I got this quick and dirty lesson on drawing planetary terminators from space artist Kim Poor. [link]
A planet's terminator (the line between day and night sides) is an ellipse. The ellipse major axis has the same length as the planet's diameter. If you extend the minor axis, it will point to the light source.
I met Kim at the El Con shopping mall in Tucson, Arizona. He had a booth where he was air brushing space landscapes.