Complex sentence: The South Pole sees the sun rise and set only once a year, technically on the September equinox and the March equinox, respectively, but atmospheric refraction means that the sun is above the horizon for some four days longer at each equinox.
Split into simpler sentences: The South Pole sees the sun rise and set only once a year. Due to atmospheric refraction, these do not occur exactly on the September equinox and the March equinox, respectively: the sun is above the horizon for four days longer at each equinox.