I live in a place where we weren't on the path of totality but were in the 90% range. I was expecting it to be a lot darker than it ended up being. What does it mean to be in the 90% range on this map? I was expecting it to block out 90% of the visible light, but while it was noticeably dimmer it wasn't close to what I thought 10% of light would look like.
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$\begingroup$ The Sun is surprisingly bright. During the 2012 annular eclipse in Albuquerque, it didn't get noticeably darker until 5 minutes before maximum eclipse. Similar effect today-- 73% eclipse, absolutely no noticeable lack of brightness. See also astronomy.stackexchange.com/questions/22230/… $\endgroup$– user21Aug 21, 2017 at 19:41
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3 Answers
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As seen in this comment, normal daylight is around 100,000 lux, but 10,000 lux is still considered daylight. So, unless you're getting close to 99%, it will still be fairly light outside.
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1$\begingroup$ There are votes for deletion of this answer because of "length and content". Sometimes short answers are OK if they are clear and this one seems perfectly clear to me. I thought about editing it to add more, but really, this is all that needs to be said. The OP has followed a recommendation by another user to post an answer to their own question, citing the user's comment. OK that comment's text could also be incorporated here. but geez, this answer is fine, and represents good practice by a new user! $\endgroup$– uhohAug 23, 2017 at 7:45
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$\begingroup$ Would it be better if I quoted that comment in my answer? $\endgroup$ Aug 23, 2017 at 22:48
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The real reason is that the human eye is not a linear sensor. A 10x decrease in illumination does not produce a 10x reduction in your perception of light, but more like a 2x reduction at most.
So yes, during the 90% phase of a partial eclipse, the level of illumination is massively reduced. You could check it with a device that could measure ambient light, and it would show you the huge decrease. But your eyes tell a different story because they're not built to operate on a linear scale.
answered Aug 23, 2017 at 23:49
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To expand on Florin Andrei's answer, it's not just that the human eye is not a linear sensor, although that is definitely an important part of the reason. The eye itself adjusts to the ambient light level by changing pupil size, which will mask overall changes in brightness. In addition, the eye, like most senses, is primarily a relative sensor. Colors are judged relative to neighboring colors, and brightness is judged relative to neighboring brightness. "Neighboring" applies to time as well as space: a sudden change in brightness from 100,000 lux to 10,000 lux would be quite noticeable, but when it happens over an hour it is imperceptible. The final change to totality is quicker, and also substantial enough that you would be able to detect it even if it were slow. When it's dark enough, the color mix is different, shadows disappear, and it's actually harder to see in an absolute way.
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$\begingroup$ Indeed, but does it change much if the ambient light drops by 90%? $\endgroup$– ProfRobAug 24, 2017 at 23:30
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$\begingroup$ No, it really doesn't, or at least not much. There is a threshold where the eye's cones start to lose effectiveness and the rods begin to do some of the work, and 10% of full daylight is above that threshold. During totality, however, it really looked like a fraction of color vision was missing. $\endgroup$ Aug 28, 2017 at 2:30