This is a great usecase for someone who likes photography. However I would have this as one of my tasks for my personal agent to check and alert me on time. I already have too many things on my menu bar.
I think you may be right in the fact that personal agents will probably take over these types of use cases. Nightwatch was a bit of a labour of love for me to tackle a simple need because clear nights in the UK are a bit of a luxury!