Errors in the Measurement of the Temperature of the Moon

Abstract

MEASUREMENT of the temperature of the Moon is generally done by absolute radiometry assuming the emissivity of the lunar surface to be independent of wave-length and to be that of a ‘black-body’, namely, e = 1.00, and calculating the temperature from Stefan's law1. This ‘black-body’ assumption is based on the 1930 interpretations of Pettit and Nicholson2 of radiometric data they obtained with crude filter monochromators. The basis of these deductions was that the ratio of the radiant energy between 8–10µ to the radiant energy between 8–14µ observed was nearly identical to the ratio calculated for a ‘black-body’ at 400° K. The temperature of the sub-solar point (400° K) was calculated in the usual manner (assuming the emissivity to be unity) after relating all the measurements to the radiometric magnitude of a comparison star. From these paradoxical assumptions they concluded that the emissivity of the lunar surface in the region 8–10µ is unity and is unaffected by the restrahlen of any silica content it may have; this is contrary to observations made on the emissivity of silica or silicates3,4. In order to reconcile this dissimilarity with their original conclusion Pettit and Nicholson decided the lunar crust was finely divided, like sand, or porous, like pumice, and that its radiating properties would therefore be nearly those of a ‘black-body’.