Abstract
A new orbit is calculated for Comet C/1861 G1 (Thatcher), associated with the Lyrid meteor shower, to replace Oppolzer's orbit of 1864. The new orbit is based upon 649 observations, 326 in right ascension and 323 in declination, made between 11 April 1861 and 7 Sept. 1861. The final orbit uses residuals calculated with the Welsch weighting function. The comet's period of 416.87 ± 0.56 yr agrees with Oppolzer's period of 415 yr athough other elements such as the inclination differ. Although the post-perihelion residuals are relatively random, 52.1% probability of randomness, pre-perihelion residuals lack randomness indicating possible deviations from Keplerian motion caused by ejection of meteoritic material. Comet Thatcher is unrelated to the Great comet of 1861.
