As the earth revolves around the sun, the angle the earth presents to the sun changes, giving each hemisphere longer or shorter days, depending which way the earth is tilted at the time, and helping to cause the seasons.
Seismic Monitor shows the dividing line between night and day (also known as the Circle of Illumination) for the exact day and hour each map is made. Because the world must be made flat in order to make an image of it (called a projection ), this day/night line no longer looks circular but is instead distorted.
Since the earth on our map is fixed, what we see is the shadow moving from north to south as the seasons change. But it is actually the earth that is tilting, not the shadow moving. Viewed from the Sun only the day circle is ever seen, and it is a perfect circle, like the illuminated side of an orange.
To help envision the day/night lines at various times of the year, here are four animations, one for each Season. Each is an animation of one day only. The yellow and red orb which moves across the world is showing the Sun's position at that time of day. Notice how the shape of the day/night line changes between Summer and Winter.
March 15th click here for animated version
June 15th click here for animated version
September 15th click here for animated version
December 15th click here for animated version