The light fixture... the realy is hot, realy will be open when it is dark but once it gets light, the realy contacts shut (NC contacts) will switch your external circuit on, and the 8 sec timer starts it's count down. Using a large enough elecrtolytic cap bleeding off to ground via a resistor can adjust to 8sec easily enough without many components.

BTW sun rise is later by about 1 minute per day. It won't matter is you use the photo sensor but if a cloud is on the horizon n it clears a bit later your shot won't be what you planed it to be.