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Extra-Galactic Sources



The Moon

The moon is a solid body with no significant atmosphere; it therefore radiates as a cool solid body (thermal radiation) and is not easy to detect at long wavelengths. It is possible for the amateur to make observations with access to an old C band (4 - 8 GHz) satellite TV antenna with a diameter of a few metres as shown in Figure 5.1.


The typical beam width of a 3m diameter dish at 4GHz is ~ 20 - rather larger than the angular diameter of the moon at 0.5°. The moon will therefore present almost a point source to the antenna and a transit scan across the moon will produce a trace with the properties of the antenna beam: i.e. a detection that is about 2° across. However it is still an interesting exercise to attempt to detect a cold, purely thermal radiating body.


Figure 5.1 3m dish with C band feed

The moon has no significant magnetic field and no ionised gaseous atmosphere containing free electrons, so there is no mechanism to generate non-thermal radio emissions. The thermal signal from the Moon is quite low and a fairly good receiver is needed to make a successful detection.

Figure 5.2 Detection of thermal emission from the Moon

© Dr David Morgan 2011