# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Compass Check**

**From:**Bill Lionheart

**Date:**2016 Mar 21, 22:07 +0000

Dear Bob 1) Correcting a compass with the sun. Its strange that I never learnt this in a navigation class but it is clearly why binnacle compasses have a spike in the middle to cast a shadow. I have an old Sestrel compass with a much bigger and better spike. 2) I had assumed that you just use it to check the compass whenever the sun casts a nice shadow and you happen to note the time and have a chance to calculate the azimuth (eg when you are doing a sun sight). Of course it is now much easier as to calculate azimuth with a computer or phone app. If you are sailing with the sun not shadowed by the sails you can sheet in to increase the angle of heal on the same course and see the healing error too (assuming you can still see the shadow when it heals!) 3) One sunny day with no sails up i decided to check what happens if I motor slowly in a circle (well it was a polygon as I used the auto pilot). What happens is of course that the shadow of the sun stays in the same place on the compass card (at least when it catches up). In practice with some deviation it moves depending on the heading. If you calculate the azimuth of the sun and know the local variation you can correct the compass from this, just making a table. 4) When you make a deviation table the deviation should be represented as the first 5 terms of a Fourier series D = A0 + A1 Cos T + B1 Sin T + A2 Cos 2T + B2 Sin 2T where T is the magnetic ships heading. If you do it quick enough that the change in azimuth is too small to measure (you can only read my steering compass to 5 degrees anyway) then this only affects A0 which is effectively the lubber line error, that the compass itself is not aligned with the ship's head. The other components are associated with soft iron A2 and B2 and permanent magnets A1 and B1. I am not sure where to suggest one reads about this (Fourier series stuff) but I read it in a very old Admiralty Manual of Navigation in our department library! I think perhaps it was first described by Lord Kelvin. I am aboard my boat at the moment so its not easy to check sources! I think in old navigation books the coefficients are labelled A to E. As I understand it a compass corrector removes the A1 B1 error by putting small magnets in the base of the compass and the A2 and B2 with the big soft iron balls you see on big steel ships. [sorry this is all from memory]. Hope that helps or is off interest Bill On 21 March 2016 at 21:23, Bob Goethewrote: >>>Even if you don't know the azimuth you have the Fourier coefficients for >>> compass correction except the constant term. Much easier than transits and >>> pelorus.<< > > Bill, I am unfamiliar with this. I have always been into transits and > peloruses. You intrigue me. Could you unpack what you are talking about > here just a bit? Or if there is a discussion of this elsewhere in NavList, > just mention that and I will dig a bit deeper. But sometimes searching for > something - which may or may not exist - can be like trying to look up a > word in the dictionary when you don't know how it is spelled. > > Thanks, > > Bob > > View and reply to this message -- Professor of Applied Mathematics http://www.maths.manchester.ac.uk/bl