Sir, In reply to Dr. Lucas's query (April 1st) regarding the orbit of Comet Hale Bopp, it is true that the last time the comet "dropped in" to our part of the Solar System was 2200 B.C. So its old period was, as Dr. Lucas correctly deduced, 4,200 years.
It is, however, in a very unstable orbit, as I pointed out in my letter of March 25th, being particularly susceptible to the gravitational pull of the giant planet Jupiter. Hale Bopp swung by Jupiter in March 1996 thereby altering its orbit so that its new period is now 2,400 years. It will therefore return, as I said, in 4400 A.D.
Your readers may be interested to know that Comet Hale Bopp is probably the third brightest comet to grace our skies in the last 600 years. Only Tycho's Comet and Comet Sarabat appear to have been brighter. We live in interesting times! - Yours etc.,
Dublin Institute for Advanced
Studies,
5 Merrion Square,
Dublin 2.