Light, sound and meteor showers

Meteors are seen instantly but heard minutes later, and some return on the same dates every year. This lesson uses waves and Earth's orbit to explain both.

Seen now, heard later

Light travels at about 300,000 km/s, so the light from a meteor reaches you almost instantly. Sound in air travels at only about 340 m/s. A large fireball that slows down low in the atmosphere can produce a sonic boom, but if it is 50 km away the sound takes over two minutes to arrive. Eyewitnesses often describe seeing the fireball fade, and then, after a long silence, hearing a rumble like distant thunder.

Showers on the same dates

Comets shed dust along their orbits, forming streams of particles. Earth crosses each stream at the same point in its orbit every year, which means on about the same date. The Perseids, from Comet Swift–Tuttle, peak around 12 to 13 August. The Geminids peak around 13 to 14 December, and their parent is the asteroid Phaethon, an unusual rocky source.

The meteors in a shower travel in parallel paths, but perspective makes them seem to spread from one point, the radiant, just as parallel railway lines seem to meet in the distance.

Colours of meteors

When atoms are heated they give out light of particular colours. Sodium gives yellow-orange light, magnesium gives blue-green, and iron gives yellow. Heated nitrogen and oxygen in the air add red. Splitting a meteor's light into a spectrum, the same way a prism splits white light, tells scientists which elements a meteoroid contains.

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