A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robert VanderPol II
Date: 2016 Mar 3, 11:27 -0800
I would cover the three big conceptual basics of Celestial that seem to be lost on Newbies:
A) When you measure the angle between the horizon and a body you are also measuring the distance between where you are and the point directly under the body. This was the big conceptual issue I had for 5yr until I worked it out myself. Once I figured that out then celestial was just like regular navigation with a lot of extra steps at various points.
B) The fact that the body is moving is irrelvant, with accurate time and an appropriate almanac you can determine where it is at any given instance. If you were sailing along a featureless coast and there was a train that passed every hour on a schedule accurate to a second or so you could use that train as a source for a line of position if you knew the schedule and had accurate time. The fact that the body is moving just adds steps to figure out where it is.
C) You can't diretly plot your measured range. Any globe big enough to to show pencil lines that are only 1nm wide is going to be too cumbersome to have aboard and flat charts showing enough of the world to draw the circle of position is going to distort the circle and it will be un-plotable. What sight reduction does is it lets you pick a point on a small area chart and calculate how high and in what direction the body should be. By comparing the observed and calc'd heights you can draw a line of position.
The following is the outline I would follow for a 1 hr presentation.
1) Does everybody understand basic nav, can they draw a line of position from a bearing or circle of position from a range.
2) Measuring the altitude of a body is the same as measuring it's range.
3) How to measure altitude. There are some corrections to the measured altitude, everything comes off of a couple, four tables.
4) The body is moving so you have to calc it's position based on the time you measured the altitude. Once again everything comes out of tables. Note that there are several forms of almanac that also trade off size, number of step and mathematical difficulty and the particular bodies that can be used.
-Nautical Almanac, Kolbe almanac, Bowditch Long-term almanac
5) Discuss plotting and the need for sight reduction (spherical trig portion). (Note that some methods don't require plotting but are special cases or are limited in some way.)
6) Discuss the various sight reduction methods, specifically noting that there are tradeoffs in book size (and number of books), number of steps, whether all the steps are addition and subtration or if multiplicaiton and division are involved and final accurace of results.
-HO208, HO211, HO229, HO249, Hav/Dinol, Davies/NASR, Pepperday, Bayless.
7) Using a form organizes all this math and the table lookups, thus greatly reducing the number of mistakes and speeding up the process.
8) Moderate amounts of practice will yield decent results.
9) This isn't rocket science, it's following a step by step recipe for the most part.
10) Other things can also be calc'd using these resources such as great circle courses and distances and direction of sun to check your compass against.