I'm using Skyfield to calculate some ecliptic longitudes for various dates. I understand that the 0° point of ecliptic longitude changes based on precession due to being the intersection between the ecliptic and the celestial equator, but as far as I can tell this should only lead to a rotation of the ecliptic, and shouldn't change the difference between ecliptic longitudes themselves, only offset them equally.
However, what I find when calculating this difference for older dates is that it actually does change, so clearly something is going on that I don't quite understand. The difference is larger the further back or forward in time from today I check. Here are some example values:
date and time: 2000-01-01 00:00:00 UTC
ecliptic longitude of Pluto as seen from Earth, with precession: 251.4423867598416
ecliptic longitude of Juno as seen from Earth, with precession: 278.0447519268405
difference: -26.60236516699888
ecliptic longitude of Pluto as seen from Earth, without precession: 251.4462722291623
ecliptic longitude of Juno as seen from Earth, without precession: 278.0486383062446
difference -26.60236607708228
difference between the differences: -9.100834006403602e-07
date and time: 1800-01-01 00:00:00 UTC
ecliptic longitude of Pluto as seen from Earth, with precession: 331.41515410502547
ecliptic longitude of Juno as seen from Earth, with precession: 310.66231298714155
difference: 20.752841117883918
ecliptic longitude of Pluto as seen from Earth, without precession: 334.21524492213473
ecliptic longitude of Juno as seen from Earth, without precession: 313.4557265008473
difference 20.759518421287453
difference between the differences: 0.0066773034035350065
date and time: 2200-01-01 00:00:00 UTC
ecliptic longitude of Pluto as seen from Earth, with precession: 144.74720464480612
ecliptic longitude of Juno as seen from Earth, with precession: 252.19120370149466
difference: -107.44399905668854
ecliptic longitude of Pluto as seen from Earth, without precession: 141.94565398823505
ecliptic longitude of Juno as seen from Earth, without precession: 249.3919999574794
difference -107.44634596924436
difference between the differences: -0.0023469125558222004
date and time: 1000-01-01 00:00:00 UTC
ecliptic longitude of Pluto as seen from Earth, with precession: 215.88352515432385
ecliptic longitude of Juno as seen from Earth, with precession: 222.06486481268652
difference: -6.181339658362674
ecliptic longitude of Pluto as seen from Earth, without precession: 229.8449049370316
ecliptic longitude of Juno as seen from Earth, without precession: 236.01560254922802
difference -6.17069761219642
difference between the differences: 0.010642046166253749
date and time: 3000-01-01 00:00:00 UTC
ecliptic longitude of Pluto as seen from Earth, with precession: 281.25959800237814
ecliptic longitude of Juno as seen from Earth, with precession: 352.8312616380944
difference: -71.57166363571628
ecliptic longitude of Pluto as seen from Earth, without precession: 267.25800738956053
ecliptic longitude of Juno as seen from Earth, without precession: 338.8213324904173
difference -71.56332510085679
difference between the differences: 0.00833853485949021
date and time: -1000-01-01 00:00:00 UTC
ecliptic longitude of Pluto as seen from Earth, with precession: 130.5063312531234
ecliptic longitude of Juno as seen from Earth, with precession: 323.32094562295737
difference: -192.81461436983398
ecliptic longitude of Pluto as seen from Earth, without precession: 172.2490220604472
ecliptic longitude of Juno as seen from Earth, without precession: 4.995252624931857
difference 167.25376943551535
difference between the differences: 0.06838380534929911
Does anyone have a good explanation for what is causing these differences? As we can see the ecliptic longitudes themselves are quite offset, by more than 10° for the furthest dates (over 40° for year -1000), but from my understanding the differences should in principle remain the same. Skyfield lists in its documentation that it's using the IAU 2000A model to account for precession, and I tried reading a bit about it, but I couldn't find anything that would explain this. I know that there's something called ecliptic precession as well, but from what I could tell this is not something that is being considered (or is it?).
'date'toecliptic_latlon. This changes the celestial equator, and thus the starting point of the ecliptic, but I don't see how the plane of the ecliptic itself would somehow be different in the two cases. $\endgroup$