The error
will be less for bodies that lie closer to the beam. Swifter work will reduce this error;
twenty minutes is a generous allowance for completing the two-sight series.
If we need to eliminate this error, we can borrow a page from aviators for whom this is a
more substantial issue. The Sight Reduction Tables for Air Navigation ( Pub. No. 249
Volume 1 [#1] include a handy table to correct a calculated intercept for “Motion of the Ob-
server.” The navigator enters the table with Speed Over Ground (SOG) and the relative
angle that a bearing line to the observed body forms with the ship's Course Over Ground.
Given these two arguments, the table provides the correction to be applied to our calculated
intercept per minute of elapsed time. At fifty to nine-hundred knots, the SOG for which
these corrections are tabulated are more representative of aircraft than of cruising sailboats.
We can, however, simply divide the SOG values and the resulting corrections by ten or one
hundred to match our earthbound realities (look for “Alternative Table 1” at the end of the
publication).
Some electronic sight reduction calculators (such as the CelNav package detailed in Celes-
tial Navigation: Useful Tools , below) will automatically correct for this error. We also find
this correction convenient to advance or retard our celestial fixes to fall on the half hour,
making them easier to reconcile with the ship's dead reckoning plot.
Another aspect to keep in mind is systematic error. Possible causes for systematic error in-
clude a misaligned sextant, misjudging the horizon in a big swell, or failing to swing the
sextant correctly to measure the exact shortest distance between the observed body and the
horizon. Picking the median intercept will help reduce random error in our sights but not a