every 76 years once ever 76 years https://www.astrosociety.org/edu/publications/tnl/01/01.html https://www.astrosociety.org/edu/publications/tnl/01/01.html https://www.astrosociety.org/edu/publications/tnl/01/01.html Halley’s Comet Seventy-six years after it last appeared