Hi Marcello To find when the sun is in opposition to a star, you must work with the Righty Ascension. All our time measurements relate to the rotation of the earth about the equator. The equation for opposition is RA_sun = RA_star -12 hrs but RA_sun = GMST - UTC - EoT +12 (all in hours) thus GMST = RA_Star + UTC + EoT and GMST = 6.6973374558 (Sun's HA at Greenwich at Epoch 2000) + 24 / 365.242191 * D_0:00 (days from Epoch 2000 to UTC midnight) + 366.24219 / 365.242191 * UTC_Hrs_since_midnight + minor precessional constant
(Epoch 2000 is noon UTC on 1 Jan 2000, EoT is the strict astronomical definition = minus that commonly used by gnomonists) With a bit of graph plotting and iteration, you can solve these two equations to get the moment of opposition in terms of Date & UTC If you want all details of all the basic theory, see http://www.precisedirections.co.uk/Sundials/ There is a document there called "Basic Solar Positional Astronomy" Best wishes Kevin On 23 Sep 2014, at 14:51, Marcelo <mmanil...@gmail.com> wrote: > Hi fellows! > Could you please solve this little doubt of mine: I’m intending to > make a cylindrical “bottle” sundial, which would show the acronychal > rising of some stars. But, how can I calculate the nearest day when > the perfect opposition between the sun and a given star occurs? I > mean, what’s the best way to define it? When there are 12 sidereal > hours of difference between their right ascensions, or when their > ecliptic longitudes have a difference of 180°? I’ve tried both > methods, and found that they can produce a difference as great as 6 > days between results. > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial >
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