It really depends on a lot of factors, how clear it is at the time, the strength of the sun's output, the angle it strikes the earth, etc... So it's hard to predict.
I've only seen them once, and that was local. (About as far south as Detroit). Going farther north doesn't matter as much as how far away from civilization you can get. The less ambient light you have around the more your pupils open so the more likely you'll notice them when they do happen against the darker sky.
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