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On Jun 6, 2006, at 1:44 PM, Greg R. wrote:

> I really didn't have a good answer for him, but said (without using
> the technical language) that I assumed that the az/int data from a
> sight reduction would still work no matter how far off from an
> actual position you started the process from.
>
> But as a practical matter (assuming that we've got a current
> almanac and can get a reasonably accurate time reading), even if we
> had no idea at all about a DR position couldn't we at least narrow
> down the starting point to ? 90 degrees of LHA (or maybe slightly
> more from an elevated position)? Seems like anything beyond that
> could be ruled out as being below the local horizon of the observer
> at that particular location, right?
>
> --
> GregR

I'm not sure the az/int data would be very good from far away, as it
assumes the earth is flat, I believe!

However, if you knew GMT, it would be duck soup to to get LHA within
3-5 hours, depending upon the azimuth of the sun or a star.  A rough
declination of the sun from 9-3 would probably get your latitude to
within 20 degrees.  You nail it with a noon shot.  At night, the
north star gives you your lat to within a degree or so, without an
almanac, or with an almanac, using the Southern Cross or similar in
south latitudes.

Lunars could give you your time within a minute or so.

No problem.

If the first DR position is off due to the round earth, chose another
DR closer in and recompute your position.

Fred

   

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