What is a meteor shower and why does it come back every year?
A meteor shower happens when Earth passes through a trail of dust left by a comet. Earth crosses the same trail at the same time each year.
A meteor shower is like a festival of shooting stars, when many more than usual appear across the sky. And the best part is that each shower comes back at about the same time every year.
Here is why. A comet travels around the Sun on a long path. As it goes, it sheds dust, rather like a car leaving a trail of crumbs behind it. Over many trips, this dust spreads out along the comet's whole path, forming a stream of dust in space.
Earth also travels around the Sun, taking one year for each trip. If Earth's path crosses the comet's dust stream, we pass through the dust at the same point every year. When we do, thousands of tiny dust bits hit our air and burn up, and we see a meteor shower. So the Perseids come every August, the Geminids every December and the Orionids every October.
All the meteors in a shower seem to come from one spot in the sky, called the radiant. Each shower is named after the constellation where its radiant is. The Perseids seem to come from Perseus, and the Leonids from Leo. It is a bit like driving through snow: the flakes all seem to come from one point ahead.