The intermediate comets and nongravitational effects

The motions of the intermediate-period comets Pons-Brooks, Olbers, Brorsen-Metcalf, and Westphal are investigated over their observed intervals. The three apparitions of comets Pons-Brooks and Olbers were successfully linked, using the now standard nongravitational-force model. The two apparitions of Comet Brorsen-Metcalf were successfully linked without the need for nongravitational effects. For the 1852 and 1913 apparitions of Comet Westphal, complete success was not achieved in modeling the comet's motion either with or without nongravitational effects. However, by including these effects, the comet's astrometric observations could be represented significantly better than if they were assumed inoperative. Comet Westphal's dynamic and photometric behavior suggests its complete disintegration before reaching perihelion in 1913. If the very large radial nongravitational parameter determined for Comet Westphal is due to the comet's disintegration into dust, then the resultant dust-particle size is of the order of 0.8 mm.