Sir, - Professor Ray says (March 25th) that Hale Bopp will return in about 2,200 years, not 4,200. But he also says that the last time it "dropped in was about 2200 B.C. If its orbital time is only 2,200 years, why was it not here for 4,200 years? - Yours, etc.,
Downings,
Co. Donegal.