That first one should have read "between 1.8 billion B.C. and 0.8 billion B.C" or "between 1.8 and 0.8 billion B.C."

And the Milky Way's speed (and other speeds of galactic things) are measured relative to the cosmic background radiation, those low energy waves from the Big Bang. I have this reference for just that question, and it's by far the best I've found that explains, at least a little, how scientists come up with these calculations.


https://nightsky.jpl.nasa.gov/docs/HowFast.pdf