NavList:

Frank,

Are you  or any other Navllist expert, able to help me out with my ealier question regarding the derivation of your version of the Thompson clearing method?

I wrote:

 I have been experimenting with the Thompson lunar clearance  method as described by you:

P1 = HP · sin(Hm) / tan(LD)

P2 = HP · sin(Hs) / sin(LD)

and

P = P1 - P2.

This is the effect of the Moon's parallax

along the lunar arc.

This I have found works perfectly to within a few arc seconds of the full methods, by either calculator or my Fuller 3 slide rule. This is definitely the easiest method for slide rules.(thanks for bringing me to this)

However, I have looked at both Bowditch 1864 and original Thompson  and cannot quite see how you got your formula  from them? (see attached).

Are you able to explain how you got  from that to your formula? ( as usual, apologies if my limited maths make this a simpleton question!).

Many thanks

Francis