"Parallaxes are measured relative to background stars. If these are not infinitely distant
themselves, then the parallax to the foreground object will be underestimated
and its distance will be overestimated.
Calculate the distance that will be measured to a star at a true distance of 40 pc if
the background stars are at a distance of 400 pc and this effect is not allowed for."
I looked through my book and even the lecture slides. It doesn't explain what true distance is...
or am I missing something really key here?