NavList:

Greg,

 

On merchant ships, az calculations are the most common, and really the only, celnav sight still used.  You need not point the ship at the body at all, you just take a bearing as you would a terrestrial sight.  You then compare the bearing to the computed Zn and you have your compass error.  We use gyro compasses and repeaters so we mark the ships gyro and magnetic headings at the same time and then compute declination.  I will post one of these problems late next week when we head out to sea and I shoot a few of them.  We typically shoot Polaris and the sun, although other stars are sometimes shot as well.

 

You can certainly take an amplitude, but this method now takes longer then a computer reduced Az.  Bowditch has 2 amplitude tables.  The primary calculation is actually easier on a calculator as it requires no interpolation.  The other table is used if the amplitude is shot with the center of the sun on the visible horizon rather then the celestial horizon (The sun is on the Celestial horizon when the LL is 2/3 of the sun's diameter above the visible horizon is the rule I learned).  If conditions permit, I will also shoot and post an exercise of a sum amplitude shot by both methods.

 

As far as timing, you actually want to shoot an Az when the body is closer to the horizon when the Zn isn't changing as rapidly in order to get a more accurate bearing.  Near transit, the body is changing directions very rapidly and even a 0.5 degree error can occur quickly. I typically like to shoot Az shots around 8-9 AM or 3-4 PM.

 

Our gyro repeaters have Az circles which use a mirror and a slit for sun sights and are accurate to about 1/2 of a degree or so.  For stars and amplitudes I use a Alidade (sp?) which is essentially a telescope with some shades that has a sight line and a mirror to see the repeater's compass card.  The latter is a bit more accurate and I can get a reading accurate to about a 1/4 of a degree (the circles themselves are marked in 1 deg incriments.

 

On a yacht, you can use a hand bearing circle or maybe it is easier to just point the bow at the body.  I am not really sure on this point.

 

Jeremy

 

I was looking at the ASA Celestial Navigation Standard (107) the other
day, and noticed that one of their requirements is "Calculate the true
bearing of a low altitude celestial body in order to determine the
error and deviation of the compass".

http://www.asa.com/asa_standards/standard_celestial_navigation.html

Back in the dark ages when I was first learning celnav, the textbook
method for doing compass checks at sea was with amplitude tables and
the sun low on the horizon (at least if I remember right).

I haven't seen amplitude tables in years (does Reed's still have them?)
- but it dawned on me that when you do sight reductions you also have a
true bearing for the celestial object (Zn), so why not just momentarily
point the bow in that direction and note what the compass reads?

Even easier, if you're doing a LAN shot the sun will be either directly
north or directly south of the boat for the time that it "hangs" in the
sky, so that really simplifies the calculations.

Can anyone think of a reason why that wouldn't work as well as using
amplitude tables, or is my info on that method several years out of
date and nobody does it that way any longer?

--
GregR
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