This graph shows the position of an eye as a function of the position of a
spot of light which acts as a target the eye is following. The target starts to move a time zero. Note that at first the eye does not move (there is a reaction time). Then is starts a slow smooth pursuit movement but soon the observer realizes that the target is moving a head of the eye so a corrective saccade is made. After that a smooth pursuit movement is made which follows the spot of light. Note that this entire process only covers an angular distance of about 3 degrees and takes about 1 second.
There are quite a few methods of recording eye movements, some more precise than others. The reader is recommended to the Eye Movements chapters referred to on the previous page.