<p>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.</p>

CC BY-SA 3.0
2017-05-02T07:08:06.000Z
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?
laune
CC BY-SA 3.0
2017-05-03T17:14:03.657Z
@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.
user25972
CC BY-SA 3.0
2017-05-01T06:59:46.000Z
2017-05-01T06:59:46.000Z
ProfRob
<p>(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.)</p>

<p>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.</p>

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

<p>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. </p>

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

CC BY-SA 3.0
2017-04-30T18:19:25.497Z
Related question: https://astronomy.stackexchange.com/questions/14901/what-is-the-effect-of-the-axial-precession-on-the-orientation-of-the-planets-or/16280#16280
Phiteros
CC BY-SA 3.0
2017-04-30T18:20:29.263Z
Do you mean why we have to include the year in our calculations, or why we use J2000 specifically?
Phiteros
CC BY-SA 3.0
2017-04-30T19:18:54.027Z
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.
laune
CC BY-SA 3.0
2017-04-30T19:27:47.473Z
Ah, so you're saying that they give the coordinates in J2000 and for something like November 2, 2008 (some arbitrary day)?
Phiteros
CC BY-SA 3.0
2017-04-30T19:53:39.140Z
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.
laune
CC BY-SA 3.0
2017-04-30T19:54:39.557Z
It would help a lot if you actually shared the algorithms and linked to the papers you're referring to.
Phiteros
CC BY-SA 3.0
2017-04-30T20:03:50.960Z
Although I think that the question is independent of particular algorithms: VSOP87 software: imcce.fr/pub/ephem/planets/vsop87/ the paper: http://adsabs.harvard.edu/abs/1988A&A...202..309B  And the other one, basis for iauPlan94 is at http://adsabs.harvard.edu/abs/1994A%26A...282..663S As for Meeus, you'll have to have the book.
laune
<algorithm><frame-of-reference>
What are the reasons for referring to equinox J2000 or the equinox of the date?
laune
# What are the reasons for referring to equinox J2000 or the equinox of the date?

(Asked by: username_0 on 2017-04-30)

(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?

**Tags:** algorithm, frame-of-reference
> **Comments:**
> **username_1**: Related question: https://astronomy.stackexchange.com/questions/14901/what-is-the-effect-of-the-axial-precession-on-the-orientation-of-the-planets-or/16280#16280  
> **username_1**: Do you mean why we have to include the year in our calculations, or why we use J2000 specifically?  
> **username_0**: 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.  
> **username_1**: Ah, so you're saying that they give the coordinates in J2000 and for something like November 2, 2008 (some arbitrary day)?  
> **username_0**: 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.  
> **username_1**: It would help a lot if you actually shared the algorithms and linked to the papers you're referring to.  
> **username_0**: Although I think that the question is independent of particular algorithms: VSOP87 software: imcce.fr/pub/ephem/planets/vsop87/ the paper: http://adsabs.harvard.edu/abs/1988A&A...202..309B  And the other one, basis for iauPlan94 is at http://adsabs.harvard.edu/abs/1994A%26A...282..663S As for Meeus, you'll have to have the book.  

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## 1 answers

### Answer by username_2 on 2017-05-01. Score: 2 | Accepted answer

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.

> **Comments:**
> **username_0**: 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?  
> **username_3**: @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.  

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