Halley's Comet's last appearance was in 1986, and its average period of revolution around the Sun is 76 years. But it turns out that the gravitational pull of the giant planets creates variations in that period of up to a few years (so the time between successive appearances isn't always exactly 76 years). Its next appearance should be in 2061.
Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.