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1667 or 1679 The inferred date of the supernova explosion which produced the Cassiopeia (Cas) A (SNR 111.7-02.1) supernova remnant, the last supernova known to have occurred in our (Milky Way) Galaxy. It has been somewhat puzzling to modern astronomers as to why there are no definitive observations by either contemporaneous eastern or western astronomers of this supernova, although it has been suggested that Flamsteed may have seen it circa 1680, e.g., by Ashworth (1980, Journal for the History of Astronomy, 11, 1), as there is a star marked in John Bevis's Uranographia Britannica (which was created in the 18th century) at the position of Cas A.

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A bright comet had appeared on 14 November 1680. It remained visible until 5 December 1680 when it moved too close to the Sun to be observed. It reappeared two weeks later moving away from the Sun along almost the same path along which it had approached. Newton found good agreement between its orbit and a parabola. He uses the orbit of this comet, and comets in general, to support his inverse square law of gravitation in the Principia.