Meteors are cometary débris. They are small and friable – usually no more than of centimetre size – and so never reach the Earth’s surface intact. There are many well-defined showers, associated with comets which can often be identified; other meteors are sporadic, not associated with any known comet, and so may appear from any direction at any moment. Meteors can, of course, occur in daylight, as was pointed out by the Roman philosopher Seneca about 20 AD, and may be tracked by radio and radar.