The formula that Planck presented on December 14, 1900, to the German Physics Society (date celebrated as the birthday of Quantum Physics) is the final answer to the Kirchoff's 1859 challenge to the physics community: to find the spectral formula for his relationship between emissivity e(L) = J(L, T), with absorptivity a(L) being equal to one in a perfect black body cavity. I use L as Lambda (wavelength).
Absorptivity a(L) of the sky can't be measured as it has no sense. There is no way to check if radiation emitted from here is actually absorbed or travels forever across the Universe.
So, I can't find any reasonable explanation by which, since 1963, this formula has been used to measure radiation from the sky and equate it as radiation within a perfect black body cavity with 100% absorptivity.
What is really funny (beside that both AT&T folks won the Nobel Prize) is how fast the original measurements (1963) about the absolute temperature of deep space was accepted by the astronomers community (cosmologists, astrophysicists, etc.) due to the fact that it validated an original explosion that created the Universe and that what was being measured was remnants of the original radiation left 200,000 years after a point-like mass-energy exploded.
When I analyze the historical development of facts around this initial "discovery", I find more questions than answers. What is clear is that billions of USD were expended at terrestrial and spatial observatories, as well as in research grants.
But the question remains: Why did they assimilated the Universe to a 1859 Kirchoff's Black Body cavity?