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R. A. Fesen (Dartmouth College)
A recent proper motion study of 40 knots in the Cas A supernova remnant indicated a knot convergent date of A.D. 1671.3 ±0.9 assuming no deceleration (Thorstensen, Fesen, & van den Bergh 2001, AJ, 122, 297). However, because these optical knots are made visible by their shock passage through the local ISM/CSM, some deceleration is expected. A deceleration of just ~1.6 km s-1 yr-1 over a 300 yr time span would yield an explosion date around A.D. 1680, consistent with a suspected sighting of the Cas A supernova by J. Flamsteed in August 1680 (Ashworth, 1980, J. Hist. Astron., 11, 1). We discuss Flamsteed's likely observations of SN 1680 in terms of their constrains on the light curve and peak brightness and possible implications regarding the Cas A SN subtype.