Christmas light circuit reverse engineer

[pyrohamish]

I'm trying to reverse engineer a set of battery / solar Christmas lights so I can program and run the lights with a ATTINY or similar.

Photo of board and the schematic I've worked out attached. I think I know mostly how it works, but, what is the purpose of the voltage divider to NPN transistor?

[exrbc]

switch on when solar voltage drops, turns on when dark.