<p>An excellent explaination found here: <a href="https://www.youtube.com/watch?v=ZD1roP5wISU" rel="nofollow">https://www.youtube.com/watch?v=ZD1roP5wISU</a></p>

<p>Goes into detail about Rayleigh dispersion.</p>

CC BY-SA 3.0
2016-03-31T16:42:52.787Z
2016-03-31T16:42:52.787Z
zundi
<p>At Noon, sunlight is coming from nearly directly above you, and goes through ~2 miles (4km) of reasonably dense atmosphere to reach you (Atmospheric pressure at 2 miles is about one half of sea level pressure). At sunset (and sunrise), since the sunlight is traveling mostly parallel to the surface, it travels through dense atmosphere for considerably longer. When traveling through dense atmosphere it's subjected to more <a href="https://en.wikipedia.org/wiki/Diffuse_sky_radiation" rel="noreferrer">scattering</a>, which decreases the observed intensity of the light. </p>

<p>Or, considered a different way, when looking at a sunset/sunrise, you don't see the sun as a single well defined point. It's hazy, with the bits of sky near to the sun also giving off a fair amount of light. That additional light is some of the light that was scattered from an otherwise direct beam. If it were all re concentrated to a single point, it would be brighter/more painful to look at (though not as bright as the noon sunlight- for shaky statistical and light absorption reasons).</p>

<p><a href="https://i.sstatic.net/J9DY4.jpg" rel="noreferrer"><img src="https://i.sstatic.net/J9DY4.jpg" alt="Creative Commons image of a Sunset... by... Mario Antonio?"></a></p>

<p>Using an <a href="https://dizzib.github.io/earth/curve-calc/" rel="noreferrer">Earth Curvature Calculator</a>, sunlight at the horizon travels through ~120 miles of 'dense' atmosphere (ie has an altitude &lt;2 miles). That will fall off rather quickly the higher above the horizon the sun is. </p>

<p>(dense atmosphere in this refers partly to just thicker air; but also to particulates/dust in the air, haze, mist, fog, etc)</p>

CC BY-SA 3.0
2016-03-11T11:31:43.517Z
What is the relevance of *Atmospheric pressure at 2 miles is about one half of sea level pressure* in this answer, and what do you mean by *has an altitude of <2 miles*?
gerrit
CC BY-SA 3.0
2016-03-11T13:19:56.783Z
I think it's because the atmosphere never really ends, it just gets less and less dense. So he's using the 2 miles as a marker where there's about half the atmospheric density to say we can ignore anything from there up.
zundi
CC BY-SA 3.0
2016-03-11T13:21:55.103Z
@CoderTao is there an equation for this? For example, at what angle above the horizon would the sun be about 50% as bright as at noon?
zundi
CC BY-SA 3.0
2016-03-11T15:04:44.863Z
@TheVoid -- Stop posting nonsense, and in this case, dangerous nonsense. It is not safe to look at the Sun for any extended period of time, even when the Sun is right on the horizon. It is not safe to look at the Sun, period, with the naked eye once the Sun is even a few degrees above the horizon.
David Hammen
CC BY-SA 3.0
2016-03-12T00:40:47.550Z
David Hammen-Why do you follow my ever post and add rude comments? and who let you be a moderator?
user5434678
CC BY-SA 3.0
2016-03-13T09:59:56.787Z
@sandy Sadly I don't have an equation for it, though David Hammen's link to the wiki article on Air Mass would be good reading, and does seem to have a few equations for magnitude vs angle above horizon. And yes, 2 miles was chosen as a rough 'going straight up, we've passed through half the air particles we're going to see'.
CoderTao
CC BY-SA 3.0
2016-04-10T15:57:27.280Z
@CoderTao Other factors that contribute to the attenuation are that at sunset, much more of the sunlight is scattered away into space--not just into different parts of the sky. As well, much more light is refracted into space at sunset. That's why the sunset is red. Refraction is a different physical mechanism from scattering.
kwknowles
CC BY-SA 3.0
2016-04-10T16:06:59.007Z
@CoderTao Your point about the ratio of air distance still holds. But, "half the atmosphere" also known as 500 millibars is actually around 5500 meters, which is approx 18,000 feet, or 3.4 miles.
kwknowles
CC BY-SA 3.0
2016-03-11T06:14:51.237Z
2016-03-11T11:23:48.970Z
2016-03-11T11:23:48.970Z
CoderTao
<p>Why can I look directly at the sun during a sunset (with a lot less pain and unconfort) but not at noon?</p>

CC BY-SA 3.0
2016-03-11T15:05:53.240Z
https://en.wikipedia.org/wiki/Air_mass_(astronomy) .
David Hammen
CC BY-SA 3.0
2016-03-12T22:10:52.110Z
Related: http://physics.stackexchange.com/q/129492
Floris
<sun>
Why can I look directly at the sun during a sunset but not at noon?
zundi
# Why can I look directly at the sun during a sunset but not at noon?

