Rotation of P/Halley

The nucleus of P/Halley rotates as a slightly asymmetric top, the orientation of the rotation axis (i.e., the orientation of the angular-momentum vector) is b = -54 deg (+ or - 15 deg), l = 39 deg (+ or - 15 deg) in the ecliptic system or R.A. = 55 deg, Dec. = -36 deg in the 1950.0 equatorial system. The region of uncertainty is a cone about this axis with a half-opening angle of 15 deg. In the rotation of an asymmetric top, the rotation axis is not fixed rigidly to the body; this means that while the nucleus rotates about one short axis with the short period of 2.2 (+ or - 0.05) days, its long axis 'nods' periodically with the long period of 7.4 (+ or - 0.05) days. The amplitude of the 'nodding' is about 14 deg (+ or - 3 deg). 27 references