The sunrise is advanced or delayed due to atmospheric refraction of sunlight. An observer on the earth sees the sun two minutes before the sun reaches the horizon because a ray of sunlight entering the earth's atmosphere follows a curved path due to atmospheric refraction before reaching the earth. The density of atmosphere varies with altitude which affects the refractive index of the atmosphere. The light from the sun enters rarer to denser medium and hence bends towards the normal. Same is the case with delayed sunset.