It may have turned out to be accurate, but for me it was still a little guesswork. From what you said, am I correct in thinking that (if we assume that the max temp of the lunar surface is 250o C), once the surface reaches that temperature, it's radiating all the energy that strikes it? With some probably fairly small loss to conduction to the lower layers?
The lunar surface doesn't get
quite that hot. It gets to a little over +100C on the equator at local noon, hot enough to still cause problems.
The surface always radiates all the energy that strikes it, either at the same wavelength (i.e., reflection) or by re-radiation at longer wavelengths you can't see. Conditions on the moon change slowly enough (the sun moves at only 0.5 degrees/hr, vs 15/hr for the earth) and the loose, well-insulated surface layer has so little thermal inertia, that it's always in near-perfect equilibrium. The sun's intensity is constant but the heat input varies as the sine of the sun's elevation angle.
But when an object is not in equilibrium, the imbalance goes to heating or cooling it, yes.
The Apollo heat flow experiments showed that the surface has very poor heat conductivity (it's a loose powder, remember) and you only have to go about a meter down before the temperature is nearly constant throughout the month. The moon has some internal heat from radioactive decay that still comes out but it's tiny relative to the solar fluxes.
You control the temperature of a spacecraft both actively (with heaters, coolers, etc) and passively, by selecting the absorptivity (a) and emissivity (e) of its surface coatings. To stay cool in sunlight, you want a low absorptivity (looks light in visible/near IR) so it reflects most of the sunlight and a high emissivity (looks dark in far IR) so it efficiently radiates its own heat. Examples of materials with low a/e ratios are aluminized Kapton, Teflon or Mylar with the aluminum on the rear surface (e.g., the blankets on the LM). To collect heat you want something with a high a/e, e.g., polished gold.
But there's a catch: high emissivity also means that it
absorbs very well in far IR. Doesn't the sun also radiate a lot there? Yes, but it's so hot (6000 K) that it radiates far more in the visible/near-IR. And because it occupies a very small fraction of the sky at 1 AU, we can pretty much ignore the sun's longwave IR.
But the moon appears quite big to someone standing on it, so it hits you with a
lot of longwave IR at local noon. You'd reach its temperature if you were in thermal equilbrium with it, so this is a real problem. Apollo avoided this problem by arriving shortly after sunrise and leaving well before noon, but the ALSEP experiments had to deal with it.
Why does the moon get so much hotter than the earth at local noon, given the same amount of sunlight? Because the moon lacks an atmosphere to redistribute heat. This also provides our solution: just hide from the lunar surface behind a reflector so we see only the sky. Even with the sun in that sky we can stay cool with a low a/e. If you look at the ALSEP experiments, particularly the central stations, you'll see reflectors doing exactly this. Some experiments overheated because the astronauts could not keep those reflectors clean.
