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Peter Wrote
>>> ...An example of how this can be useful is
>>> when you are trying to make an observation at prime vertical - unlike
>>> a meridian passage the body just zooms past this due east or west
>>> point of azimuth. It is the only way I know of being sure to be able
>>> to adopt that precise moment.
>
> Bill inquired:
>> Would you please go into more detail on the above paragraph?
>
> If you can manage to make an observation with an azimuth of precisely
> 090d or 270d then you can establish longitude with that one sight,
> just as latitude can be established with a meridian sight. But how to
> ensure that the sight is recorded at the right moment?
>
> The moment of prime verticality can be calculated for the body
> (usually the sun) but its a bit tricky to make that the moment when
> the altitude is recorded. By making a series of sights over a few
> minutes before and after this moment then plotting the slope the
> appropriate moment can be adopted for sight reduction. As a bonus all
> the other advantages of this slope analysis of the individual sights
> are also made available.

Peter

I am missing something so obvious I am *almost* ashamed to ask.

First, to calculate the precise moment a body will be at 90d or 270d do I
not need to know my latitude exactly?  If I am that sure of my latitude, why
not use a time site to extract longitude directly? Or created an LOP near
90/270.  Again, if I know my latitude, the LOP will give me longitude.

Second, if I understood you correctly am am not clear how the slope is used
to determine the exact moment, or a are you saying with the slope you can
use a point between observations that  corresponds to the calculated moment?

May I impose on you to give me a cook-book version of the method?

Thanks

Bill


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