Observation of a shadow of the Moon in the underground muon flux in the Soudan 2 detector

A shadow of the Moon, with a statistical significance of 5σ, has been observed in the underground muon flux at a depth of 2090 mwe using the Soudan 2 detector. The angular resolution of the detector is well described by a Gaussian with a sigma <~0.3^∘. The position of the shadow confirms that the alignment of the detector is known to better than 0.15^∘ and has remained stable during ten years of data taking.