
More about Parallax
Astronomers can measure parallax by measuring the position of a nearby
star very carefully with respect to more distant stars behind
it, then measuring those distances again six months later when the Earth
is on the opposite side of its orbit. The shift is tiny... less
than an arcsecond even for the nearest star (an arcsecond is 1/60 of an
arcminute, which is 1/60 of a degree).
The formula for finding the distance to a star, measured in parsecs, using
the star's parallax, measured in arcseconds, is:
