How are interstellar distances measured? For measuring enormous distances, astronomers are aided by the method of triangulation, widely used in ordinary Earth geodesy.
Choose a segment of known length, the base, measure from its ends the angles under which the inaccessible point can be seen, and then simple trigonometric formulas give the distance we are looking for. When we move from one end of the base to the other, the apparent direction to the point changes, it shifts against distant objects.
This is called parallactic displacement, or parallax. It is smaller the farther away the object is, and larger the longer the base.
