Observations of the Lunar Eclipse of 4 April 2015 with Grove Creek Radio Telescope

Joachim Köppen DF3GJ Kiel 2015

Radio emission from the Moon was first picked up in the late 1940. It is the thermal radiation produced by the lunar surface material which heats up under sunlight. In 1949, soon after the detection, Piddington and Minnett (Australian Journal of Scientific Research A, Vol. 2, p 63) found out that contrary to the naive expectation that the Moon would appear to be warmest at Full Moon, the surface temperature does not peak until 4 to 5 days later. The reason is that the lunar surface consists not of solid rock, but of small pebbles, sand, and dust. Thus the heat from the top layers passes only slowly to the other layers, and the surface as a whole takes a longer time to heat up.
During a lunar eclipse the Moon's surface is shielded from sunlight, albeit for only a few hours. So it is interesting to observe what happens to the radio brightness of the Moon. This is best done at high frequencies, such as 10 GHz, as the Moon is brighter than on longer waves. But the Small Radio Telescope at Grove Creek Observatory operates on 1.4 GHz, where the Moon is just detectable. Nonetheless, let's give it a try ...
To capture the weak radio noise from the Moon on a wavelength of 21 cm, the beamswitching technique is used: The telescope is pointed to the Moon, then to the empty sky left of the Moon, then back to the Moon, then to the sky on the right hand side, and so on ... Everytime the telescope points to the Moon, it receives noise slightly higher than when it points to the sky. The plot shows the difference between the Moon and the sky to the left (blue) and to the right (green). While the signal level itself amounts to about 4500 counts, there exists a genuine difference of about 30 counts, which is only 0.07 percent of the signal. The plot shows that before UT 12:00 there is always such an excess signal, and the same from the sky measureument at either side, despite the appreciable fluctuations of about 10 counts.
The three red marks indicate the first contact of the Moon with the Earth shadow, the time of maximum eclipse, and the last contact with the shadow. Until maximum eclipse the signal remains constant, so that the obscuration of the lunar surface by the shadow has no effect on the radio noise level, hence on the lunar surface temperature. Unfortunately, after maximum eclipse the signal gets unstable most probably due to an increase of the local electronic noise level (see below).
This tracing of the measured signals at the Moon (red), to the left (blue), and to the right (green) shows that at about UT 12:30 the signal level increased quite rapidly. Such a drastic change indicates an increase of electronic noise. In the data there is no evidence for any radio interference by distinct signals, as it may happen at times. The telescope pointed high in the sky, and not to the nearby observatory building which contains computers. Their presence would have been obvious by their spectral signatures. Thus, there is no identifyable cause for the rise of electronic noise, it just happens as it sometimes does!
Despite the difficulties in measuring the weak signal from the Moon, and despite the obvious occurance of the rise of local electronic noise, the observations confirm that the radio brightness of the Moon does not drop during the eclipse: shadowing the lunar surface from the sunlight for few hours is too short for the temperature of the lunar soil to fall measurably.
While a thermometer placed on the surface of a rock or pebble would certainly indicate a drop in local temperature, the emission from the entire surface layer of the whole visible hemisphere hardly changes. The instruments deposited on the surface by the Apollo 15 lunar landing measured temperatures during an eclipse. To quote from the Apollo 15 Mission Report: Six days after startup, on August 6 [1971], the experiment package was subjected to its first lunar eclipse. This was a total eclipse and the package was closer to the center of the umbra than any previous Apollo lunar surface experiments package during any previous eclipse. During the eclipse, sun shield temperature of the central station dropped from plus 140 F [60ºC] to minus 143 F [-97ºC] with accompanying rates of change of temperatures up to 260 F [144ºC] per hour. [...] The instrument measured a lunar surface temperature change of 330 F [180ºC] during the eclipse.

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