Forked from Mike Bostock's Solar Terminator gist. Changed map projection to Waterman's Butterfly.
It would be nice to:
- fade transition from day to night
- use satellite image as background (one for day area, the other with lights for the night area)
- plot the position of the Sun [done] and the Moon, like here or here
- use astronomical symbols for Sun [done] and Moon (there are SVG versions...)