The concept of apparent black-body temperature is used to describe the radiation received from the moon and the planets. The received radiation is compared with the radiation from a hypothetical black-body which subtends the same solid angle as the visible disk of the planet. The apparent black-body disk temperature is the temperature which must be assumed for the black body in order that the intensity of its radiation should equal that of the observed radiation. The use of this concept does not specify the origin of the radiation, and only if the planet really radiates as a black body, will the apparent black-body temperature correspond to the physical temperature of the emitting material.

The radio radiation of the sun which is reflected from the moon and planets should be negligible compared with their thermal emission at centimeter wave lengths, except possibly at times of exceptional outbursts of solar radio noise. The quiescent level of centimeter wave-length solar radiation would increase the average disk brightness temperature by less than 1 ` K. At meter wave lengths and increase of the order of 10 ` K in the average disk temperatures of the nearer planets would be expected. Therefore, neglecting the extreme outbursts, reflected solar radiation is not expected to cause sizable errors in the measurements of planetary radiation in the centimeter -- and decimeter wave-length range.