<p>Albedo is the fraction of light that is reflected.  A mirror has an albedo of nearly 1, but a tiny mirror at a distance of 400000km won't do much.</p>
<p>Similarly bond albedoor geometric albedo: this is a statement about the colour of the object, and doesn't directly correspond to brightness.</p>
<p>Apparent magnitude and Apparent brightness are different ways of measuring the same thing. Apparent magnitude is a logarithmic scale, that better matches how our eyes work. These measure how bright the object is in the sky</p>
<p>Lux is a measure of how much light there is at the surface, per square meter.  If you want to define reading light, this is the easiest. But it is not a property of the object, it is measured at surface level.</p>
<p>For example, the sun has a magnitude of -27 and illuminates the ground at 100000 lux. The moon has a magnitude of -12 and illuminates the ground at less than 0.1 lux.</p>

CC BY-SA 4.0
2021-02-18T23:53:54.717Z
I want to +1 but all facts are currently *unsupported/unsourced.* I'm confident that you've got it right but if all users did this...
uhoh
CC BY-SA 4.0
2021-02-18T23:57:10.900Z
Yikes! If that means what I think it means, *good luck!*
uhoh
CC BY-SA 4.0
2021-02-19T00:48:40.313Z
If I have understood it correctly Apparent Magnitude measure on a logarithmic scale. Does that mean that the difference between say 1 and 2 on this scale is 1000 times?
Constantthin
CC BY-SA 4.0
2021-02-19T01:01:05.600Z
https://www.astro.indiana.edu/novasearch/magnitude.html The above link states that each step on the scale is just 2.5. If it really is that little I can’t understand why bother making a logarithmic expansion at all?
Constantthin
CC BY-SA 4.0
2021-02-20T08:20:00.573Z
It's a logarithmic scale because that better matches the way our vision works.  It is 2.5 to roughly fit the logarithmic scale to the pre-existing lists of stars of different magnitude.  The magnitude scale started out as a classification, it was only later turned into a scale.  and 2.5 is quite a lot. It means that there is more than double the amount of light coming from a magnitude -12 object than there is from a magnitude -11 object!
James K
CC BY-SA 4.0
2021-02-18T23:52:37.850Z
2021-02-18T23:52:37.850Z
James K
<p>Readability requires taking into account the sensitivity of the human eye to various wavelength. This is called <a href="https://en.wikipedia.org/wiki/Illuminance" rel="nofollow noreferrer">Illuminance</a>.</p>
<p>Physics knows <a href="https://en.wikipedia.org/wiki/Lumen_(unit)" rel="nofollow noreferrer">Lumen</a> (flux weighed by human perceptibility) and <a href="https://en.wikipedia.org/wiki/Lux" rel="nofollow noreferrer">Lux</a> (flux per unit area weighed by human perceptibility).</p>
<p>Quoting that, different light sources have roughly these values:</p>
<pre><code>Source             Lux
Full daylight   10,000
Overcast day     1,000
Very dark day      100
Twilight            10
Deep twilight        1
Full moon            0.1
Quarter moon         0.01
Starlight            0.001 
</code></pre>

CC BY-SA 4.0
2021-02-18T23:52:48.980Z
It's surprising that there is approximately a factor of 10 difference between quarter Moon and full Moon. Is there any info on a threshold for being able to read something (like a star map, or in this case a newspaper)?
uhoh
CC BY-SA 4.0
2021-02-18T23:44:55.183Z
2021-02-18T23:44:55.183Z
planetmaker
<p>Which unit is best to use, and why, when determining how many times easier it is to read a newspaper (or any other printed text) on the moons surface at night under a full earth, compared with reading it outdoors on  earth during a cloudless full moon night: Albedo, Bond Albedo, Apparent Brightness, Apparent Magnitude, Geometric Albedo, Lux, or something else?</p>

