Abstract
The variation of the absolute magnitude H0 of Halley's comet, 1910II, as a function of the apparition number, A, (1910 ≡ −1) is given by $$H_0=(6.028\pm0.329)+(0.0205\pm 0.0210)A$$. The comet was about 0.5 ± 0.5 mag brighter 2000 years ago and is losing about 2.3 ± 2.3 per cent of its mass at each apparition.