The same trick your two eyes use measures the distance to stars
Glance at a car's speedometer from the passenger seat and the needle seems to point at a slightly different number than the driver sees. That shift is parallax, the apparent movement of an object when viewed along two lines of sight. Nearer things shift more, so the size of the shift reveals how far away something is.
Measuring distance this way is really triangulation. Know the length of one side of a triangle and the angles at both ends, and the rest follows. Astronomers make the baseline as long as possible by observing a star from opposite sides of Earth's orbit, six months apart. Even so, the resulting triangle is absurdly thin: the angle at the star always comes out below one arcsecond. The payoff is a wonderfully simple rule. Distance in parsecs equals one divided by the parallax in arcseconds, so Proxima Centauri, with a parallax of 0.7687, lies about 1.3 parsecs, or 4.243 light-years, away. These measurements form the bottom rung of the cosmic distance ladder, the chain of techniques used to gauge ever more remote objects.
Animals exploit the same geometry. Two forward-facing eyes with overlapping views give the brain slightly different pictures, and it merges them into depth perception, a process called stereopsis. Pigeons lack that overlap, so they bob their heads to create shifting viewpoints instead, a strategy known as motion parallax. Computer graphics borrow the idea in wiggle stereoscopy, animations that fake depth by rocking the viewpoint.
Parallax is also a nuisance. Reading a pointer against a scale from an angle gives a wrong value, so some analogue meters print their scale over a thin strip of mirror; line the pointer up so it hides its own reflection and your eye must be looking straight on. Cameras with a separate viewfinder above the lens tend to frame close subjects slightly low, famously cropping the tops of heads. Stitched panoramas can show seams for the same reason.
Turned around, the error becomes a tool. In aerial photography, tall buildings seem to lean away from the centre of each frame, and measuring that lean, with flying height and baseline known, reveals their height. Surveyors, rangefinders and naval fire-control systems all rely on the same angles.
Source: Parallax
