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Re: longitude positive west?
From: Marcel Tschudin
Date: 2008 Jun 12, 08:27 +0300
From: Marcel Tschudin
Date: 2008 Jun 12, 08:27 +0300
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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---