NavList:

Dear Geoffrey,

Clever remarks of yours, Thanks to you.

Nothing being perfect down here on Earth, Tangent screw errors do exist. I have not seen them extensively addressed anywhere, at least under the form of some "correction tables" which would be very much time consuming tasks to complete.

So we are left with assuming that they are negligible while in fact they may not be at all. By character, the overall result of such tangent screw errors may be considered as "random" since these errors are directly linked to the manufacturing (i.e. cutting/grinding) process.

Any different view point here ?

Eccentricity errors do exist and they are measured at the end of the manufacturing process. Once you know the required 2 unknowns which can be "derived" from the discrete Eccentricity Data published for each sextant, you can "mathematically" predict/reconstruct such eccentricity errors over the whole continuous range of observations. Once such 2 initial unknowns" are solved, instead of being "random" such eccentricity errors can thus be considered as having a systematic and fully predictable character over the full range of the observations.

What I am also saying here is that if you need to perform eccentricity corrections the Manufacturer published discrete data are only halfway of what you actually need.

From such discrete data :

(1) - you first need to "linearly interpolate" - which may not always be the best approximation - and:

(2) - you then need to transform you arcsecond result into a tenth of arcminute correction.

Given the high manufacturing laser assisted/guided manufacturing standards achieved to-day, many new sextants show eccentricity errors not exceeding 2 arc seconds, i.e. totally negligible for our day-to-day CelNav requirements.

This also explains why Manufacturers keep not caring - and have they ever even cared in the past ? - about publishing more than the strict minimum, i.e. some discrete values.

For you Doug, if you could publish a picture of the eccentricity correction table of this sextant you have shown us here, that would be great.

From such example, It might then be possible to elaborate this chapter in further and deeper detail.

I know ... that's splitting hair ... but that's fun anyway :-)

Thanks in advance to you Doug.

Antoine M. "Kermit" Couëtte