THE main problem in the radiometric programme of 1924 at the Lowell Observatory, as stated in the article in NATURE of September 19, p. 439, was the measurement of the radiation from Mars. For this purpose most of the measurements were made at the 640-inch focus of the 40-inch Lowell reflector, which gave an image of the disc of Mars about 2 mm. in diameter. The receiver of the vacuum thermocouple was 0.23 mm. in diameter, or about 1/8 of the diameter of the disc of Mars at opposition. This permitted the isolation of small areas on the disk of the planet and revealed hitherto unobserved and perhaps unexpected temperature conditions.
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COBLENTZ, W. Radiometric Determination of the Temperature of Mars in 1924. Nature 116, 472–474 (1925). https://doi.org/10.1038/116472a0