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(Background: Trying to learn something about the mathematical and computational part of astronomy I have acquired a copy of Meeus' "Astronomical Algorithms" and implemented the material up to Chapter 33 in Java, with only one major sidestep: VSOP87, for which I obtained the full data set and "translated" the FORTRAN program.)

VSOP87 exhibits the reason for my question: you can compute rectangular/spherical heliocentric positions either for the reference frame J2000 or the one of the given date. Clearly, if I want to observe some planets tonight, I'll compute the data for today's equinox. On the other hand, there is the possibility of a transformation between equinoxes of different dates.

So why are these methods (here and for other methods, too) provided for both reference frames? Was it the (then) weaker computer performance?

Only a little later, iauPlan94 provides the result only for J2000 although Meeus in the chapter dedicated to the paper of J.L.Simon e.al. provides tables with data from their theory for J2000 and the equinox of the date.

What would be good reasons for today and the years to come to refer a computation to J2000?

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  • $\begingroup$ Related question: astronomy.stackexchange.com/questions/14901/… $\endgroup$ – Phiteros Apr 30 '17 at 18:19
  • $\begingroup$ Do you mean why we have to include the year in our calculations, or why we use J2000 specifically? $\endgroup$ – Phiteros Apr 30 '17 at 18:20
  • $\begingroup$ It is clear that you have to use a Julian day as the basis for all planetary positions etc. One question is why providers of algorithms provide one where the result is referred to J2000 and the other one to the equinox of the given date, i.e., in two different coordinate systems. $\endgroup$ – laune Apr 30 '17 at 19:18
  • $\begingroup$ Ah, so you're saying that they give the coordinates in J2000 and for something like November 2, 2008 (some arbitrary day)? $\endgroup$ – Phiteros Apr 30 '17 at 19:27
  • $\begingroup$ I think both algorithms produce the coordinates for the position on the given date. But both the ecliptical and the equatorial coordinate systems shift with time, and therefore a coordinate referred to J2000 will not be valid today. $\endgroup$ – laune Apr 30 '17 at 19:53
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Maybe you are interested in watching the occultation of a star by a solar system object. In which case you will need to compare their positions. Catalogues of stellar positions are most commonly given with J2000 positions.

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  • $\begingroup$ So, more generally, any complex scenario where you want to incorporate the "constant background" of the stars is better computed in J2000, rather than chasing the sum of a planet's displacements (orbit, precession) with the displacement (due to precession) of the star. If you think you have found a solution you would have to recalculate everything to date, right? $\endgroup$ – laune May 2 '17 at 7:08
  • $\begingroup$ @laune: You have the idea. Just imagine if every entry in every catalogue were given relative to the equinox at the time of measurement. That would create a lot of extra work for anything involving multiple objects. $\endgroup$ – user25972 May 3 '17 at 17:14

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