(Asked by: username_0 on 2016-03-10)

Why can I look directly at the sun during a sunset (with a lot less pain and unconfort) but not at noon?

**Tags:** sun

> **Comments:**
> **username_1**: https://en.wikipedia.org/wiki/Air_mass_(astronomy) .  
> **username_2**: Related: http://physics.stackexchange.com/q/129492  

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## 2 answers

### Answer by username_3 on 2016-03-11. Score: 9 | Accepted answer

At Noon, sunlight is coming from nearly directly above you, and goes through ~2 miles (4km) of reasonably dense atmosphere to reach you (Atmospheric pressure at 2 miles is about one half of sea level pressure). At sunset (and sunrise), since the sunlight is traveling mostly parallel to the surface, it travels through dense atmosphere for considerably longer. When traveling through dense atmosphere it's subjected to more [scattering](https://en.wikipedia.org/wiki/Diffuse_sky_radiation), which decreases the observed intensity of the light.

Or, considered a different way, when looking at a sunset/sunrise, you don't see the sun as a single well defined point. It's hazy, with the bits of sky near to the sun also giving off a fair amount of light. That additional light is some of the light that was scattered from an otherwise direct beam. If it were all re concentrated to a single point, it would be brighter/more painful to look at (though not as bright as the noon sunlight- for shaky statistical and light absorption reasons).

[![Creative Commons image of a Sunset... by... Mario Antonio?](https://i.sstatic.net/J9DY4.jpg)](https://i.sstatic.net/J9DY4.jpg)

Using an [Earth Curvature Calculator](https://dizzib.github.io/earth/curve-calc/), sunlight at the horizon travels through ~120 miles of 'dense' atmosphere (ie has an altitude <2 miles). That will fall off rather quickly the higher above the horizon the sun is.

(dense atmosphere in this refers partly to just thicker air; but also to particulates/dust in the air, haze, mist, fog, etc)

> **Comments:**
> **username_4**: What is the relevance of *Atmospheric pressure at 2 miles is about one half of sea level pressure* in this answer, and what do you mean by *has an altitude of <2 miles*?  
> **username_0**: I think it's because the atmosphere never really ends, it just gets less and less dense. So he's using the 2 miles as a marker where there's about half the atmospheric density to say we can ignore anything from there up.  
> **username_0**: @username_3 is there an equation for this? For example, at what angle above the horizon would the sun be about 50% as bright as at noon?  
> **username_1**: @TheVoid -- Stop posting nonsense, and in this case, dangerous nonsense. It is not safe to look at the Sun for any extended period of time, even when the Sun is right on the horizon. It is not safe to look at the Sun, period, with the naked eye once the Sun is even a few degrees above the horizon.  
> **username_5**: username_1 username_1-Why do you follow my ever post and add rude comments? and who let you be a moderator?  
> **username_3**: @sandy Sadly I don't have an equation for it, though username_1 username_1's link to the wiki article on Air Mass would be good reading, and does seem to have a few equations for magnitude vs angle above horizon. And yes, 2 miles was chosen as a rough 'going straight up, we've passed through half the air particles we're going to see'.  
> **username_6**: @username_3 Other factors that contribute to the attenuation are that at sunset, much more of the sunlight is scattered away into space--not just into different parts of the sky. As well, much more light is refracted into space at sunset. That's why the sunset is red. Refraction is a different physical mechanism from scattering.  
> **username_6**: @username_3 Your point about the ratio of air distance still holds. But, "half the atmosphere" also known as 500 millibars is actually around 5500 meters, which is approx 18,000 feet, or 3.4 miles.  

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### Answer by username_0 on 2016-03-31. Score: 1

An excellent explaination found here: <https://www.youtube.com/watch?v=ZD1roP5wISU>

Goes into detail about Rayleigh dispersion.


---

