By taking the size of the Oort cloud

Which comets have long periods?

Long-period comets tend to be the most spectacular comets we see in the night sky, with the two most recent ‘great’ comets – comet Hale-Bopp (1997) and comet Hyakutake (1996) – having predicted orbital periods of several thousand years.

How many periodic comets have been discovered?

Thousands of comets have been seen by astronomers, but only around 190 are classified as periodic. The most famous periodic comet is Halley’s Comet, which returns every 76 years.