Friday, June 4, 1999 Published at 09:01 GMT 10:01 UK
Moon's underground shadow
The Moon casts a muon shadow - underground
By BBC News Online Science Editor Dr David Whitehouse
A shadow cast by the Moon has been detected 700 m underground.
The Soudan Mine in northern Minnesota contains a detector sensitive to sub-atomic particles called muons. Having the detector underground means that it is isolated from interference that it would be plagued with on the surface.
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 the Soudan mines detectors 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.
Although the Moon can sometimes seem to dominate the sky, it is relatively small - 0.5 degrees 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!
The Soudan 2 detector is run by a team from the Argonne National Laboratory, and Minnesota, Tufts and Western Washington Universities US, and Oxford University and the Rutherford Appleton Laboratory in the UK.
The detector, key features of which were developed at Oxford University, was built originally to search for the decay of protons, although no evidence for this phenomenon has been found.
However, the detector is ideal for detecting muons that penetrate hundreds of metres below ground.