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My apologies to Nigel Gardner, Steven Webster and the list.

I meant to reply to Nigel's question via the list, but - by hitting the reply
button - the message was mistakenly sent to Steven directly. Hence he commented
on a message of mine that nobody on the list had seen. Having been cruising
since July 4th, I did not notice and could not rectify the problem earlier.

Although Steven kindly quoted the whole message without omissions, I post it
here for the sake of good order and hope to answer to the received comment
shortly.

Steven Wepster wrote:

>> the altitudes of the bodies need not be taken very accurate. <<

As a matter of fact, the altitudes don't have to be taken at all, as they
can be computed. However, the problem with this is that actual refraction
might differ from the tabulated values and hence measuring the altitude
might result in higher accuracy. Has anybody ever investigated this?

As to the question of an algorithm itself, this depends on the purpose of
the algorithm. For historical research, often the original methods have to
be emulated and the then available ephemeris be used. For modern use, the
fastest and safest algorithm would be right in the spirit of St. Hilaire:
heuristic and iterative. The distance is a function of time. Starting from
a reasonable "assumed time", compute the corresponding "computed
distance", compare it against the "observed distance", make a correction
and iterate until the difference between computed and observed value is
small enough.

Best regards

Herbert Prinz (from 1368950/-4603950/4182550 ECEF)

   

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