ON JULY 19th 1862 Horace Parnell Tuttle, an assistant at Harvard
College Observatory, spotted a comet in the constellation of
Camelopardalis. After he announced his discovery, Lewis Swift,
an observer from Marathon, New York, piped up to say that he had
seen it three days earlier. After a fair amount of to and fro,
the comet came to be called Swift-Tuttle. Or now, more fully,
109P Swift-Tuttle. The alphanumeric addition recognises it as
the 109th comet to have been shown to be in an orbit that P-for-
periodically brings it back into the view of Earth's
astronomers.  Brian Marsden always liked the alphanumerics. As
the head of the Central Bureau for Astronomic Telegrams of the
International Astronomical Union (IAU), adjudicating between
discoverers on the naming of comets was part of his job, and
observers with a shot at immortality could get fractious over
the process. The numbers were cleaner. In the case of Swift-
Tuttle he might be allowed a particular bent towards the
numeric: the eventual return that made