Transform one Geocentric to multiple earth-locations

Hi,

First thanks for your work, your library is really amazing.

But for a specific application, i try to generate astrometric table and graph in alt/az for earth observers. I know the dates in advance, but not the locations. To gain some time and I/O reads from the Kernel file i try to find a way to pre-compute Geocentric position and use them on demand to transform from Geocentric to a Apparent with the observer coordinate.

I try adding/substrating different positions object with no success. I think i have to Positions class directly, but it’s not clear in my head, and the documentation is not talking about this approach.

How can we use an existing Geocentric Position (Apparent or not) and transform it into multiple Earth locations ? I guess it’s just a Vector transformation, but don’t know how to handle it.

Sorry i don’t have a lot of code to show.

from skyfield.api import load, wgs84

def astrometric_to_apparent_with_topo(astrometric, t, location):
    #don't know how to take this function ...
    pass
    return

if __name__ == "__main__":
    planets = load('data/de440.bsp')
    timescale = load.timescale()
    t0 = timescale.now()

    astrometric_mars = planets["EARTH"].at(t0).observe(planets["MARS BARYCENTER"])

    geographic_toulouse = wgs84.latlon(43.6167, 5.7667)

    print(astrometric_to_apparent_with_topo(astrometric_mars, t0, `geographic_toulouse))

[zaltanar]

You are too reactive for me.

I had my response since the third post, the discussion after it’s just bonuses and some curiosity.

1. Yes, in my initial thought i didn’t take into account for diurnal aberration
2. I didn’t know at first the caching mechanism
3. No, i’m still in api testing, i plan to tests different methods.
4. After some calculations, as the moon travels 550 mas per second in the sky, i can play with this accuracy error.

I get everything i need now, thanks a lot, i learn everything :

1. Skyfield can substracts Positions
2. The api load data in ram
3. Diurnal aberration exists

I close the issue, i as i have my response. I will share tests results later here. But as it’s a side project it may take some days. Thanks again.

[brandon-rhodes]

As shown just above, I’ve committed a fix for vector subtraction that will use more specific vector classes like Geocentric if subtraction indeed results in a vector whose origin is the geocenter. To try the new feature out:

pip install -U https://github.com/skyfielders/python-skyfield/archive/master.zip

As for your original question, is this an accurate summary of the discussion so far?

1. Because each location on Earth (besides the poles) has its own velocity around the Earth’s axis in addition to the velocity of the geocenter itself, the aberration of light will be different; and unless it happens to be at the exact same distance from the target as the geocenter, the light-travel-time will be different and therefore so will the position of the target; and so vector subtraction alone won’t produce a topocentric vector from a geocentric vector.
2. As mentioned above, there is no I/O penalty for computing planet positions once the data has been loaded into RAM; they are one of the fastest things Skyfield does.
3. Which might mean that your application is being slowed down by something else. Have you tried profiling the code to see which Skyfield routine is taking the most time?
4. I assume that a 600 mas error is unacceptable for your requirements, but it is not clear what your requirements are — you might want to review the precision to which coordinates will be given in your output table, so that we can know what the acceptable error is, which would suggest which effects can be ignored while still giving an accurate enough answer.