Moonshadow - in muons
2 June 1999
A shadow cast by the Moon has been detected in the Soudan 2 detector, 700 m underground the Soudan Mine in northern Minnesota. The shadow occurs in the "rain" of cosmic rays - particles from outer space which create charged particles known as muons when they collide with the Earth's atmosphere. It appears on a map of the sky as "seen" by muons through the detector. The observed shadow results from approximately 120 muons missing from a total of 33 million detected in Soudan 2 over its 10 years of operation.
The cross at the centre of the map indicates the true position of the Moon. The shadow is slightly offset, due to the small bending of the electrically charged cosmic rays in the Earth's magnetic field. (The light blue areas on the map are "noise" - statistical fluctuations in the uniform illumination of the cosmic rays.)
Although the Moon can sometimes seem to dominate the sky, it is relatively small - 0.5 degrees across in angle, or about the same width as a thumb held at arm's length. So the Moon's effect in shielding the Earth from cosmic rays, which bombard the planet from all directions, is rather like using your thumb as an umbrella!
For a Postscript version of this image see here.
See also pictures of the Soudan Mine; preparing part of the detector; and a selection of other images
Credit: Email J.Cobb, Oxford University
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