During the summer months of both I942 and I943, the writer and certain of his associates observed a small but measurable amount of microwave radiation coming from the sun. This appeared as random noise in the output of a conventional double-detection radio receiver designed to work at centimeter wave lengths. Other observations made at the time showed that the noise output of such a receiver was sensibly less when it was pointed at open sky than when pointed at nearby objects. A high noise level also prevailed when the receiver was located in a closed room. All of these results are in general accord with ordinary black-body radiation theory. In this connection, it is convenient to regard the room as a black-body enclosure in which the receiver is in thermal equilibrium. When the receiver is pointed at the sun, equilibrium conditions are such that more energy is absorbed than is radiated and when it is pointed at interstellar space, conditions are reversed and more energy is radiated than is absorbed.* In the latter case, the result is almost as though a sensitive pyrometer calibrated t~ work at high temperatures had been turned suddenly on a block of ice.
NATURE OF THERMAL RADIATION.
Before considering further the experiments and their results it may be well to review briefly certain well-known facts about thermal radiation and follow with an account of some of the more important steps that have led to our present knowledge of the sun's spectrum.
According to the accepted views of radiation, most solids and liquids and also a few of the gases radiate energy when heated. In order to distinguish this form of radiation from that associated with fluorescence and light from ionized gases it is generally known as temperature radiation. The relationship between the energy radiated and temperature is generally rather complicated except for a certain class of surfaces, known as black bodies. The latter are not characterized so much by their apparent color as by their ability to absorb completely a wide range of wave lengths.
* While these simple statements serve a useful purpose as regards orientation of ideas, it is quite possible that they do not apply strictly to the apparatus actually used. \Ve can be certain only that as the apparatus was pointed alternately at the sun and at interstellar space, there was a net differential in favor of energy coming from the sun. 285 * The possibility of circuit noise being radiated into space by an antenna must certainly have been entertained by many radio engineers and physicists in the past. Reber for ore' pointed out this possibility in one of the articles cited above.