Can astronavigation be used during flight?
You sure can! It's not really done all too much any more in the days of GPS but it was done quite a bit in the early days of aviation. Historically, this was done by the "flight navigator" a position that no longer really exists. Some aircraft even had mounted sextants.  



(source)

Here is a video of it being done in practice (fanta optional)

This was even automated on the SR-71's Inertial Astro-Navigation system which used the stars to align itself.