Dear Wee-Meng You raise some very interesting points...
> When I read about longitudes/latitudes in GPS articles, > there are loads of different types of projection used. Indeed there are. These all represent different models of the shape of the Earth and the best one is kept secret by the U.S. Military who probably know the shape of the Earth better than anyone else! > In my GPS, if a point is specified using the wrong > projection, it may be way off. Sadly this is true. We in the U.K. like to think that Longitude 0 degrees has been fixed since 1884 by the position of Airy's Transit Circle telescope at Greenwich. For some purposes this is still true but certainly not for all purposes. Even the much-acclaimed British Ordnance Survey Maps use a different Longitude 0 (for the simple reason that the Ordnance Survey started long before 1884 and they haven't wanted to change!). The WGS84 model was established by the U.S. who used a secondary longitude (probably one in Washington) as a reference during the refining of the model. When they finished, it turned out that 0 degrees on the WGS84 model is about 6 arc-seconds to the EAST of the longitude of the Airy Transit Circle. I am writing all this from memory so someone else may correct this figure of 6 arc-seconds. If my memory is right this translates into about 0.4 seconds of time or (very roughly) 100 metres at the latitude of Greenwich. I don't think diallists should worry too much about this error. UTC is allowed to differ from UT1 by over twice this amount and I expect most diallists use UTC for checking sundials without correcting to UT1. [A few serious pedants like me make this correction!] To those readers who are familiar with the difference between UT1 and UTC I should like to have it confirmed that UT1 STILL uses the Airy Transit Circle as defining reference longitude 0 degrees. In short: is it still the case that 12h UT1 is the instant of superior transit of the mean sun at Airy's Transit Circle? I am fairly sure the answer is yes but I would be happier if some expert could confirm this. Frank King Cambridge, U.K. -
