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In the incoming April total eclipse in North America, if a city's sun is said to be 99.5% blocked, What darkness can it reach compared to the period of dawn or dusk or evening?

I ask this question because I live in a city where the sun is said to be 99.5% blocked during the eclipse. So if the darkness is similar to a predawn level (dark enough), there is no need to travel to a place (50-100 kilometers away) where the sun is 100% blocked, especially on a cloudy day.

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    $\begingroup$ A closely related question on our sister site: How different is a 99.5% eclipse from a total eclipse? $\endgroup$
    – PM 2Ring
    Mar 22 at 8:24
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    $\begingroup$ The sun will be too bright to look at even at 99.5%. Anywhere more than a mile outside of totality the corona won't be visible. You'll want to get inside the path of totality if at all possible. $\endgroup$ Mar 22 at 16:02
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    $\begingroup$ The light from a 99.5% obscured sun high in a clear sky is nothing like the diffuse glow of twilight from the western horizon. If you took a photo using your phone or a point-and-shoot camera during the 99.5% eclipse, you would later have a hard time convincing anybody that you didn't simply take it in broad daylight on an ordinary day. It looks like broad daylight, except... well... without as much light. I expect you'll find out in a couple of weeks what I'm talking about (if the weather cooperates.) $\endgroup$ Mar 23 at 15:20
  • $\begingroup$ Yes, you are right. I already found a webpage which answers it and confirms yours. So it is necessary to go to places of total eclipse to witness all the effects. 99.9% is not enough. $\endgroup$
    – E Zhang
    Mar 23 at 15:30
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    $\begingroup$ Having experienced a total solar eclipse, I can tell you it is nothing like normal darkness. It's spooky. Animals do weird things. Shadows are strange. $\endgroup$
    – Schwern
    Mar 25 at 2:41

7 Answers 7

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This misses the point.

During totality the sun's corona becomes visible. If any part of the photosphere is exposed, then the corona isn't visible.

The experience of a total eclipse is not just "darkness" (you get about 12 hours of that every day!) It is the ring of the solar corona.

To answer your question, 99.5% of the sun is obscured a few seconds before sunset. But this isn't a fair comparison since at sunset the sun is obscured by the Earth which is inside the atmosphere, and during an eclipse the sun is obscured by the moon, which is beyond the atmosphere.

My granddad used to say "a miss is as good as a mile" If the sun is not covered by the moon, it is a partial eclipse and those are pretty common. You will only experience totality if the sun is covered by the moon. 99.5 % is not totality and is a completely different thing.

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    $\begingroup$ This does not answer my question. I do not care whether I can see the solar corona with the naked eye or not because I have an eclipse glass. Nor do I care if it is obscured by the Earth's atmosphere or others. I just want to know if the 99.5% blocked sun would like a night, or closer to nightlight than daylight. For me, the most important thing is to experience a sudden change to the night environment from daylight. $\endgroup$
    – E Zhang
    Mar 22 at 17:39
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    $\begingroup$ I've edited. If all you care about is the change from light to dark, I suggest you go out at between 6 and 7 pm then. You will experience a change from day to night. Or go out between 5 and 6 am. You will experience a change from night to day. These are not special. If you are not in totality you are not experiencing a total solar eclipse. And, frankly, who cares about a partial eclipse? Those happen every couple of years. Totality is special. 99.5% is rubbish by comparison. $\endgroup$
    – James K
    Mar 22 at 17:54
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    $\begingroup$ The change of light at sunset/sunrise takes some time. In a total eclipse the change is much quicker I think. Put in another perspective, can stars be seen under the 99.5% block of the sun? I know they can be seen under 100% block of the sun. Is 99.5% still belonged to partial eclipse? $\endgroup$
    – E Zhang
    Mar 22 at 18:09
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    $\begingroup$ Feel free to self answer. $\endgroup$
    – James K
    Mar 22 at 18:17
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    $\begingroup$ @EZhang you state in your question that "there is no need travel to a place where the sun is 100% blocked". James' answer explains why that's not the case. If all you want to see is darkness, just wait until night time. $\endgroup$ Mar 22 at 19:10
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I agree with James K's observation that the main point of a total eclipse is not the darkness but the visibility of the corona.

However, to answer your original question, blocking 99.5% of the Sun* would reduce the daytime illuminance by a factor of 200, roughly from 20,000 lux (full daylight) to 100 lux. Even so, this would not feel very dark, because our eyes have a very wide dynamic range.

At dawn or dusk (in the absence of an eclipse), natural illuminance of 100 lux occurs during civil twilight shortly before sunrise or shortly after sunset. You can still see well. For additional context, this is comparable to a day with very heavy overcast clouds, or to the indoor lighting in an office building.

So the change in general brightness during a 99.5% eclipse is noticeable, but not dramatic. Even if the eclipse is covered by clouds, there would be a much bigger perceived change in brightness where the eclipse is total. And if the eclipse can be viewed clearly, then of course there is no comparison.

