SIMBAD references

Published images of the Crab Nebula have been scanned and measured to derive proper motions, the age of the nebula, and its point of origin. Astrometric reductions provided rms errors ranging from 0".14 to 0".31 in both axes for a given image. Proper motions of 50 optical filaments with a 53 yr baseline projected backward indicates the mean date of the supernova event as A.D. 1130±16 yr, in good agreement with previous investigations by Trimble and Wyckoff & Murray but 76 yr after the reported outburst recorded by the Chinese astronomers in A.D. 1054. This result confirms the well-known acceleration in the Crab's expansion. The image-scanning technique demonstrates that accurate astrometric reductions can be achieved on a personal computer.