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I have a bit more to add on this topic, which I had triggered by writing in 
5399
"it seems madness to measure hour-angles as positive Westwards, and 
longitudes as positive Eastwards. Can anyone really justify it?"

to which Herbert Prinz relied-
"The fact that now we all measure longitudes the same way is probably the 
best possible justification. Speculating about the philosophical reason is 
likely to be counter productive: It may weaken the consensus. Someone might 
come up with the really bad idea to change the convention again in the 
interest of "better science".

Herbert says that "now" we all measure longitudes the same way, but if there 
is any consensus, it's a recent one, dating from a IAU resolution in 1983, 
and which has not been accepted by all; in particular, not by Meeus. Meeus 
provides several references about the matter, chosen, no doubt, to support 
his case, which have sent me to my own bookshelves. The results may be of 
interest. Here's what I've found, in chronomlogical order.

Chauvenet, W, "A manual of Spherical and Practical Astronomy", 1891 (vol 1, 
page 317), defining L, the difference of longitude between two points by 
reference to their local times, states "L is the west longitude of the 
point".

Newcombe, Simon, "A Compendium of Spherical Astronomy", 1906, page 118, 
says- "Astronomical custom is divided as to whether East or West longitudes 
should be considered positive; the West are positive in the American 
Ephemeris".

"Explanatory Supplement to the Astronomical Ephemeris", 1961, refers on 
page7 to a resolution of the International Meridian Conference in 
Washington, 1884, one of its resolutions being- (referring to Greenwich) 
"That from this meridian longitude shall be counted in two directions up to 
180 degrees, East longitude being plus and West longitude minus". However, 
it goes on to say- "Although the other resolutions are now in use, it has 
been customary for many years in astronomy, but not in all other related 
sciences, to treat west longitudes as positive and east longitudes as 
negative. That is the convention adopted in the Ephemeris." And indeed, in 
the ephemeris for 1971 and 1976, positions of observatories are given with 
their longitudes marked as + or -, not W or E, and clearly the 
West-is-positive convention is applied.

However, following that IAU resolution in 1983, the author of the successor 
to that Explanatory Supplement, Siedelmann, (1992) is in no doubt, saying 
that longitude is "...measured eastward around the Earth from 0� to 360� 
...". And indeed, my Astronomical Ephemeris for 1985 gives observatory 
positions which are described, specifically, as "Easterly Longitude", with 
the appropriate sign.

That might be that, you would think, but for the rejection of that change by 
the influential Jean Meeus, who points out, on page 93 of "Astronomical 
Algorithms", that for all other planets, longitudes are measured in the 
opposite direction to their rotation, but for some reason, not so for the 
Earth.

So there is hardly the clear-cut consensus that Herbert would wish for.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.



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