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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?

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!

With the "positive east" convention it's easier (for me) to remember how
to compute local hour angle: "add everything". That is, LHA = GHA body +
lon and LHA = GHA Aries + SHA star + lon.

In addition, converting from spherical coordinates to rectangular or
vice versa is easiest if the longitude angle increases eastward.

The Astronomical Almanac's table of major observatories gives their east
longitudes, increasing to 360. A few years ago that caused trouble when
I tried to use those values in an online calculator. It didn't allow
longitude greater than 180, so I had to fix the coordinates by hand.
Grrr. The USNO's MICA program is also guilty of this over-zealous input
validation.

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