The program initially appears iconic, showing a small map in a 126 by 63 pixel double-sized icon with the local date and time at the bottom. If opened, a 640 by 320 pixel map is displayed which shows both the local and Universal (GMT) time to the second. While the window is open, pressing the right mouse button displays a menu which lets you step the clock from the present time forward or backward in increments of one hour, day, week, month, or year. If you select ``Animate'' from this menu, the program steps continuously every half second. Try selecting ``Week'' then ``Animate'' to watch the seasons pass. Select ``Year'' and ``Animate'' and you can see the (ever so subtle) precession of the equinoxes. Selecting ``Real time'' restores the current time display.
Astronomical Formulae for Calculators by Jean Meeus, Third Edition, Richmond: Willmann-Bell, 1985.
and Mercator projects the illuminated area onto map image. The Sun's position is calculated to better than one arc-second in accuracy.
The illuminated area shown is the area which would be sunlit if the Earth had no atmosphere. The actual illuminated area is larger because of atmospheric refraction and twilight.
Only works for dates between 1970 and 2034 due to limited range of Unix time representation.