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The astrolabe is the item stolen in The Dead of Night. Astrolabe is an anagram for "a stale orb". It is revealed that it was a part of Machina Fini Mundi, a machine designed by Archimedes. Vesper One uses it by altering the earth's magnetic poles and using the major subduction zones of the world to cause disasters such as earthquakes, tsunamis, etc, by building a dooms day device.
The astrolabe is a bit like a compass.