<p>The difference between the equinox of J2000 and the equinox of date is precession. You can read about precession, and how to calculate precession of coordinates between epochs, in chapter 20 of the Meeus book you mentioned.</p>
<p>You always need the actual Julian Day number (JD) to calculate a planets position, but the epoch defines which reference directions are used for the coordinates.</p>
<p>If you want to plot an object, such as a planet, on a star chart you would use the chart's epoch, usually J2000. If you want to point your telescope at an object in the sky you would use coordinates for the epoch of date. I have not used any of the VSOP models recently, but I recall they provide ecliptic coordinates in either spherical or rectangular format. They also provide these coordinates in both epoch J2000 and those already precessed to the epoch of observation date.</p>
<p>Precession of the equinox is a bit under 50 arc seconds, or about 0.8 arc minutes per year. So it does not take too many years for positions in the sky to be slightly shifted from the J2000 epoch.</p>

CC BY-SA 4.0
2023-10-03T16:51:27.637Z
[Bretagnon and Francou 1988](https://ui.adsabs.harvard.edu/abs/1988A%26A...202..309B/abstract) §4.3 says they made VSOP87C by applying the [Laskar 1986](https://ui.adsabs.harvard.edu/abs/1986A%26A...157...59L/abstract) precession model to VSOP87A.
Mike G
CC BY-SA 4.0
2023-10-02T21:08:01.397Z
2023-10-02T21:08:01.397Z
amateurAstro
<p>When I read the description of VSOP87, I find out that there is a file with &quot;variables of date&quot; and a file with &quot;variables J2000&quot;. In Astronomical Algorithms of J. Meeus I can find out how to use the variant &quot;Variables J2000&quot;. But I don't know, for what is &quot;variables of date&quot; good.</p>
<p>The description just says &quot;apply for each term the formulae (1) or (2) with T=(JD-2451545)/365250&quot;. So I have always to calculate the JD for both variants.</p>

CC BY-SA 4.0
2023-10-02T23:42:10.813Z
FYI, implementations in most popular languages are available here: https://github.com/gmiller123456/vsop87-multilang
Greg Miller
CC BY-SA 4.0
2023-10-07T17:45:42.673Z
Thanks for Info, but it isn't fun to *use* other program, it's fun to write myselft my program and check it with data of e.g. NASA :)
lazyboy
CC BY-SA 4.0
2023-10-08T02:30:51.240Z
there certainly is value in implementing it yourself.  But test it first with the test data provided by the authors (not NASA).  It's the vsop87.chk file here: https://cdsarc.cds.unistra.fr/viz-bin/cat/VI/81#/browse
Greg Miller
<mathematics>
Meaning of "variables of date" in VSOP87 files
lazyboy
# Meaning of "variables of date" in VSOP87 files

(Asked by: username_0 on 2023-10-02)

When I read the description of VSOP87, I find out that there is a file with "variables of date" and a file with "variables J2000". In Astronomical Algorithms of J. Meeus I can find out how to use the variant "Variables J2000". But I don't know, for what is "variables of date" good.

The description just says "apply for each term the formulae (1) or (2) with T=(JD-2451545)/365250". So I have always to calculate the JD for both variants.

**Tags:** mathematics
> **Comments:**
> **username_1**: FYI, implementations in most popular languages are available here: https://github.com/gmiller123456/vsop87-multilang  
> **username_0**: Thanks for Info, but it isn't fun to *use* other program, it's fun to write myselft my program and check it with data of e.g. NASA :)  
> **username_1**: there certainly is value in implementing it yourself.  But test it first with the test data provided by the authors (not NASA).  It's the vsop87.chk file here: https://cdsarc.cds.unistra.fr/viz-bin/cat/VI/81#/browse  

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## 1 answers

### Answer by username_2 on 2023-10-02. Score: 3 | Accepted answer

The difference between the equinox of J2000 and the equinox of date is precession. You can read about precession, and how to calculate precession of coordinates between epochs, in chapter 20 of the Meeus book you mentioned.

You always need the actual Julian Day number (JD) to calculate a planets position, but the epoch defines which reference directions are used for the coordinates.

If you want to plot an object, such as a planet, on a star chart you would use the chart's epoch, usually J2000. If you want to point your telescope at an object in the sky you would use coordinates for the epoch of date. I have not used any of the VSOP models recently, but I recall they provide ecliptic coordinates in either spherical or rectangular format. They also provide these coordinates in both epoch J2000 and those already precessed to the epoch of observation date.

Precession of the equinox is a bit under 50 arc seconds, or about 0.8 arc minutes per year. So it does not take too many years for positions in the sky to be slightly shifted from the J2000 epoch.

> **Comments:**
> **username_3**: [Bretagnon and Francou 1988](https://ui.adsabs.harvard.edu/abs/1988A%26A...202..309B/abstract) §4.3 says they made VSOP87C by applying the [Laskar 1986](https://ui.adsabs.harvard.edu/abs/1986A%26A...157...59L/abstract) precession model to VSOP87A.  

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