GeoClock Tour - Seasons

GeoClock shows the current actual sunlight and twilight curves. As the seasons change, it highlights some unexpected characteristics. In the summer and winter, the curve shows increased sunlight in one hemisphere, and decreased sunlight in the other. Also, the twilight area has a similar shape but is displaced to the south or north. However, during the spring and fall equinoces, when the sun is very near the equator, note that both the north and the south poles are in sunlight. This is because the sun appears to just break the horizon when in fact the center is almost one degree BELOW then horizon. The non-zero diameter of the sun, and the refraction of sunlight by the atmosphere, are responsible for this effect. GeoClock takes great care to model these effects accurately.