CC BY-SA 4.0
2021-02-18T22:56:17.097Z
Can you give some more context.  Why do you need to make this comparison.  Given that no newspaper has every managed to reach the moon, and no human has ever experienced lunar night.
James K
CC BY-SA 4.0
2021-02-18T23:15:36.447Z
@James K. I don’t need to make that comparison. It is just a hypothetical issue, like most other issues here on this site. Various comments and answers, here and elsewhere, have made me curious, that’s all. By the way, note that “newspaper” stands for text in general.
Constantthin
CC BY-SA 4.0
2021-02-18T23:23:46.490Z
@JamesK OP is right, questions do not need to justify themselves. Previous and well-received newspaper-reading-on-the-Moon question by the same OP: [Does lack of atmosphere on the moon make earth-shine more, or less, bright?](https://astronomy.stackexchange.com/q/41308/7982). However, this question seems open-ended, poorly defined and in need of being more specific. Right now it's about human vision, photometry and physical units and not really related to Astronomy or space at all. It's one of those questions where "...in space?" is added at the end arbitrarily.
uhoh
CC BY-SA 4.0
2021-02-18T23:24:56.513Z
@Constantthin if you really want to know about the best units, I'd say let's close this here and ask to have it migrated to Physics SE where I think you can get a fairly quick and helpful answer.
uhoh
CC BY-SA 4.0
2021-02-18T23:27:55.197Z
Sure they don't need to justify themselves, but if there is context, that helps. The OP has obviouly done some research, it would be helpful to include it. Thats all. I think this is quite on topic and answerable here.
James K
CC BY-SA 4.0
2021-02-18T23:38:46.327Z
Lumen: https://en.wikipedia.org/wiki/Lumen_(unit) or Lux https://en.wikipedia.org/wiki/Lux
planetmaker
CC BY-SA 4.0
2021-02-18T23:39:25.640Z
@Uhoh. I thought this question would be of interest to an Astrophysicist?
Constantthin
CC BY-SA 4.0
2021-02-18T23:42:43.430Z
@Constantthin it might be but so would a good recipe for [Tabbouleh](https://en.wikipedia.org/wiki/Tabbouleh) :-) There are [almost 200 SE sites](https://stackexchange.com/sites#name) to choose from, and each site has a well-defined scope that we all agree to stick to. If the question is not *about astronomy* as defined in the help center, then it's off-topic and should not be asked here in Astronomy SE.
uhoh
CC BY-SA 4.0
2021-02-18T23:49:15.290Z
@JamesK okay you and (at)planetmaker (below) have convinced me that it can be on-topic here, but what kind of *context* could be added? Assuming the person survives, they have 14 days of night to get through, and if they've got a newspaper about, wouldn't they *really, really want* to read it? Would it matter which newspaper it was given the particular circumstances?
uhoh
CC BY-SA 4.0
2021-02-18T23:56:15.963Z
I think the OP has answered my question  in a comment
James K
CC BY-SA 4.0
2021-02-19T00:20:32.207Z
“Theoretical” may be a better word than “hypothetical” in my first comment.
Constantthin
<the-moon><earth><visible-light>
Which unit is most suitable in comparing needed light intensity for night reading on the Moon, vs on Earth?
Constantthin
# Which unit is most suitable in comparing needed light intensity for night reading on the Moon, vs on Earth?

(Asked by: username_0 on 2021-02-18)

Which unit is best to use, and why, when determining how many times easier it is to read a newspaper (or any other printed text) on the moons surface at night under a full earth, compared with reading it outdoors on earth during a cloudless full moon night: Albedo, Bond Albedo, Apparent Brightness, Apparent Magnitude, Geometric Albedo, Lux, or something else?

