Gianni wrote: > As in almost all Web sites, also you take as positive the Longitudes for > places West of Greenwich. > Despite the opinion of the known astronomer J. Meeus, with which also Davis > agrees in his Sundial Glossary, even if a secular tradition justifies this > definition, it is NOT correct. > The Explanatory Supplement to the Astronomic Almanac (USNO 1992) at page 203 > affirms: > "The geocentric longitude is defined by the angle between the reference (or > zero) meridian and the meridian of point p, measured EASTWARD around the > Earth from 0 to 360 deg (IAU, 1983, p.47) " > > Perhaps it would be opportune, at least in new programs and in new > Web sites , to use this correct definition
Just because there is a definition which professional astronomers have adopted for their purposes, I don't see any reason to assume automatically that it is the correct one for practical sundialling. It seems to me that for our purposes a range -180 to +180 is more convenient than 0 to 360, especially when trying to visualise what's going on. For instance, in the morning the sun has a negative Hour Angle which links conveniently to the idea that the East has less longitude than Here. Another example is that for calculating my standard time, I can just add my longitude number and my time zone meridian to get the adjustment required. That is, I'm at 64.50°W in the Atlantic time zone which is -4. So I divide +64.5 degrees by 15 to get 4.3 hours and add -4, and I know I'm 0.3 hours from my TZ meridian. I'm to the West, which is consistent with positive sense of the number I calculated. If I treat my longitude as 295.5 measured going East, it is a harder sum. I have to multiply my TZ meridian by 15 and subtract that from 360 to get 300, then subtract my longitude of 295.5 to get 4.5 and divide that by 15 to get the 0.3 final result. It is positive which fits with positive now being to the East, so it is still consistent and the final result is the same, but it more work mentally and thus I suspect more error prone for those of us who are still learning or who don't work routinely with these concepts. I'd like to know what other people think about the right convention to use. Steve ---- Want to know who's going to win in your constituency? Try my UK Tactical Voting Wizard at http://users.eastlink.ca/~srgl/election2001.htm
