I have a few ideas in mind how to make the algorithm to determine the dawn/duck time (don't forget this algorithm has to be lightweight enough to fit on low-power MCU), but taking a measurement in single place, fit algorithm to it and call it done is not the way I like to do things. I decided to find a few "pen friends" and send them loggers to make record of light intensity and then, return it back with measured data. For now, the logger will not and can't do any geolocation, as I still don't have any algorithm, but those raw data will help me to find one and validate it. For now I have two volunteers in here, in Slovakia; one in Germany (thank you @davedarko ) and one in Spain.

For now I have four loggers almost done and ready to be sent tomorrow, I hope post service will not be overloaded with annoying Christmas cards and stuff, interfering with my project.
In next log I'll show you how the logger looks and works.