[Certified]

I really like all the caveats and the time taken to explain things in the first part of that document, but later it starts to rush and gloss over important details and caveats. On page 151 of that link, when it starts talking about using parallax to measure the distance to nearby stars, it says "However, if one takes measurements six months apart, one gets a distance separation of 2AU." This is obviously incorrect because the whole solar system is orbiting around the galactic core, which itself is moving with respect to the CMB rest frame. I did a quick calc based on the 552.2 km/s galactic velocity value from Milky Way wiki [1] and found that it moves an additional 0.97AU in 6 months. I am assuming that this has been accounted for by scientists, and is being simplified to make it more digestible for the reader, but it hides a rather large dependency for every higher rung on the cosmic distance ladder. A cosmic velocity ladder that seems to be based off of Doppler CMB measurements [2]. If we are indeed using measurements many months apart and under or overestimating our velocity through the universe, even a little bit, every higher rung of the ladder would be affected wouldn't it?

In the process of writing this, I thought "Surely we have launched a satellite pair that can take parallax measurements at similar times in different places!" They could range off of each other with Time of Flight, be positioned much further apart than a few AU, and take parallax star measurements at more or less the same time without atmospheric distortion, but it doesn't seem like we have. Both Hipparcos and Gaia were satellites that were deployed to measure parallax, but not as a pair. My reading suggests they used multi-epoch astrometric observations (speed ladder dependent) to generate their parallax measurements and it seems our current parallax and star catalogues are based on the measurements taken by these two satellites. New Horizons got the most distant parallax measurements by comparing simultaneous* earth observations, but it was limited to Proxima Centauri and Wolf 359, far from a full star catalogue.

I would love if someone more knowledgeable can steer me towards a paper or technique that has been used to mitigate the cosmic distance ladder's dependency on this cosmic speed ladder. Regardless of how certain we think we are of our velocity through the universe, it seems to me that sidestepping that dependency through simultaneous* observations would be worthwhile considering how dependency laden the cosmic distance ladder already is.

[1] https://en.wikipedia.org/wiki/Milky_Way

[2] https://arxiv.org/pdf/astro-ph/9312056

* Insert relativity caveat here for the use of "simultaneous". What I mean in this context is more simultaneous than waiting months between measurements.