Periodic behaviour of the April Lyrids

Appearances of the April Lyrid meteor shower have been recorded since as far back as 687 BC. However, in most years the shower is of very low activity, but with bursts of activity at least one order of magnitude higher than normal occurring. During the 20th century these outbursts have occurred at time intervals that are multiples of 12 yr, and earlier outbursts have also occurred with a frequency consistent with this period. Since the orbital period of the parent comet is of the order of 400 yr,the explanation for these outbursts cannot be simply in terms of a burst of activity being seen every time the comet is close to the Earth. In this paper we produce a computer model for the stream by the ejection of test meteoroids from the nucleus of Comet C/1861 G1 (Thatcher) at past perihelion passages. We then follow the evolution of these meteoroids to the current epoch under the influence of perturbations from the major planets. Analysis of the location of the crossing points with the ecliptic suggests that a ring structure is formed, with clumps of meteoroids apparently moving around this structure with an approximate 12-yr period, and in this we feel lies the explanation to the observed periodicity that is claimed to exist in the observations of the April Lyrids.