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  • $\begingroup$ Depending on the size of the object, there are things of thermal significance four meters above the surface of the Moon that you did not mention, which are the interplanetary medium and the Moon's very, very thin atmosphere. These too are way out of equilibrium. $\endgroup$
    – David Hammen
    Commented Feb 29 at 15:36
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    $\begingroup$ @DavidHammen Both are too thin to matter. We don't even include the thicker exosphere in LEO in spacecraft thermal balance calculations. $\endgroup$
    – John Doty
    Commented Feb 29 at 15:38
  • $\begingroup$ They're not too thin to matter if your "thermometer" is the size of an atom. Even the very, very thin intergalactic medium has a "temperature", and it is not 3 kelvins. It is instead in the tens of millions of kelvins, if not higher. That very, very thin intergalactic medium can support sound waves, but only at extremely low frequencies. Astronomers have even detected those extremely low frequency sound waves propagating through that very sparse medium. $\endgroup$
    – David Hammen
    Commented Feb 29 at 17:05
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    $\begingroup$ @DavidHammen Well, sure, for a tiny enough thermometer with negligible capability to interact electromagnetically. As for the intergalactic medium, I helped show its emission is thermal almost half a century ago articles.adsabs.harvard.edu/pdf/1976ApJ...205L..65S. $\endgroup$
    – John Doty
    Commented Feb 29 at 17:18