How do I get the position of the light source at the specified point in time?
So the only light source in Cesium is the sun, and you can get its position using this class:
https://cesiumjs.org/Cesium/Build/Documentation/Simon1994PlanetaryPositions.html
There’s a discussion about adding more light sources, or controlling the light position, which has an example of using that class:
https://github.com/AnalyticalGraphicsInc/cesium/issues/6553
Hope this helps!