Flickering sky

I recently observed that the sky starts to flicker at night.
Unfortunately I can't give any further informations except that it only happens at night and that it affects the blueprints in a manner that they start to flicker as well, when the sky is behind them.


I hope I could explain it sufficiently.
Edit: Here you can see what's happening to the blueprints:


Unfortunately I can't give any further informations except that it only happens at night and that it affects the blueprints in a manner that they start to flicker as well, when the sky is behind them.


I hope I could explain it sufficiently.
Edit: Here you can see what's happening to the blueprints:


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