**Tags:** the-moon, earth, visible-light

> **Comments:**
> **username_1**: Can you give some more context.  Why do you need to make this comparison.  Given that no newspaper has every managed to reach the moon, and no human has ever experienced lunar night.  
> **username_0**: @username_1. I don’t need to make that comparison. It is just a hypothetical issue, like most other issues here on this site. Various comments and answers, here and elsewhere, have made me curious, that’s all. By the way, note that “newspaper” stands for text in general.  
> **username_2**: @username_1 OP is right, questions do not need to justify themselves. Previous and well-received newspaper-reading-on-the-Moon question by the same OP: [Does lack of atmosphere on the moon make earth-shine more, or less, bright?](https://astronomy.stackexchange.com/q/41308/7982). However, this question seems open-ended, poorly defined and in need of being more specific. Right now it's about human vision, photometry and physical units and not really related to Astronomy or space at all. It's one of those questions where "...in space?" is added at the end arbitrarily.  
> **username_2**: @username_0 if you really want to know about the best units, I'd say let's close this here and ask to have it migrated to Physics SE where I think you can get a fairly quick and helpful answer.  
> **username_1**: Sure they don't need to justify themselves, but if there is context, that helps. The OP has obviouly done some research, it would be helpful to include it. Thats all. I think this is quite on topic and answerable here.  
> **username_3**: Lumen: https://en.wikipedia.org/wiki/Lumen_(unit) or Lux https://en.wikipedia.org/wiki/Lux  
> **username_0**: @username_2. I thought this question would be of interest to an Astrophysicist?  
> **username_2**: @username_0 it might be but so would a good recipe for [Tabbouleh](https://en.wikipedia.org/wiki/Tabbouleh) :-) There are [almost 200 SE sites](https://stackexchange.com/sites#name) to choose from, and each site has a well-defined scope that we all agree to stick to. If the question is not *about astronomy* as defined in the help center, then it's off-topic and should not be asked here in Astronomy SE.  
> **username_2**: @username_1 okay you and (at)username_3 (below) have convinced me that it can be on-topic here, but what kind of *context* could be added? Assuming the person survives, they have 14 days of night to get through, and if they've got a newspaper about, wouldn't they *really, really want* to read it? Would it matter which newspaper it was given the particular circumstances?  
> **username_1**: I think the OP has answered my question  in a comment  
> **username_0**: “Theoretical” may be a better word than “hypothetical” in my first comment.  

---

## 2 answers

### Answer by username_1 on 2021-02-18. Score: 1 | Accepted answer

Albedo is the fraction of light that is reflected. A mirror has an albedo of nearly 1, but a tiny mirror at a distance of 400000km won't do much.

Similarly bond albedoor geometric albedo: this is a statement about the colour of the object, and doesn't directly correspond to brightness.

Apparent magnitude and Apparent brightness are different ways of measuring the same thing. Apparent magnitude is a logarithmic scale, that better matches how our eyes work. These measure how bright the object is in the sky

Lux is a measure of how much light there is at the surface, per square meter. If you want to define reading light, this is the easiest. But it is not a property of the object, it is measured at surface level.

For example, the sun has a magnitude of -27 and illuminates the ground at 100000 lux. The moon has a magnitude of -12 and illuminates the ground at less than 0.1 lux.

> **Comments:**
> **username_2**: I want to +1 but all facts are currently *unsupported/unsourced.* I'm confident that you've got it right but if all users did this...  
> **username_2**: Yikes! If that means what I think it means, *good luck!*  
> **username_0**: If I have understood it correctly Apparent Magnitude measure on a logarithmic scale. Does that mean that the difference between say 1 and 2 on this scale is 1000 times?  
> **username_0**: https://www.astro.indiana.edu/novasearch/magnitude.html The above link states that each step on the scale is just 2.5. If it really is that little I can’t understand why bother making a logarithmic expansion at all?  
> **username_1**: It's a logarithmic scale because that better matches the way our vision works.  It is 2.5 to roughly fit the logarithmic scale to the pre-existing lists of stars of different magnitude.  The magnitude scale started out as a classification, it was only later turned into a scale.  and 2.5 is quite a lot. It means that there is more than double the amount of light coming from a magnitude -12 object than there is from a magnitude -11 object!  

---

### Answer by username_3 on 2021-02-18. Score: 1

Readability requires taking into account the sensitivity of the human eye to various wavelength. This is called [Illuminance](https://en.wikipedia.org/wiki/Illuminance).

Physics knows [Lumen](https://en.wikipedia.org/wiki/Lumen_(unit)) (flux weighed by human perceptibility) and [Lux](https://en.wikipedia.org/wiki/Lux) (flux per unit area weighed by human perceptibility).

Quoting that, different light sources have roughly these values:

```
Source             Lux
Full daylight   10,000
Overcast day     1,000
Very dark day      100
Twilight            10
Deep twilight        1
Full moon            0.1
Quarter moon         0.01
Starlight            0.001 

```

> **Comments:**
> **username_2**: It's surprising that there is approximately a factor of 10 difference between quarter Moon and full Moon. Is there any info on a threshold for being able to read something (like a star map, or in this case a newspaper)?  

---

