One hundred and sixty five years ago, while scanning the night sky from his home in New York, Alfred E. Thatcher saw an object with an apparent diameter three times the size of the planet Jupiter approaching the sun.
What he saw was a comet. It flew by Earth, swerved around the sun, and then exited the solar system, never to be seen again. But scientists analyzed its trajectory and orbit, and estimated that this comet, now called Comet Thatcher, will return about every 415 years.
