Parallax is an everyday phenomena. It is the apparent change
in the position of a nearby object relative to a distant object
when the observer moves to a new position.
Try viewing your finger at arms length with only one eye.
Switch eyes and see how the position of your finger appears to change
with respect to the distant background.
The Earth moves in a circle 300 million kilometers in diameter
as it orbits the Sun.
One would expect the positions of the nearby stars
to change with respect to the distant ones if viewed 6 months
apart.
Such motion is observed, but it is miniscule.
The more distant an object, the smaller its parallax
motion.
The tiny parallax observed for the stars attests to their
tremendous distance from our Solar System.
Ancient Greeks used the absence of observed parallax to argue that the stars are
all at a fixed (close) distance to the Earth.
The angular displacement of the nearest stars due to parallax
is less than 1 arcsecond.
One arcsecond ( 1" ) is the apparent diameter
of a dime viewed from a distance of one mile.
By definition, a star which exhibits a parallax motion of 1
arcsecond is 1 parsec away
1 Parsec = 3.26 Light Years
The nearest star (Proxima Centauri) has a parallax of 3/4" .
Hipparcos measured accurate parallaxes of stars within 200 parsecs (one million stars).
