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?
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$\begingroup$ 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. $\endgroup$– James KFeb 18, 2021 at 22:56
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$\begingroup$ @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. $\endgroup$– ConstantthinFeb 18, 2021 at 23:15
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$\begingroup$ @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?. 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. $\endgroup$– uhohFeb 18, 2021 at 23:23
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$\begingroup$ @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. $\endgroup$– uhohFeb 18, 2021 at 23:24
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$\begingroup$ 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. $\endgroup$– James KFeb 18, 2021 at 23:27
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2 Answers
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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.
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$\begingroup$ 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... $\endgroup$– uhohFeb 18, 2021 at 23:53
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$\begingroup$ Yikes! If that means what I think it means, good luck! $\endgroup$– uhohFeb 18, 2021 at 23:57
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$\begingroup$ 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? $\endgroup$ Feb 19, 2021 at 0:48
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$\begingroup$ 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? $\endgroup$ Feb 19, 2021 at 1:01
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$\begingroup$ 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! $\endgroup$– James KFeb 20, 2021 at 8:20
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Readability requires taking into account the sensitivity of the human eye to various wavelength. This is called Illuminance.
Physics knows Lumen (flux weighed by human perceptibility) and 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
answered Feb 18, 2021 at 23:44
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$\begingroup$ 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)? $\endgroup$– uhohFeb 18, 2021 at 23:52