NavList:

Dear Bill,

> Come on Alex, do I quibble about your 0.0 error averages with 1.0 SD's ;-)
> Of course you have the excuse that your sextant only reads to 1!0.

I did not quibble:-) I just brought to your attention
the fact that SD characterizes the precision of an individual
observation.
In my case, when the sextant reads to 1', the theoretical
limit is .5', and SD=1'  seems OK.
But I would not be completely satisfied with a series taken with
SNO and having SD=0.6', because SNO's theoretical limit
(accuracy of reading the scale) is 0.1'.

In one of your message of yesterday, you again expressed
your worries about almanac's rounding of Sun
SD (semidiameter) to 0.1'. Unfortunately I cannot site that message
literally, because we are on the NavList now, instead
of the good old Nav-L.

So let me explain why I think that this rounding
(and the whole procedure of determining IC from Sun)
seems correct to me.

Sun's SD changes slowly and regularly over the year.
So it is safe to assume that the almanac's value of
Sun's SD is within 0.05' of its real value.
(When you round a number, you introduce an error of 1/2
of the last preserved digit).
Thus the maximal rounding error in 4SD is (4 times 0.05)=0.2
(not 0.4 as you assume!).
Now when you do IC test, you ADD two observations, and
compare the result with the almanac value.
If the dirreference is greater than 0.4'
the test results should be rejected.
(This is what the rule says).

Thus if each of your measurements is within 0.1' of the
true value, the result will never be rejected.
It is true, there is an unlikely situation when you
are within 0.2' of the true value, and the test is
rejected, but such combination is very rare indeed,
and repetition of the test will not hurt anyway.

Alex.


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