Our Astronomical Column

Abstract

MEASUREMENTS OF LUNAR RADIANT HEAT.—Numerous measurements of lunar radiant heat have been made at Birr Castle Observatory by Lord Rosse and Dr. Copeland, and the results obtained have been published from time to time. During the total lunar eclipse of October 4, 1884, Dr. Otto Boeddicker, Lord Rose's present assistant, carried out a series of observations for the purpose of testing the striking result previously arrived at by Dr. Copeland, viz., that “the maximum of heat seemed to occur somewhat before full moon.” It was then found that “The heat as before diminished, and increased again nearly proportionally to the light, becoming inappreciable on reaching the limits of totality. The minimum of heat apparently fell later than that of illumination. But the most remarkable thing was that while during the short interval between the first contact with the penumbra and the commencement of total phase, all appreciable radiation vanished, between the end of total phase and the last contact with the penumbra, and even forty minutes later, the heat had not returned to the standard for full moon, being deficient by about 12 per cent.” These facts are remarked upon by Lord Rosse in an introduction to a paper by Dr. Boeddicker, giving the results obtained during the lunar eclipse of January 28, 1888 (Transactions of the Royal Dublin Society, Series III., vol. iv., Part ix., 1891). The measurements of radiation were commenced about an hour before the first contact with the penumbra, and a decrease of heat seems even then to have set in. But excluding this diminution of heat exhibited by the curve connecting the observations, there is indisputable evidence that the decrease had definitely commenced about three minutes before the eclipse began, and probably fifteen minutes before. This indicates, therefore, that the terrestrial atmosphere extends to a height of not less than 190 miles, and intercepts the sun's rays before any part of the moon has entered the earth's shadow. In 1888, as in 1884, the anomaly of the heat not returning to its standard value even I hour 40 minutes after the last contact with the penumbra, was observed. Dr. Boeddicker enumerates the series of observations required to elucidate these interesting points, and hopes soon to publish some further results of his investigations.