Lunar occultations
 
Accurate positions and the angular extent of some radio sources have been obtained from observations at lunar occultations. As a radio source is occulted, the Moon’s limb acts as a diffracting edge. During

Figure1. Parkes steerable 210 ft radio telescope, Australia. (By courtesy of the Royal Astronomical Society.)

Figure 2. The 1000 ft radar-radio telescope (Cornell University) at Arecibo, Puerto Rico. (By courtesy of the NAIC-Arecibo Observatory, a facility of the NSF.)

Figure 3. The Ryle Telescope in compact configuration. (Photograph by Keith Papworth, reproduced courtesy of Mullard Radio Astronomy Observatory.)

Figure 4. A simple helical antenna.
the progress of the occultation, the diffraction pattern is effectively scanned across the aperture of the telescope and measurement of the variations of the strength of the signal with time allow the size and position of the source to be determined with good precision.
The typical scale of a fringe on the Earth’s surface is given by

Because of the wavelength values in the radio region, the timescales for the passage of the passage of the pattern over a telescope are of the order of several minutes relative to the millisec times for measurements undertaken by optical telescopes. This method of pinpointing a source is, however, limited to the band of sky through which the Moon appears to travel.