ABSTRACT
But unless we include Lexell's Comet, which has been so roughly treated by Jupiter, all the parent comets Listed here, apart from Thatcher's, have relatively short periods. We see Lyrids every year, even though tbe ZHR is variable. This means that there must be debris spread all round the orbit, and this in tum means that Thatcher's Comet has made many revolutions round the Sun. Since it takes over four centuries to complete one orbit, we may therefore assume that it is very ancient indeed.
