ARTIFICIAL HORIZON MATH

[pabrides]

It's probably an old discussion, but I still dont have this clear in my head

Of course we all know that using an artificial horizon doubles the actual elevation of a body which at some point requires division by two.

The worksheets show that when taking a shot with an artificial horizon one must add or subtract index error to hs before dividing.

Im sorry but this needs explaining.  Because index error is like a constant it does not change with elevation so why add it to hs before an AH division?  The index error is then halved.   Seems to me all corrections to hs should be made after division, that means altitude correction and index error.

Stupid me just doesnt see how index error can double just because the height is doubled while using an AH.

Lets just say your index error is 20 degrees on the arc and the AH elevation is 60 degrees. So according to the work sheet you subtract 20 from 60 then divide by 2 which equals 20 degrees elevation. Dividing first then subtracting index error equals 10 degrees elevation. You can see there is a difference although index error is not likely to be this bad.

I dont currently have any ie in my sextant so it's difficult to experiment. Someone with two sextants, one way off, might have an opportunity to check out the math with observations.

Cheers

[Lipr]

Pabrides

When using an AH divide the Index Error by 2 and subtract or add (depending upon the IE sign) it to the figure you obtain after dividing Hs by 2.

Try using a spreadsheet like this one;
https://thenauticalalmanac.com/Sunsight.xls

The figures you enter for an example don't have to be realistic but it'll show you that when using an AH the IE gets halved.

Larry

[pabrides]

Hello Larry

I just got it... I was making some wrong assumptions within the example about the absolute angle between objects when there is index error.

I worked it out graphically and came to the realization that if ic is added to hs then THAT number represents twice the altitude of the body.

So now I fully comprehend the math - add index error to AH and divide by 2.... or half each and add together. Works the same either way.

This whole exercise was well worth the effort. It's nice to be able to bounce ideas off like minded folks.

Thanks
Cheers.

[c_davidson]

Now you fellas have me confused.

To account for IC when using an AH what are we supposed to do?

Thanks,

Clen

[pabrides]

Now you fellas have me confused.

To account for IC when using an AH what are we supposed to do?

Thanks,

Clen

Lol, Clen,

Apply index correction to AH altitude then divide by 2... There is no dip and altitude correction comes after the division.

I really muddied the waters this time.

Cheers