Reciprocity Failure?

[Speed]

This is a fairly long (2.5 hours) exposure of Polaris, the North Star. I have taken several shots of Polaris over the last month or so, and I have noticed that in my 10, 20, and 30 minute exposures, the sky is blue. In my exposures of 60 minutes and longer, the sky is green.

I will be taking more shots of Polaris in the future, and I intend to take some shots from darker sites. I'll be taking shots without any trees in them, and I'll be taking some shots using slide film as well.

So my question is, is this reciprocity failure, or is this green cast from the trees in the bottom of the frame, or is it the culmination of all the sodium vapor lights in Havelock (which is north of me)? I will find out eventually through experimentation, but in the meantime, give me your best guess.

K1000, Superia Xtra 400, 50mm lens @ f2.0

Would you believe it was handheld? No? ;-)

Bogen 3011 tripod