Is ECI a pseudo-inertial frame?

Question

The Python poliastro package documentation states that, when propagating from position and velocity vectors, the orbit's reference frame will be "one pseudo-inertial frame around the attractor," which in the case of earth-orbiting objects, means some form of ECI. However, I want to make sure I am matching up coordinates because I am using another bit of code to convert a position on Earth to ECI using the IERS.

I have already checked answers to a similar post and found the clarity to be lacking. I want a specific answer in this case from someone who has used poliastro in the following form:

from astropy import units
from poliastro.bodies import Earth
from poliastro.twobody import Orbit
from poliastro.twobody.propagation import kepler

ephemerisPosition = (ephemerisPosition * units.earthRad).to(units.meter)
ephemerisVelocity = (ephemerisVelocity * units.earthRad / units.day).to(units.meter / units/second)
seconds = ephememerisStart + secondsOffset

for i, time in enumerate(seconds):
    ss = Orbit.from_vectors(Earth, ephemerisPosition, ephemerisVelocity).propagate(time * units.second, method=kepler)

Answer 1

First things first: Software questions are off-topic at this site.

So I'll focus my answer from the perspective of the title question, with a hint toward poliastro.

Is ECI a pseudo-inertial frame?

This question is being asked from a Newtonian perspective. (Poliastro is fully Newtonian.) The answer is of course it is a pseudo-inertial frame. The Earth orbits the Sun. QED. Or, as someone challenged me forty years ago: "There is no such a thing as a Newtonian inertial frame of reference. Name one if you think otherwise."

Using a coordinate system with the origin at the center of a solar system body is inherently non-inertial. The central body is accelerating toward all the other bodies in the solar system. This acceleration is why any frame of reference whose axes are aligned with the ICRF axes but whose origin is at the center of a solar system body is pseudo inertial frame rather than an inertial frame.

Poliastro even provides a function, poliastro.core.perturbations.third_body, that very explicitly takes into account that a non-rotating body-centered frame is by definition a pseudo-inertial frame of reference. What solar system dynamicists and spacecraft engineers call the "third body effect" explicitly captures the non-inertial nature of a body-centered frame of reference.