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George, you are not the only one noticing those different conventions
in using coordinates.

> George Huxtable wrote:
>  > Additionally (and this is a separate matter) Andres' program appears to
>  > treat all longitudes as Easterly, so that a Westerly longitude is
> shown as
>  > negative. To me, this seems somewhat perverse, though I am aware that
> many
>  > programs (particularly for astronomy) do so. We navigators work in hour
>  > angles (GHA and LHA) for our positions of bodies in the sky, and
> those hour
>  > angles are always measured Westwards, so that they always increase with
>  > time. An hour angle is nothing else but the longitude of the body,
> measured
>  > Westerly from Greenwich, 0 to 360. Why don't we measure our geographical
>  > longitudes exactly the same way, so that we simply difference the
> longitudes
>  > to get local hour angle? Meeus is an astronomer who sets us a sensible
>  > example. It seems madness to measure hour-angles as positive
> Westwards, and
>  > longitudes as positive Eastwards. Can anyone really justify it?

On Thu, Jun 12, 2008 at 12:00 AM, Paul Hirose  wrote:
> I'll try. First off, "positive west" is odd enough nowadays that Meeus
> felt obliged to defend it in a sidebar. Admitting it was contrary to the
> IAU convention, he growled, "We shall *not* follow this IAU resolution."
> (Astronomical Algorithms, 1991)
>
> And if you want to do things the Meeus way, note that he measures
> azimuth from the south!

Yes, Meeus, and still also other textbooks (at least those which I
used when setting up my own program for calculating astronomical
positions) use longitude Westerly and Azimuth from South. This made me
believe that it would correspond to a particular astronomical
convention. I was surprised when being told later that azimuth South
would not be an astronomical but rather a surveyor's convention.
Paul's comment "is odd enough nowadays" indicates that the astronomers
apparently dropped some time ago their hour angle oriented convention
in order to replace it against the "normal" geographer's(?)
convention.

Marcel

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