1
$\begingroup$

If the Moon is visible against a clear blue sky with a lux value of around 30,000 does it stand to reason that the lunar surface must be brighter than that?

$\endgroup$
1
$\begingroup$

It is better to use the unit of luminance, cd/m2. Wikipedia gives examples of luminances:
2.5 kcd/m2 Moon surface;
5 kcd/m2 Typical photographic scene in full sunlight;
7 kcd/m2 Average clear sky;
10 kcd/m2 White illuminated cloud.

These values imply that the luminance of the moon surface is less than that of the clear sky, even though the moon is visible against the sky. The lunar highlands are clearly visible against the sky; the mare are less visible.

Surface brightness does not depend on distance, so in the photo below the brightness of the moon can be directly compared to that of the sky and terrestrial rock. No correction for distance is needed. Evidently, the brightness of the lunar highlands (albedo 0.2) is comparable to rock.

enter image description here Photo from wikimedia commons

$\endgroup$
10
  • $\begingroup$ Thanks for the replay. However: $\endgroup$
    – Solon
    Jan 12 '20 at 0:11
  • $\begingroup$ Thanks for the replay. However:"Surface brightness does not depend on distance".is dependant on distance if you use an online calculator. "2.5 kcd/m2 Moon surface;" How is this figure arrived at? Calculated using the solar constant and lunar albedo or measured at the lunar surface? $\endgroup$
    – Solon
    Jan 12 '20 at 0:20
  • $\begingroup$ @Solon The photo is a striking and thoroughly convincing illustration that surface brightness is independent of distance, no matter what you may conclude from an online calculator. And the Moon is at the essentially the same distance from the Sun as we are. $\endgroup$
    – user137289
    Jan 12 '20 at 0:26
  • $\begingroup$ @Pieter. Looking at the Apollo mission video cameras it can be seen that the expected light levels were very low and NASA had to get permission from the D.O.D to be able to use the then classified very low light level capable SEC vidicon tube on Apollo 11 onwards. Why would they need very low light level cameras? hq.nasa.gov/alsj/ApolloTV-Acrobat7.pdf $\endgroup$
    – Solon
    Jan 12 '20 at 19:03
  • $\begingroup$ @Solon For larger dynamic range, because shadows are very dark on the Moon. $\endgroup$
    – user137289
    Jan 12 '20 at 21:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.