What is the luminance of the star background, i.e. of all the objects outside of the Solar system? E.g. if we measured average luminance of the sky from the far side of the Moon (so that solar and lunar contributions, as well as Earth's atmosphere, didn't count) with the brightest planets being below the horizon, what quantity would we get?

If there haven't been any measurements of luminance, maybe illuminance in the similar conditions has been measured?

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    $\begingroup$ There are sky brightness numbers in some Wikipedia articles somewhere I remember, and one for the average Milky Way as well. But I don't remember exactly where. update: there's some information in this answer $\endgroup$
    – uhoh
    Jan 1 '21 at 0:26
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    $\begingroup$ @uhoh that answer mentions "Scattered starlight" contribution to the brightness, but I'm not sure how to understand this: is this starlight scattered by the atmosphere? or scattered by the interplanetary dust? What is the brightness of the actual unscattered light then? $\endgroup$
    – Ruslan
    Jan 1 '21 at 0:31
  • $\begingroup$ good questions and I don't know, so I just left a comment as a potential lead/starting point $\endgroup$
    – uhoh
    Jan 1 '21 at 0:41
  • $\begingroup$ @Ruslan In the answer uhoh linked to, "scattered starlight" means scattered by the Earth's atmosphere (it's for the case of mid-latitude on the surface of the Earth). The "zodiacal light" comes from Sunlight scattered off of interplanetary dust. $\endgroup$ Jan 4 '21 at 21:07
  • $\begingroup$ Note that no one in astronomy uses "luminance" (or "illuminance"), so it would be difficult to provide an answer in those terms. $\endgroup$ Jan 4 '21 at 21:19

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