NavList:

Frank wrote:
"Let's suppose in a typical case we start twenty minutes before noon, take
four Sun altitudes over the course of ten minutes. Then around noon we take
four more sights over the course of ten minutes. Finally starting about ten
minutes after noon, we take four more sights over ten minutes. So that's a
time period of forty minutes. And notice that there's really no significant
distinction among the individual sights. I've grouped them into three sets
just for the explanation. Is "forty minutes" (total) what you mean by a
"long period on either side of noon"? If so, how is that horribly
impractical? After we have the sights, each consisting of an altitude and a
Greenwich time, we correct for the motion of the vessel and changing
declination, which is quite easy and we plot them out on graph paper. Then
we fold our graph paper in half (so that we can see through it if we hold it
up to a light) and we line up the sights so that all of them, before noon,
near noon, and after noon, all lie on one nice half of a parabola. Unfold
the paper and the crease gives the actual time of LAN. Read off the best
estimate of altitude there and process that as a normal noon Sun sight for
latitude [important note: this is the ONLY altitude that has to be corrected
for index error, dip, refraction, and semi-diameter --the others can be used
raw]. Read off the GMT corresponding to the crease, correct that GMT with
the equation of time to get Greenwich Apparent Time, and then find the
difference between 1200 and that GAT and convert to longitude at the rate of
15 degrees per hour of difference (if GAT is earlier than 1200, then the
longitude is east, else west). In typical cases, the accuracy of the
latitude is about five times better than the accuracy (in miles) of the
longitude. So if I can get my latitude to 1 n.m. accuracy and my longitude
to 5 n.m. accuracy without buying, carrying, or learning any real sight
reduction tables, is that impractical?? Of course, some students of
navigation will prefer the more elaborate approaches involving tables and
LOPs. Nothing wrong with their preferences."

 

This I have to try.  Fortunately my "free time" at sea is from 1200-1600 so I have time to shoot and graph.  I know I have some graph paper somewhere in my office...  I will note all data pertaining to ship's course and speed as well as GPS fixes.  We are getting underway next week, so I should have a good opportunity to try it underway if the clouds cooperate.

 

Jeremy

 

Jeremy