The parallax distance to an object is the distance you get by moving your (single) eye's position by a transverse distance x, measuring the angular displacement θ of the object given that move, and dividing the transverse distance by the angle.
Problem-set problem: You are outside after the rain late in the day and you see a rainbow. What is the parallax distance to the rainbow?
(Thanks to Andrei Gruzinov at NYU.)
