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Wikipedia's opposition surge is a short article and forwards shadow hiding and coherent backscattering as proposed mechanisms, but it doesn't really explain how much the brightness of the Moon actually "surges" and how quickly it happens.

I was reminded of this by this interesting answer.

Question: Are there measurements and good quality plots of lunar opposition surge? How fast is it? How long does it last?


I don't think the coherent backscattering is the predominant mechanism for visible light; it's primarily seen in radar1. I'll be happy to be proven wrong, but the shadow-hiding seems to be pretty easy to embrace in photographs.

Images from https://space.stackexchange.com/q/20071/12102 and answers there. First is taken by the Yutu rover on the Moon (Chang'e 3 mission) second is a famous image (it's cropped, a reflection in a visor of the photographer, first lunar selfie!) and the third is from What causes Hyabusa-2's close-up images of Ryugu to be dark in the corners?

See also

Yutu rover on the Moon shadow Apollo 11 famous visor reflection photo and shadow Hayabusa 2 shadow

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The lunar opposition surge has been well studied, likely because we can study it in detail, we have surface samples, and so it serves as a baseline for other bodies in the solar system (as it does for many other kinds of surface studies). It is quite substantial in visible light.

Probably Clementine data are the oldest-modern reference, and among others, Buratti et al. showed that from a baseline normalized reflectance of ~0.6 at a phase angle of 10°, it increased to 1.0 at a phase angle near 0° (hence, normalized reflectance). Their Fig. 6 shows that this increase happens over just the last few degrees, which is typical of opposition surges. Shkuratov et al. (1999) also published based on Clementine data and their Fig. 5 again shows a ~10s% increase over the last ~10° of phase angle.

What you really want is an expanded plot of just the last few degrees of phase angle, which they include in their Fig. 10 (I'm not reproducing the images here due to © concerns). They show that the Buratti et al. (1996) results increase by ~20% in brightness when going from just a phase angle of 0.5° to 0°. The paper compares this to numerous other materials. That's all in UV to visible light.

Velikodsky et al. (2016) also looked at this from LROC-WAC data. For the UV (415 nm) and red (689 nm) bands, they show (Fig. 6) that from a phase angle of 1° to 0°, the increase can be around 50%. Going from 5° to 0° in a different part of their paper (Fig. 9), the increase can be a factor of ~35–45%. This is again wavelength- and material-dependent. I would say that that paper is probably going to be the best quality plots, or at least a paper that uses LROC-WAC data.

As the Shkuratov et al. (1999) paper notes: "Below about 10° the brightness of the Moon grows more sharply than at large phase angles. The lunar opposition effect is about 1.25 at 1°–10°." So it could be considered that the "effect" starts <10°.

Those all seem to tell a reasonably consistent story that, within those last few degrees, the phase function increases the visible light returned by up to about 50%.

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  • $\begingroup$ Wow! Excellent, thorough, and well sourced answer, thanks! Hopefully it will receive an equally dramatic surge in up votes :-) $\endgroup$
    – uhoh
    Mar 30 at 4:53
  • $\begingroup$ Searching Clementine reveals quite a fascinating mission, I've added a Celmentine link in the question because interestingly it mentions "coherent backscatter opposition effect" in relation to its bistatic radar experiments! fyi I have just asked Has lunar opposition surge ever been observed from Earth? From Earth orbit? The more I think about how it would have to be done, the more interesting this becomes. (e.g. this) $\endgroup$
    – uhoh
    Mar 30 at 5:11

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