If the parallax angle is too small to measure because the object is so far away, then the surveyors have to increase their distance from each other. The farther away the object is, the less it appears to shift. Since the shifts of the stars are so small, arc seconds are used as the unit of the parallax angle. There are 3,600 arc seconds in just one degree. The ball in the tip of a ballpoint pen viewed from across the length of a football field is about 1 arc second.
Trigonometric parallax is used to measure the distances of the nearby stars.
The stars are so far away that observing a star from opposite sides of the Earth would produce a parallax angle much, much too small to detect. As large a baseline as possible must be used. The largest one that can be easily used is the orbit of the Earth. The parallax angle p is one-half of the total angular shift. A parsec is the distance of a star that has a par allax of one arc sec ond using a baseline of 1 astronomical unit.