The terminator is the dividing line between points on the earth that are illuminated by the sun, and points that are not illuminated. That line is drawn as white in the terminator overlay.
The fuzzy shading on each side of the terminator corresponds to the location of the terminator 30 minutes before and after the current time. The current time is shown at the top right corner. The current time is automatically updated every 1 minutes unless the Automatic Terminator checkbox is turned off; then you can enter any time of year into the date picker.
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