Lack of agreement with real data

[KDarshan20]

Hello, I'm trying to calculate the position of the Moon with the Earth's center as the origin, using this code:

import spiceypy as spice

spice.furnsh("../meta.txt")
# print distance of Moon for every second in the day  
for i in range(86400):
    date = 2457708.5 + (i / 86400)              # 2457708.5 corresponds to 14 November 2016, 00:00 UTC, the date at which the Moon was at its perigee
    x, y, z = spice.spkpos("Moon", date, "J2000", "NONE", "Earth")[0][0:3]
    print("distance:", ((x ** 2 + y ** 2 + z ** 2) ** 0.5))

meta.txt:

\begindata
KERNELS_TO_LOAD=(
'<path>/de438.bsp')
\begintext

But all the values I get are around 403,087.56 km, while Moon's perigee is around 363,228.9 km. Where am I wrong?

[jdiazdelrio]

Hi @KDarshan20,

the date (ephemeris time) shall be expressed as seconds past J2000 TDB (see https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/spkpos.html).

In your code, your are computing times starting at 2000-01-29 22:41:48.500000 UTC, for fractions of a second.

You'll need to convert the Julian Date (or UTC calendar time) you have to seconds past J2000 TDB (use str2et for that).

Hope this helps. Jorge

[KDarshan20]

That cleared it. Thanks for the quick reply!