Halley's Comet  StarDate: December 25, 2008   audio/mpeg icon
Edmund Halley's resume is one of the most impressive in the
history of science. He compiled the first atlas of the stars of
the southern hemisphere. He measured Earth's magnetic field, and
even invented the basis for life insurance.  Yet Halley's
enduring claim to fame was solidified well after his death. In
fact, it happened 250 years ago today.  Halley was a friend of
Isaac Newton, who had devised laws of motion and gravity. The
planets and moons all followed these laws just fine, but no one
was sure if comets would do the same thing.  Comets are
mountain-sized balls of rock and frozen gases that grow long
tails as they approach the Sun. In Halley's time, though, their
nature was poorly understood. In fact, no one was sure if they
continuously orbited the Sun, as planets do, or if they
disappeared into space after a single appearance.  Halley
plotted the motions of about two dozen comets that had appeared
in recent centuries. He found that the c