Quick snippet to use astropy to convert from heliocentric times to barycentric and vice-versa

helio_bary.py

from astropy.coordinates import SkyCoord, EarthLocation
from astropy import units as u
from astropy.time import Time


def helio_to_bary(coords, hjd, obs_name):
    helio = Time(hjd, scale='utc', format='jd')
    obs = EarthLocation.of_site(obs_name)
    star = SkyCoord(coords, unit=(u.hour, u.deg)) 
    ltt = helio.light_travel_time(star, 'heliocentric', location=obs)
    guess = helio - ltt
    # if we assume guess is correct - how far is heliocentric time away from true value?
    delta = (guess + guess.light_travel_time(star, 'heliocentric', obs)).jd  - helio.jd
    # apply this correction
    guess -= delta * u.d

    ltt = guess.light_travel_time(star, 'barycentric', obs)
    return guess.tdb + ltt


def bary_to_helio(coords, bjd, obs_name):
    bary = Time(bjd, scale='tdb', format='jd')
    obs = EarthLocation.of_site(obs_name)
    star = SkyCoord(coords, unit=(u.hour, u.deg))
    ltt = bary.light_travel_time(star, 'barycentric', location=obs) 
    guess = bary - ltt
    delta = (guess + guess.light_travel_time(star, 'barycentric', obs)).jd  - bary.jd
    guess -= delta * u.d
    
    ltt = guess.light_travel_time(star, 'heliocentric', obs)
    return guess.utc + ltt

Thanks Stuart, I like the idea of checking the guess and correcting it slightly. We don’t have to run this process too many times in the code, only in setup in fact, so I might put this in for completeness. Thanks for your help, it’s much appreciated. Ian