* A technicality: The most commonly quoted measure of a partial eclipse, the "magnitude", is the maximum fraction of the Sun's apparent diameter (not area) that is covered by the Moon. So for a precise answer, you would need to ensure which number you read. I will assume here that 99.5% is the fraction of area, known as the "obscuration". The issue does not affect the qualitative conclusions.

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There's no equivalent. At 99.5% coverage, the overall light level is comparable to that of twilight a few minutes after sunset. However, the majority of the light is still coming from a point source, so you get phenomena such as sharp shadows that aren't present after sunset. The overall result is an impression of "wrongness" to the light, but it's nothing like the experience of totality.

During the 2017 eclipse, I recorded a video of the final 30 seconds before totality in an attempt to catch the Moon's shadow sweeping across the ground. It mostly didn't work, but I did catch a phenomenon nobody had mentioned before: the crowd cheering the moment totality started. Below is a frame from 25 seconds before the cheering, showing how a camera sees 99.5% coverage.

enter image description here

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My experience with the last total eclipse was that it got noticeably dimmer and instantly cooler. It was not like night at all. I would say the level of light was probably closer to dawn, or like when a heavy cloud goes over the sun.

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This webpage answers my question completely. Even a 99.9% block of the sun belongs to a partial eclipse, you can not experience any of the following:

  • temperature & wind changes
  • dramatic brightness changes
  • sunset colors around horizon
  • bright stars and planets
  • corona, diamond ring effects, and others

So a 99.9% eclipse will not be enough at all. You need a 100% eclipse to enjoy those experiences.

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    $\begingroup$ This may be the answer you wanted, but it is not an actual answer to your stated question of what "night hour" or "evening/morning hour" has equivalent darkness. My answer does address that. $\endgroup$
    – nanoman
    Mar 22 at 19:40
  • $\begingroup$ No, it answers exactly what I want to know. It says that no dramatic brightness changes, no sunset colors around the horizon, and no bright stars and planets under the 99.9% eclipse, which means people can not really experience the change of daylight and night even under the 99.9% eclipse. $\endgroup$
    – E Zhang
    Mar 23 at 1:33
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    $\begingroup$ What you want to know is not what you actually asked though. $\endgroup$ Mar 23 at 15:24
  • $\begingroup$ @EZhang Welcome to Stack Exchange! It's always OK to answer your own question, and to accept the answer if it's the best one. However we really avoid changing the question once answers start to be posted. Try to avoid doing that in the future, thanks! $\endgroup$
    – uhoh
    Mar 24 at 0:24
  • $\begingroup$ @EZhang I also find it frustrating when answers suggest the question is not the right question to ask. It happens occasionally, I just ignore it. Sometimes answer posts are written more for the benefit of future readers, or general interest, than they are the question author's specific interest. I call this uhoh's lemma #3 Stack Exchange is BOTH a floor wax, AND a desert topping :-) $\endgroup$
    – uhoh
    Mar 24 at 0:38
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Snakes!

In addition to @Mark's answer that everything looks "wrong" with a near point-like dim source (basically this is what daylight looks like at 15 AU, (between Saturn and Uranus)) there are other effects you can think about. I am not sure 99.5% coverage is enough, but when the Sun is covered except for a very, very thin sliver, atmospheric effects on Earth called "shadow snakes" or what Wikipedia calls shadow bands become visible.

Slightly related: Would an extremely bright star produce same kind of shadow band effect as seen in solar eclipses just before totality?

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The eclipse as seen in Toronto, Ontario, Canada reached a maximum magnitude of 0.998, reported on a Toronto and Region Conservation Authority website as "a magnitude of 99.56%" (they presumably actually meant obscuration; this is from a vantage point that is probably about as close to the path of totality as it's physically possible to get in Toronto while still being on land).

This was enough, per CBC News coverage, to have noticeable effect on animal behaviour and cause a large drop in temperature - presumably substantively the same drop as observed within totality (since, although the human ocular response to light is highly non-linear, the same can't really be said about the Earth's temperature response to incident infrared radiation). A temperature drop is detectable with even a fairly small amount of obscuration, although it isn't very impressive. But from my own experience within totality, it was not noticeably colder at totality than several minutes before. (It did seem to warm up more quickly afterward, but that may have been psychological.)

I was also able to find some independent local footage (timestamped at or near the moment of maximum eclipse). It's dark enough to impress viewers and commentators, and quite noticeable on film, but still bright enough to see clearly in an area with tons of light pollution. It looks like moderately deep twilight rather than late night. I know there is some other footage I saw by the CBC taken from a rooftop of their building, which looked somewhat darker than this (away from street-level lights); but I can't seem to find it now.

Here's another timestamp from the same CBC News coverage, allowing us to see the contrast between the darkness of totality (as seen in Kingston) and the near-totality in Toronto.

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  • $\begingroup$ This webpage is completely right. In London, Ontario, the eclipse was at a maximum of 99.8%. But there was no corona, no evening effect, no temperature drop noticeable. But the light was darker enough to cause the road lights to turn on. So it needs 100% eclipse and the longer lasting time, the better effects. $\endgroup$
    – E Zhang
    Apr 10 at 23:20

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