
Eclipses and Decibels - ash
http://apenwarr.ca/log/?m=201708#27
======
Stratoscope
As anyone who has experienced totality knows, the difference between a total
eclipse and the deepest partial eclipse is not just a quantitative change.

At the deepest partial phase, you see Baily's Beads and the Diamond Ring. This
is still the sun's photosphere peeking through the moon's valleys, bright
enough that it washes out the corona, and bright enough that you had better
use eye protection. Only when the moon completely blocks the photosphere do
you see the solar corona.

This is totality. This is where you no longer need (and can't use) the
protective glasses or filters or pinhole projectors. During totality, you can
view the corona with your own eyes, with no protection, and even with
binoculars.

The difference between a 99.9% partial eclipse and a 100% total eclipse is not
0.1%. It is infinity.

~~~
Florin_Andrei
> _This is still the sun 's photosphere peeking through the moon's valleys,
> bright enough that it washes out the corona, and bright enough that you had
> better use eye protection._

The photosphere (diamond ring, etc) is not dangerous if you only see it for 1
or 2 seconds. It is prolonged, central-vision, fixed staring that takes more
than a few seconds that is dangerous.

So when the diamond ring comes out, take one look at it. But that's it, one
look. Then turn away and put your goggles on. You'll be fine as long as you
don't linger beyond a couple seconds.

The problems begin when people keep staring at it fixedly beyond a handful of
seconds.

> _The difference between a 99.9% partial eclipse and a 100% total eclipse is
> not 0.1%. It is infinity._

Well put.

~~~
Stratoscope
Finally, someone who knows the truth!

Indeed, at the end of totality in the 1979 eclipse, I kept looking at the
diamond ring and Baily's beads for a second or two _with my binoculars_. It
did not damage my vision at all.

In the weeks leading up to this year's eclipse, I almost felt like it was my
civic duty to correct some of the misinformation that was going around in the
media. I had a feeling that many thousands of people who went to considerable
trouble and expense to go to the path of totality would miss it entirely,
because they got scared into keeping their eclipse glasses on the whole time.

There was such a fear campaign going on, just like the one in 1979 when Oregon
schools changed their hours to keep the children indoors during totality.

This TV report was a good example:

[http://www.kptv.com/story/36143862/portland-man-shares-
warni...](http://www.kptv.com/story/36143862/portland-man-shares-warning-
after-being-partially-blinded-by-1963-eclipse)

The gentleman in the report did unfortunately damage his vision by staring at
the 1963 partial eclipse - for 20 seconds! Truly a sad thing, but in the video
they kept talking about the dangers of staring at a partial eclipse, while
showing photos of a total eclipse - as if there was no difference between the
two!

I tried to set the record straight in the comments, but man, it was an uphill
battle. Read the comments and weep...

~~~
Florin_Andrei
This is why:

[http://phdcomics.com/comics/archive.php?comicid=1174](http://phdcomics.com/comics/archive.php?comicid=1174)

------
idlewords
It's not true that you don't perceive the darkening of the sun until just near
totality. Instead, what you get is a really weird subliminal creepy feeling
that something is _wrong_.

You're seeing things with the brightness of a rainy day, and then with the
brightness of sunset, but the color balance is that of full sunlight. For me
at least it's very unsettling, and I wonder if other people experience it that
way.

The uncanny feeling is quite visceral and a big part of the reason I think
eclipses just have to be experienced. All kinds of alarms go off in your
animal brain when the moon eats the sun.

This is not to take away from the author's very correct observation that the
light effects only really kick in close to totality, I'd say with over 90% of
the sun covered.

~~~
ctdonath
Yup, it's a "had to be there" thing. Complete eclipse, start to finish, took 3
hours. Totality was just over 2 minutes. Most of the time, lighting can only
be described as "odd" \- as you put it, color balance of full sunlight for
brightness of sunset. Bugs & animals picked up on it, rapidly heading into
night mode. Humans tended to stand around slowly realizing "huh? that's odd".
Lighting starts as a sense of just wrong. Then at about 95% totality (!) you
finally perceive the rate of change, as it exponentially plunges into
darkness. Then you realize the darkness isn't, you're seeing with the
visibility of a nightlight (but color balance still that of full sunlight),
and proceed to freak over the visibility of the corona surrounding a big,
well, black hole where the sun was. The horizon is also oddly bright.

