To measure a length, the positions of both ends are read at the same time. Length contraction is a statement about values obtained this way.
What the eye takes in is light that left both ends and arrives at the eye together. Light that arrives together did not leave together.
Light from the end farther from the eye travels a longer way, so it must leave earlier than light from the nearer end to arrive at the same time. The shape the eye sees is put together from light that left at different times.
Place a cube at rest in a drawing seen from directly above. The eye is toward the bottom of the drawing, far away compared with the size of the cube.
The path from the top left corner to the eye is longer than the path from the bottom right corner by the width of the cube. For the two beams to arrive together, the light from the top left corner must leave earlier.
While the cube is at rest, early and late light both come from the cube in the same place, so this has no effect on the shape that is seen.
Now move the cube to the right at speed . In the time between the light leaving the top left corner and the light leaving the bottom right corner, the cube moves to the right.
These two beams are what reach the eye, so the outline seen combines the cube at two positions. The left face, on the side opposite to the motion, comes into view from the side as a strip of width .
At rest, the left face is never visible to the eye directly below. This strip follows from nothing but the finite speed of light; no relativity has been used so far.
Now add length contraction. The front face, turned toward the eye, has both ends at equal distance from the eye and is seen by light that left at one time, so it appears with its measured, contracted width .
The outline that reaches the eye is the front face of width beside the left face of width . Take the angle with and , and these two widths are exactly those of an uncontracted cube turned by .
The cube the eye sees is not flattened but turned. This is called Terrell rotation, shown independently by Terrell and by Penrose in .
The cube in the figure moves at , and is about degrees. At it looks turned by about degrees.
The change in the direction light arrives from affects the whole surrounding view. Look at the sky while moving at speed and the stars shift toward the direction of travel.
A star seen at angle from the direction of motion while at rest is seen at the angle given by . At , a star that was directly to the side moves to within degrees of straight ahead.
This shift in direction is called aberration. It happens with the Earth's orbital motion () too, where the directions of the stars shift by up to about arcseconds.
The measured shape is set by length contraction; the shape the eye sees also depends on the different times light takes to reach the eye. When a moving body is said to contract, it is the measured shape that is meant.