Underway - Pacific Crossing Notes: A Sailor's Guide to the Coconut Milk Run - page 125

Figure 4: Picking the right stars
Step 2 : At twilight, take an uneven number of sights for each of the two bodies in quick
succession. Five sights each work very well. We have found little improvement by taking
seven or more sights.
Step 3 : Use your preferred method for calculating the intercept for each sight (ten inter-
cept values if you took five sights of each of the two bodies). An electronic sight reduc-
tion tool will help do this quickly and easily.
Then, for each body, sort the intercept values and take the sight with the median (middle)
intercept value to plot the LOP. For example, if the sights of one of our two observed bod-
ies yielded a sorted intercept series of:
4.2 away
3.9 away
1.2 away
0.3 toward
2.8 toward
we would take the middle value of 1.2 away and its associated azimuth angle to plot the
LOP for this body.
Step 4 : Plot the two LOPs to determine the fix latitude and longitude (some electronic cal-
culators will do this automatically).
Step 5 : Evaluate your fix against your GPS position, looking for ways to improve your
sights by reducing any systematic error in your measurements (more on that later).
In addition to providing robust estimates of the observed bodies' true altitudes (and thus
high-quality LOPs), this approach also allows us to collect a comprehensive set of sight
data. We can later analyze this data to help us improve the quality of our sights.
Five sights each for two bodies may sound like a lot, but it is actually quite quick and
easy. Since we stick with the same star or planet for each sight series, we don't have to re-
set the sextant and bring the body all the way down to the horizon every time. Nor do we