= Solution
Measure the absolute <spectral radiance> over a range of <frequencies> and fit the <Planck spectrum>
$$
B_\nu(T)=\frac{2h\nu^3}{c^2}\frac1{e^{h\nu/(k_BT)}-1}.
$$
For example, the <COBE FIRAS blackbody spectrum experiment> used a differential <Michelson interferometer> to compare the sky with an internal <blackbody>; an external calibrated <blackbody> could cover the sky entrance. Cold <bolometers> measured the difference spectrum, and foreground separation distinguished <Milky Way> emission from the nearly isotropic background. The fitted background temperature is approximately $\boxed{T_0=2.73\,\mathrm K}$.<The COBE Far Infrared Absolute Spectrophotometer by NASA LAMBDA>[Instrument overview: differential internal reference, external calibrator and absolute CMB temperature]{ref} The result relies on an absolute spectral comparison, rather than merely measuring a map's hot-to-cold contrast.
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