system
Given the following post, respond as if you're commenting on Reddit:
human
I thought the sky would get much darker yesterday during the eclipse. How can almost 90% of the Sun be occluded, but the daylight is hardly affected?
gpt
Your eyes are essentially optical sensors, and by their design, they have a very large *dynamic range*.

Dynamic range is a term used to describe the ability of sensors to function in the presence of signals whose strength can vary tremendously. For example, your ears are sensors for sound; their sensitivity in terms of frequency is in the range 20:20000 range, which is to say that the highest signal they can pick up has 1000x the frequency of the lowest signal, the dynamic range (for signal frequency) is therefore 1000.

The dynamic range of your ears is even more impressive when looking not at the frequency, but at the signal strength. The ratio of the highest vs the lowest sound level your ears can pick up is something like 10,000,000,000.

Your eyes have limited dynamic range in terms of light frequency (about a factor of 2), but their dynamic range in terms of optical energy is about 1,000,000,000 -- the lowest light level that can be detected is one billionth of the highest light level.

Your eyes achieve this impressive range by adapting to the light environment. In high light conditions, your pupils go small and you squint to limit the amount of light falling on your retina, and the detectors (cones and rods) in your retina will saturate -- extra light will not lead to more signal.

In conditions of low illumination, your pupils open fully to allow as much light to enter as possible, and chemicals will be produced to highten the sensitivity of your rods (the most light sensitive optical cells, that cannot distinguish color). In darkness, this process takes about 15 minutes.

So what happens during a partial solar eclipse is this: the illumination level goes from broad daylight (the highest level of illumination that your eye is equipped to handle) to 10% of that. That is still A LOT of light to work with, and compared to the enormous dynamic range of your eye, it barely makes a difference.


I personally had the pleasure of witnessing a full solar eclipse in Northern France in 1999. Indeed up until the last 10 minutes of so before the actual full phase of the eclipse, the effect was surprisingly small. One interesting phenomenon is that shadows become sharper -- the sun normally takes up about 0.5 degrees in the sky, and due to this its shadows are never sharp; there is always a transition area where the sun is only partially illuminating the surface. So when the sun becomes smaller (just prior to an eclipse), almost becoming a "point source", shadow edges become unnaturally sharp.

The last few minutes prior to an eclipse are quite weird. The illimunation levels go down really fast, in a way you're not used to. In the last 10 seconds before totality, your vision becomes almost colorless (because the cones stop picking up color information), and it feel as if a blanket is pulled over the sky. Then totality sets in, and the corona emerges. It is quite a sight.