This is an example of using D3's orthograph projections to visualize moon phases. My starting point for this was http://bl.ocks.org/mbostock/3795040.
There is a nice geojson wsyiwig editor here: http://geojson.io/#map=1/16/-68.
When defining points, the first value is the angle in degrees from the prime meridian (-180, 180). The second is the angle in degrees from the equator [-90, 90]. So, any value [x, 90] is at the North pole, and any value [x, -90] is at the south pole. I was getting some funkiness when I put things directly on the poles, so I fudged it a little bit and went with 89.5.
You can read hemisphere.json like this: