Question

Why comets appear periodically in the sky?

Answer 1

The "new and improved" orbit for the comet may no longer carry it back into deep space. It might end up in an elongated orbit that brings it back near the sun every 76 years like comet Halley. Or, perhaps, it could end up in a more circular orbit out among the asteroid "belt" where most asteroids are found (between the orbits of mars and These are the periodic comets, so called because they come back near the sun "periodically." There are a few hundred of these known, and we can easily predict when they will be visible from earth. The periodic comets are given a "periodic number" by the IAU, which is usually followed by a "P". An example is comet 2P/Encke, periodic comet number 2. Encke takes a little over 3 years to orbit once around the sun.