What is the cause of advance sunrise and delay sunset?

advanced sunrise and sunset is due to the phenomenon of atmospheric refraction. we see the sunrise two min. before the actual sunrise. at that time the sun is just below the horizon. so , the rays from below the horizon come towards our eyes in a bent form but as you know our eyes see the image in a straight line.. therefore, the bent rays seem to be coming from a straight line above the horizon. same is the case for late sunset. hope its clear to you a bit!! enjoy