The American Kepler
Across the Atlantic, with the element of national pride removed, there was less enthusiasm than in the U.S. On the one hand, David Brewster in his 1850 presidential address to the British Association for the Advancement of Science applauded Kirkwood's discovery as a work of genius. On the other hand, Heinrich Schumacher, editor of the Astronomische Nachrichten, where Walker had published his letter, declined comment on the subject beyond a scathing note suggesting that readers could judge for themselves "how much . . . is really factual and how much is simply hypothetical." Most European astronomers seem to have sided with Schumacher, if, indeed, they commented on the matter at all.
Eventually, however, all the high praise died down, and, it seems, less and less was heard about Kirkwood's analogy, until today it is all but completely forgotten.
Still, Kirkwood received an honorary doctorate from the University of Pennsylvania in 1852 and appointment to the chair of mathematics at Indiana University in 1856, events that one presumes probably would not have happened without all the previous praise and publicity.
For one so modest and diffident, it is remarkable that the setback in fortune of the n-D relation did nothing to dissuade Kirkwood from publishing later research. Three books and well over 100 papers came later. The most famous paper was published in 1866 when he examined the statistics of the 80 or 90 asteroids then known and found that essentially none of them was located where its period of revolution about the sun would have been a simple fraction (one-half, one-third, three-fifths etc.) of Jupiter's period. These were the now-famous gaps in the asteroid belt. Kirkwood recognized that asteroids in such resonances would encounter the full effect of Jupiter's pull more frequently than otherwise, and so were soon removed from those positions. In fact, he realized more sophisticated effects were present as well, and, indeed, research on the subject has continued into our own times. Kirkwood also pointed out the same year that the main gap in Saturn's rings (Cassini's division) is owing to the same phenomenon: Ring particles at that location would have one-third the period of the satellite Enceladus and nearly one-half that of Mimas. In 1861 he gave the first convincing evidence of a connection between meteor showers and comets, and over the years made several other solid contributions to solar-system science.
So in the end Kirkwood could look back on a very satisfactory career, even if he wasn't the American Kepler. He retired to California in 1886, becoming for a while an astronomy lecturer, aged 77, at Stanford University, and eventually died in 1895 a few months short of his 81st birthday. His friend and former colleague, Joseph Swain, said of him at the time of his retirement that "during his fifty years as a teacher, he has gained from his students such universal love and admiration as few men can enjoy, and while ? he has made many valuable contributions to science, as a genial, temperate, and genuine man, he has solved the problem of gracefully growing old." Kepler should have been so lucky.
© J. Donald Fernie