Data from an extensive air shower detector of ultrahigh-energy cosmic rays shows shadowing of the cosmic-ray flux by the Moon and the Sun with signficance of 4.9 standard deviations. This is the first observation of such shadowing. The effect has been used to determine that the angular resolution of the detector is 0.75${\mathrm{°}}_{-0.09\mathrm{}\mathrm{°}}^{+0.13\mathrm{}\mathrm{°}}$.

• Received 18 June 1990

DOI:https://doi.org/10.1103/PhysRevD.43.1735