Why does Halley’s Comet return at slightly different intervals?

Although Halley’s Comet returns to the inner solar system on average every 76 years, its orbital period is not fixed. The interval between its returns varies from approximately 74.5 to slightly more than 79 years.1

This variation is primarily caused by the gravitational influence of the massive planets in our solar system, such as Jupiter. As the comet travels along its highly elongated orbit, these planets exert forces that perturb its motion, effectively speeding it up or slowing it down.2

The ability to account for these gravitational perturbations was a significant milestone in the history of science. When astronomers calculated the comet's return in the 18th century, they had to incorporate these complex forces into their models to predict the arrival date accurately, which served as a major triumph for Newtonian theory.2