Worked example - when will Halley's Comet return? Halley's Comet has an orbital period of about 76 years and last reached perihelion (closest to the Sun) in 1986. Step 1: add one period: 1986 + 76 = 2062 (refined calculations give mid-2061). Step 2: note why it varies - each passage near the Sun vaporises ice unevenly, acting like tiny thrusters that slightly change the orbit. So the answer: expect it around 2061. Mark Twain was born in 1835 (a Halley year) and died in 1910 (the next Halley year) - he famously predicted he would 'go out with the comet'.