Possible issue with Moon positions

@mkbrewer, @dhomeier - I’m moving our discussion in #11753 to a new issue, since I convinced myself that at least it has nothing to do specifically with the use of moon98. But there does seem something weird with the Moon. Somewhat similarly to @dhomeier’s worry, I had just been checking what happens if I adjust the time a bit, and I find the match is significantly better for a time offset of 0.65 s. It is independent of whether I use the new moon98 or our own implementation, and does not seem to depend on time span I try to cover. Note that there is no such offset for body='earth'.

This is my test programme:

import numpy as np
from astropy.time import Time
from astropy import units as u
from astropy.coordinates import get_body_barycentric_posvel, get_body_barycentric

t = Time('1990-03-20T12:30:00') + np.linspace(0, 20*u.yr, 800)
time_diffs = np.linspace(-1*u.s, 1.*u.s, 21)
body = 'moon'
dp, dv = get_body_barycentric_posvel(body, t, ephemeris='de432s')
bp = get_body_barycentric(body, t, ephemeris='builtin')
for time_diff in time_diffs:
    pos_diff = (bp + time_diff * dv - dp).norm().to(u.km)
    print(f'Δt={time_diff:5.2f}, rms={np.sqrt((pos_diff**2).mean()):.2f}')

(Note I use the velocity from JPL so I can also check current master - the velocities are close enough it does not matter much.)

Output

Δt=-1.00 s, rms=53.25 km
Δt=-0.90 s, rms=50.52 km
Δt=-0.80 s, rms=47.82 km
Δt=-0.70 s, rms=45.16 km
Δt=-0.60 s, rms=42.54 km
Δt=-0.50 s, rms=39.96 km
Δt=-0.40 s, rms=37.45 km
Δt=-0.30 s, rms=35.02 km
Δt=-0.20 s, rms=32.67 km
Δt=-0.10 s, rms=30.43 km
Δt= 0.00 s, rms=28.33 km
Δt= 0.10 s, rms=26.41 km
Δt= 0.20 s, rms=24.69 km
Δt= 0.30 s, rms=23.22 km
Δt= 0.40 s, rms=22.07 km
Δt= 0.50 s, rms=21.27 km
Δt= 0.60 s, rms=20.87 km
Δt= 0.70 s, rms=20.90 km
Δt= 0.80 s, rms=21.34 km
Δt= 0.90 s, rms=22.18 km
Δt= 1.00 s, rms=23.37 km

[mkbrewer]

@mhvk I believe that this issue can now be closed.

[mhvk]

Just for completeness in case someone stumbles on this in the future: #11753 now uses moon98 correctly and will thus fix this issue.