And yes, there's a huge difference between any perceivable percentage away
from totality, and totality itself. We're talking fractions of a second making
a difference between direct viewing vs rapidly going blind.

~~~
dredmorbius
NB: The colour-balance of night, say, under a full moon, is actually pretty
close to that of daytime.

It's just ... well ... a _lot_ darker.

Shooting long-exposure slide film is a good way to confirm this (no colour
adjustment possible).

Moonlight is has a _slightly_ lower (that is, optically "warmer") colour
temperature -- 4100K rather than 5000K.

Contrast with "daylight" vs. "tungsten" film colour balance: 5500K vs. 3200K.

[https://physics.stackexchange.com/questions/244922/why-
does-...](https://physics.stackexchange.com/questions/244922/why-does-
moonlight-have-a-lower-color-temperature/244929)

[http://www.apogeephoto.com/white-balance-and-color-
temperatu...](http://www.apogeephoto.com/white-balance-and-color-temperature-
in-digital-photography/)

~~~
Florin_Andrei
> _Shooting long-exposure slide film is a good way to confirm this (no colour
> adjustment possible)._

Or shooting long exposure with a digital camera that does not automatically
adjust the color balance. E.g. take the raw data from the sensor, instead of a
camera-generated JPEG.

~~~
dredmorbius
Right. I was also contrasting _print_ film (negative process) as virtually all
film-development would attemtp to "correct" shots for exposure and colour
balance.

(Much of my experimental night photography was conducted ... some time ago.)

~~~
Florin_Andrei
Hey, I did some of that too. Had my own little dark room and everything.

Working with film was, um, exciting? Not sure if that's the right word.
Anyway, mistakes were final. I remember deliberating on the settings for a
really long time before pushing the button.

------
teraflop
A slight correction: according to some measurements that I found [1], the
ambient illumination during totality is roughly 2-5 lux, which is "only" about
45dB below normal. (Full daylight is about 10^5 lux.)

I started researching this a few weeks ago after seeing a Reddit comment
asking (paraphrased) "my house is just a couple miles outside the path of
totality, is it really worth the trouble of finding a different spot to view
the eclipse?" The answer is that even at 99.5% occlusion, the sky is still
hundreds of times brighter than complete totality.

[1]:
[http://www.strickling.net/sofi_eng.htm](http://www.strickling.net/sofi_eng.htm)

~~~
russdill
I think for me the think that it really drove home is that the sun is really
really really really bright.

------
mjhoy
Interesting. I was in Wyoming and noticed this as well. During the partial
eclipse it was impossible to tell that the sun was dimming except that I could
measure this with my camera's light meter. My friend hardly believed me when I
informed her the sunlight had been halved three times (losing three stops) --
except that we could _feel_ this difference in how chilly it had become. It is
only as the last bit of sun is obscured that the world goes dark in any
noticeable way.

~~~
athenot
It was also fun to notice the street lights & home outdoor lights all come on
in what looked like sunny day. The brain really compensated for the dimness.

------
idlewords
The main source of light during totality is the ring of daylight around the
horizon. It looks like sunset in every direction.

I wonder how dark an eclipse could get under optimal conditions (wide track,
and heavy overcast all around except in the path of the shadow).

~~~
russdill
It looks weirdly different as everything about it is backwards. Blue sky is
beyond the sunset, but the light is coming from above you.

------
madengr
I have a plot of WWVB radio propagation here, for several days about the
eclipse:

[https://zenodo.org/record/851609](https://zenodo.org/record/851609)

The ionosphere was doing strange things the entire day, and even past local
sunset.

Does anyone know of a tool that plots the earth/moon/sun position on arbitrary
days? I'd like to go back and figure out what was going on.

~~~
th0ma5
Probably not ideal, but I know that PyEphem can be used to figure out relative
positions of all of these at arbitrary times from a given location.

------
marcosscriven
I went to view totality for the first time, in Salem, and thought exactly the
same thing; I ended up putting a jumper on as I felt a little chilly, even
though there was a mere sliver of the photosphere visible.

It's curiously satisfying (I suppose because it's affirmative) to see one's
subjective experience described objectively like this though.

------
learn_more
You must remember that the irises in your eyes are dialating as the sun is
being eclipsed. If that was not happening, you would undoubtedly notice the
change in visible light like you do the heat.

~~~
rcthompson
The dilation of your pupils is actually only responsible for a small portion
of your eyes' ability to adjust to ambient brightness. The rest is your rods
and cones adjusting their sensitivity biochemically.

------
nayuki
> When the sun is completely blocked by the moon (which appeared about 3%
> larger than the sun during this particular eclipse), the total power
> delivered becomes 0% for a while, which is -infinity dB

The total power delivered during a total solar eclipse is obviously not zero.
The author even admits a few paragraphs down that the corona is visible - and
it's what people enjoy viewing during an eclipse. This one numerically
inaccurate phrase spoiled my enjoyment of the article, unfortunately.

