A lot of people mentioned blood pressure, which is the cause of what you’re talking about. However, there’s another neat thing you can do that is similar – some time, when the sky is very clear, look at it for a while and let your eyes unfocus. You’ll start to see dots wandering around your vision, even though you can’t really focus on them. What you’re actually seeing there are the white blood cells in your eyes! When you look at “blank” space, like the sky, your eyes don’t really know what to focus on, so they focus on the thing changing the most, which are the constantly moving blood cells in your own eye.
Doesn’t answer your question (since others have already done that), but it’s still neat and it demonstrates that our sense of sight is very odd sometimes.
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