Radio Observations of a Lunar Occultation of the Crab Nebula

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Abstract
Observations were made at wave-lengths of 3·7 m and 7·9 m of the lunar occultation of the Crab Nebula on 1956 January 24. At 3·7 m, observations were made throughout the occultation, and the distribution of radio brightness across the nebula was derived. The position of the radio source agrees closely with that of the optical nebula but its size is somewhat larger. At 7·9 m, severe man-made interference occurred during the emersion but observation of the immersion suggested that at this wave-length the source was considerably more extended. Recent optical evidence has suggested that the emission from the Crab Nebula is due to the acceleration of high energy electrons in weak magnetic fields; a variation of the spectral distribution of the radio emission from centre to limb may be associated with a radial variation in the energy spectrum of the high energy particles or of the magnetic field. By comparing the time of obscuration at 3·7 m with the computed value, an estimate was made of the refraction occurring in the lunar atmosphere; an electron density at the Moon's surface of 104 cm–3 was derived, a figure which corresponds to a surface density of the lunar atmosphere about 10–13 of that of the Earth's.

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