What effect causes a moving object to appear shorter in the direction of motion?

The effect that makes a moving object appear shorter in its direction of motion is length contraction.

In special relativity, an object moving relative to an observer is measured as shorter along the axis of motion than it is in its own rest frame. The relationship is described by the Lorentz factor: the faster the object moves, the greater the contraction. At everyday speeds the effect is far too small to notice, but it becomes significant when speeds approach that of light.

Length contraction is not ordinary physical squeezing caused by pressure or friction. It results from how space and time measurements change between inertial frames. Crucially, only the dimension parallel to motion contracts; dimensions perpendicular to the motion do not.

The effect is closely linked to time dilation and the relativity of simultaneity. To measure an object's length, an observer must record the positions of its front and back at the same time in their frame. Because simultaneity is relative, different observers can obtain different lengths without any contradiction.