Meteor showers

A bright streak flashes across the night sky, and you get to make a wish. But what burns up there is not a star, it is usually a grain of dust, hardly bigger than a grain of sand. It plunges into Earth's atmosphere at several dozen kilometres per second and is slowed and heated so violently that it burns up as a glowing streak at an altitude of about 80 to 100 kilometres. While the grain is flying through space it is called a meteoroid, the streak of light is called a meteor, and if a larger chunk makes it all the way to the ground, it is a meteorite. On some nights it does not stop at a single shooting star: then dozens fall every hour, a meteor shower.

A comet's dust trail

The dust comes from comets. Every time such a dirty snowball gets close to the Sun, part of its ice evaporates and it sheds enormous numbers of dust grains. These grains do not simply fly away though, they keep orbiting the Sun, roughly on the same path as the comet itself. Visit after visit they spread out further, until after centuries the entire orbit is filled like a dusty tube. If Earth's path crosses such a tube, our planet sweeps right through it once a year and collects millions of grains on the way.

Why showers keep a calendar

Earth needs exactly one year for one lap around the Sun. It therefore reaches the crossing point with a dust trail on the same date again and again, and that is exactly why meteor showers are marked in the calendar. The famous Perseids peak around the 12th of August every year, under a dark sky up to 100 shooting stars per hour are possible then, and their grains hit the atmosphere at about 59 kilometres per second. The dust comes from comet Swift-Tuttle, which itself only passes by about every 133 years. The shower still falls every year, because the dust trail stays put even when its comet is long gone far out again. The Geminids around the 14th of December show that there is another way: their dust comes not from a comet but from the asteroid Phaethon.

The radiant, a trick of perspective

If you trace the streaks of a shower night backwards, they all seem to come from the same region of the sky, the radiant. That is pure perspective: in reality the grains fly on parallel paths, just like snowflakes on a night drive all seem to stream out of one point in front of the windscreen. Showers are named after the constellation of their radiant: the Perseids radiate from Perseus, the Geminids from Gemini. So the radiant tells you which direction the streaks come from, but they can flash up anywhere in the sky.

How to catch your shooting star

To watch a shower you need no telescope, quite the opposite: a scope shows only a tiny patch of sky, but shooting stars can appear anywhere. Find a dark spot away from street lights, lie back comfortably and give your eyes about 20 minutes to adjust to the darkness. A bright Moon washes out the faint streaks, so you see the most on a night when the Moon sets early. The best hours come after midnight: then your side of the Earth turns into our planet's direction of flight and meets the dust trail head on, just as the windscreen of a moving car catches more raindrops than the rear window. After that it is all patience, and do not forget to make a wish.