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At a total solar eclipse the sun is barely covered, like right after sunset. So why is it much darker than right after sunset (which allows us to see the corona)?

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The most significant difference is that in a total eclipse the moon obstructs the sun's light outside of earth's atmosphere whereas at sunset, the light is obstructed by the horizon within the atmosphere. With the sun just below the horizon, sunlight still hits the atmosphere above the horizon and even above you, and it's scattered all around.

Even around the total eclipse's trail there's partial shadowing, considerably reducing the amount of sunlight into the atmosphere that could be scattered.

enter image description here

(image from Wikipedia)

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    $\begingroup$ Additionally, the atmosphere over earth is about 100 km thick. At dusk, the light hit at a more oblique angle and have much more depth to go through, increasing the scattering effect. $\endgroup$
    – Criggie
    Jan 2 at 2:45
  • $\begingroup$ Thanks. I believe that the partial shadowing plays no big role, because it doesn't reduce sunlight very much. $\endgroup$
    – George Lee
    Jan 3 at 15:15
  • $\begingroup$ @Criggie: The total scattering is indeed more, but is the scattering per area also more? My intuition guesses no. $\endgroup$
    – George Lee
    Jan 3 at 15:19
  • $\begingroup$ @GeorgeLee Well, the partial shadow does make a difference. Sunlight decreases from 100% at the penumbra edge the to 0% at the core - in your direct vicinity. It might not make much difference for seeing the stars near the umbra (1% sunlight is still vastly too much light for seeing the stars) but it does reduce scattered light substantially, darkening the sky above you. $\endgroup$
    – Zac67
    Jan 3 at 17:15
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    $\begingroup$ @GeorgeLee Yes, that's possible - however, total eclipses just over the horizon are very rare as the umbra needs to graze the Earth's disc just where you're standing. Remember that the umbra is cone and narrows with increasing distance from the moon. Greatest totality is only possible at or very near noon. $\endgroup$
    – Zac67
    Jan 7 at 16:15
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Right after you see the sun disappearing below the horizon, the air above you is still illuminated and scatters light back to the ground. That is why it is not completely dark during dusk. During a solar eclipse, the moon casts a shadow on both you and the air above, so there is no scattering going on.

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    $\begingroup$ There is of course still scattering going on, but given the size of the umbra, it’s “quite far away”. A picture to scale of the atmosphere- both for a recently-set sun, and for an overhead-but-eclipsed sun, would really drive that point home. $\endgroup$
    – Floris
    Dec 31, 2021 at 17:54
  • $\begingroup$ Thanks. I have a follow up question that I planned to include in the original question, but I forgot: Can a solar eclipse after sunset enable us to see the corona? My guess is that yes, if the umbra is located in the atmosphere above us. Is my guess right? $\endgroup$
    – George Lee
    Jan 3 at 19:14
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Additionally, its quite a quick change. The moon is doing about 1 km/s orbital velocity, and so is its shadow (approximately)

The length of Dusk can vary, but somewhere between 15-60 minutes at most commonly populated latitudes.

An average adult's horizon is around 5 km away on flat land. So the "edge" of the shadow will pass over an outdoor observer and their surroundings in just 10 seconds. This is not instant, but one's eyes take a period to adjust. Its somewhat akin to turning off a room light and being unable to see anything until one's night vision returns.

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    $\begingroup$ If the darkness is perceived due to the rapidity of its onset not allowing our eyes to adjust, then how do we see the stars during a solar eclipse? $\endgroup$
    – dotancohen
    Jan 2 at 12:48
  • $\begingroup$ Totally wrong, as the previous comment pointed out. My question was "why" is it getting dark more quickly than at dusk. $\endgroup$
    – George Lee
    Jan 3 at 15:27

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