Why is there a difference between the visible sun rise/set timing and the one given in the timetable on some days?
There is always a difference between visible and actual (Mathematical) Sun rise/set timings. There are certain factors, which cause such a difference, of which the main factor is REFRACTION of suns rays due to atmospheric belt around earth. Refraction in its turn depends upon temperature, moisture and density of atmosphere at a point of visible horizon. On land the difference between actual (mathematical) Sun rise/set time and visible Sun rise/set time may have some consistency as the curvature of earth and apparent horizon remains on the same point. Only density of atmosphere and temperature may change. But this is not the case when we are observing the Sun rise/set on horizon on sea. Sun rise/set point on horizon on sea is fluctuating due to low and high tides and curvature of sea. Due to these factors, difference between actual and visible time may vary. Another factor is the different concepts of sun rise/set. Different people follow different concepts e.g. Upper rim of the Sun dis