~~~
dahart
Bummer that a nit that small would change your summary and overall emotional
response, and prevent you from enjoying a great blot post, especially when the
author pointed it out. "0%" is numerically accurate to at least 3 significant
figures, isn't it?

For me, it's really interesting that there's a discrepancy between the heat
you feel and the light you see, and I love the writeup that is explaining the
primary reason why -- because our perception of brightness is logarithmic, but
our sensation of heat seems more linear in this case.

The experience of being there and having it go cold before it got dark was
startling, and I have to agree with the author, it's easy to see why this
stuff caused so much emotional stir before science showed us how it all works.

There's a much larger source of error, btw, in using a straight decibel scale
as a proxy for brightness. (Also admitted & linked in the article.) It just
doesn't matter, it's close enough, it's true, it's interesting, and it
supports the narrative.

~~~
nayuki
The author's rushed approximation of "-infinity dB" is disingenuous, and
discounts the efforts of people who handle low-level signals.

For example, his claim makes it impossible to compare the ambient light level
of an eclipse versus the light level from a full moon or new moon.

For example, not knowing the noise floor will make it impossible to tell
whether a 16-bit audio recording, or 20-bit, or 24-bit is the best choice.

What is the "larger source of error" in using a decibel scale for brightness?
Maybe the fact that human eyes are not perfectly logarithmic? But such a scale
is very appropriate for calculating camera exposures.

~~~
dahart
Aww, come on, don't double-down, just let it be. If you want some benefit of
the doubt, feel free to give some benefit of the doubt. We can nit pick each
other to death, but that isn't going to be productive or fun at all.

You can't take something out of context and claim that it's insulting people
that he wasn't talking about. He wasn't discounting anyone. Yours is a bigger
and more intentional error than his statement that totality is 0% energy.
You're picking a fight instead of acknowledging that totality really is
approximately 0% solar power for the purpose of figuring out how much heat
you're going to feel on your arm during the eclipse, compared to before the
eclipse.

He admitted the noise floor is not really zero. He wasn't contradicting
himself, he was clarifying for the sake of the nit pickers. You're using that
against him instead of accepting that both of his statements can be true at
the same time. He was never claiming absolute zero with infinite precision,
that is completely clear from the article.

He also wasn't asking what bit depth to use for audio recording. That would be
weird, wouldn't it? Because he's talking about the sun's heat and not the
sound it makes. He wasn't talking about photographing it either, or even
measuring the heat precisely. He was showing why it gets cold before it gets
apparently dark. You can do that in even less than 16 bits.

Yes, human eyes are not perfectly logarithmic. Interesting that you don't care
for the sake of camera exposures in the middle of a conversation about the sun
and moon, where you just cited bit depths and people who handle low-level
signals. The logarithmic approximation isn't particularly good, which the link
in the article discusses in detail. Log is good enough for this article, but
not for vision scientists. The logarithmic brightness approximation really
truly breaks down when you look at the sun or the night sky, human vision does
not give an approximately logarithmic response to either the sun or the night
sky, not even close. Camera film and camera sensors also break down and begin
to clamp at either end of a moderate log range that is narrower than human
vision. Photographing the corona well is really hard due to the extreme
dynamic range and the way that camera response stops being logarithmic at the
ends.

~~~
ars
The person you are replying to is not wrong though. The author of the article
is.

Even during totality it's not as dark as night. It's around the illumination
of just after sunset, and he could have definitely plotted that - it's not
that hard to lookup how much there is then, he doesn't have to measure it
himself.

~~~
dahart
Ugh. Be very precise and tell me, what does it matter? What about the story
would change if he'd used super extremely precise numbers? How would the
conclusion change, exactly?

Does the correct number for solar power during totality explain why it gets
cold before it gets dark?

The article was not wrong, it was an approximation, which he stated clearly.
Did you perhaps not understand this sentence: "Anyway, there is never really
"zero" radiant energy to detect. In reality there's always some kind of
background noise, maybe at -90 dB or so."?

If he had used the correct number, you would see that it is approximately 0%
power. Again, and I'll state it as fact now rather than rhetorical question,
the article was accurate to more than 3 significant figures.

BTW, the discrepancy between night time levels and the darkness of totality is
due in part to atmospheric scattering during the eclipse, so you're
introducing a red herring.

> he could have definitely plotted that

Given that the heat plot goes from 0% to 100%, and that the brightness plot
goes from -70dB to 0dB, will you estimate for me how many pixels in both plots
would be different if he used really precise numbers for solar power during
totality? My current estimate is 0 pixels would change across both plots. Do
you think the number is higher? If so, why?

------
Trombone12
Excellent science writing! It is rare to see such quality here.

