Shadow of the Moon

Viewed from the Earth, the Moon subtends an angle of 1/2 degree. Cosmic rays coming at us from this direction are absorbed by the Moon and do not reach the earth. So if looks at the region surrounding the Moon one will observe a deficit of cosmic rays. However, the earth's magnetic field bends the cosmic rays that do reach us (like a magnetic spectrometer the bending depends upon the momentum and the charge of the incoming cosmic ray) so that the position of the shadow is displaced from the direction of the Moon. This displacement can be used to calibrate the absolute energy response of the detector and to search for an antimatter component to the cosmic rays. Antimatter cosmic rays will cast a shadow on the opposite side of the Moon from the shadow cast by cosmic rays. The figure below illustrates how the Moon casts a shadow in the cosmic ray flux.

The picture below is data taken with the Milagrito detector in the region around the Moon. The data has been background subtracted and smoothed by averaging the event density over a 1 degree circle. The shadow is evident as the dark blue region to the southwest of the center of the figure. The scale on the right is in standard deviations, which is related to the statistical significance of the deficit.