As the earth rotates the sun seems to move across the sky. So the shadow of a gnomon (the shadow caster) will move across the hour band of a sundial. By noting the hour line closest to the edge of the shadow – or to centre of the shadow on an armillary sphere – you can read the time.
To work properly the angle of the gnomon and hour lines on a sundial must be calibrated for the latitude in which the sundial is being used. This is why, here at Border Sundials we only make dials to commission for their intended location.