BornHarford County, Maryland, USA, 27 September 1814
DiedRiverside, California, USA, 11 June 1895
Daniel Kirkwood’s most important contribution to astronomy was his discovery, published in 1866, of gaps in the distribution of orbits of the asteroids. His interest in the origin and evolution of the solar system was clearly evident in his books and papers on asteroids, comets, and meteors that were important contributions on these topics. Born in Bladensburg, Maryland, to John Kirkwood, a farmer, and his wife Agnes (née Hope) Kirkwood, Daniel was the 12th of 13 children. His early education was limited to a nearby country school. Kirkwood began his career as a teacher at the age of 19 when he took a teaching position at a country school in Hopewell, Pennsylvania. He enrolled at the York County Academy, York, Pennsylvania, in 1834, majoring in mathematics. Following his graduation in 1838, Kirkwood was appointed first assistant and instructor in mathematics at the York County Academy. In 1843 he became principal of the Lancaster, Pennsylvania, High School, and in 1845 he married Sarah A. McNair.
Kirkwood became interested in the rotations of the planets in 1839, during his first year as instructor in mathematics at York County Academy. In August 1843 he derived a mathematical analogy relating the rotation and revolution periods of the planets based on the nebular hypothesis of Pierre de Laplace . A year later, he described his work to the eminent astronomer Sears Walker . At the second meeting of the American Association for the Advancement of Science [AAAS], at Cambridge, Massachusetts, in August 1849, Walker presented Kirkwood’s letter, dated 4 July 1849, to the meeting as a formal paper. Benjamin Gould asserted that Kirkwood’s analogy supported the Laplace nebular hypothesis, while Walker proclaimed it “the most important harmony in the Solar System discovered since the time of Kepler.” Thus, Kirkwood’s letter brought instant international fame to the 35-year-old principal of the Pottsville Academy. David Brewster called it “a work of genius” in his 1850 presidential address to the British Association for the Advancement of Science. The Kirkwood analogy became irrelevant when the Laplace nebular hypothesis was temporarily abandoned in favor of Chamberlin-Moulton hypothesis, but it is noteworthy that Kirkwood in his later years became one of the leading critics of Laplace’s nebular hypothesis.
During his 5 years in Lancaster, Kirkwood published seven scholarly papers on astronomical topics including one in which he analyzed reports of a very bright meteor that had been seen in Maryland, Pennsylvania, New Jersey, Delaware, and Virginia on 13 July 1846. Kirkwood collected and compared “as many newspaper descriptions of the appearance as possible” and also “corresponded with scientific gentlemen residing in various parts of the country.” Using this information he calculated a height of 62 miles, a track length of more than 200 miles, and a velocity of 13 miles/second. Kirkwood’s efforts were reminiscent of a similar effort by Nathaniel Bowditch for the Weston, Connecticut, meteorite observed widely all over New England in 1807. Both cases were valuable because too few such well-documented path observations and calculations had accumulated since the first coordinated attempts to determine meteor altitudes were made by Johann Benzenberg and Heinrich Brandes in Germany in 1798.
In 1849, Kirkwood accepted an appointment as the principal of the Pottsville Academy. Near the end of his second year in this position, Kirkwood gave some of the first public demonstrations of the Foucault pendulum in the United States. Kirkwood left the Pottsville Academy in 1851 to become professor of mathematics at Delaware College. He was elected by the faculty to be its president in 1854. After 2 years as president, he resigned to accept an appointment as professor of mathematics at Indiana University.
Kirkwood’s interest in asteroids can be traced to the announcement of the discovery of minor planet (5) Astraea by Karl Hencke in Marienwerder, Germany, in 1845. There had been no such discoveries after the first four asteroids were discovered between 1801 and 1807. Thus, the announcement of Hencke’s discovery was a significant event, as was the announcement, 2 years later, of Hencke’s second asteroid discovery (6) Hebe. At the time of the announcement of Hencke’s first discovery, Kirkwood was principal of the Lancaster High School. Announcements of additional asteroid discoveries came in fairly rapid order, stimulating Kirkwood to study the orbits of this emerging new class of solar system object. By 1857, a year after Kirkwood arrived on the Indiana campus, 55 asteroids with computed orbits were known to exist, and it was at about that time that Kirkwood first realized the existence of the gaps with which his name has since been associated. Kirkwood found an absence of asteroids with orbital periods that were one-half, one-third, two-fifth, etc. of the orbital period of Jupiter.
Kirkwood formalized this most important of his contributions to solar system astronomy at the AAAS meeting in 1866, in a paper that also dealt with a theory of meteors and with the gaps in Saturn’s rings. Kirkwood generalized the problem to some degree by noting that both the Cassini and Encke divisions in Saturn’s rings would be populated with bodies with periods that would be in resonance with the periods of various Saturnian satellites.
Kirkwood’s continued study of the asteroids led to several other important discoveries based on resonances of their orbital periods with that of Jupiter. This led to his prediction of the existence of what is now known as the Hilda group of asteroids at the two-thirds resonance. In 1892, Kirkwood identified some 32 other possible groups based on this concept.
Another aspect of solar system dynamics that attracted Kirkwood’s attention was the relationship between these various minor solar system objects and other phenomena. He was the first to recognize and convincingly demonstrate that the orbits of certain periodic comets and those of certain meteor showers coincide and were likely related, a fact borne out in later studies. His speculations regarding a possible relationship between comets, asteroids, showers of meteors and stony meteorites, and the origin of fireballs in asteroids were controversial but also productive.
Richard Proctor , a British astronomer and leading writer of popular books on astronomy, frequently called Kirkwood “the Kepler of our day” in his books. Proctor spoke in Indianapolis in 1873 while on a lecture tour of the United States. After the lecture he was approached by a delegation from Greencastle, Indiana, who requested that he lecture at DePauw University the next evening. Proctor replied, “No I cannot do so. I came from England to America to see Daniel Kirkwood. Tomorrow is my opportunity and I am going to Bloomington to see him.”
Indiana University had a faculty of six in 1856, and this had increased to 23 in 1886, the year Kirkwood retired. He served under five presidents, including zoologist David Starr Jordan.
In 1889 Kirkwood and his wife moved to Riverside, California, where Mrs. Kirkwood died the next year. Their only child, Agnes, had died in 1874 after many years as an invalid. Shortly after their arrival in California, Kirkwood joined the Astronomical Society of the Pacific – unusual for the society at the time, given that he had performed the majority of his work outside California. He promptly published three papers in the Publications of the Astronomical Society of the Pacific, Volume II, followed by another in Volume III, and two more in Volume IV. David Starr Jordan became the founding president of Stanford University in 1891. He showed his high regard for Kirkwood by appointing him to the original Stanford Faculty as nonresident professor and lecturer in astronomy. Kirkwood was then 77 years old.
Kirkwood was a prolific scholar, publishing a total of 133 papers and 3 books during his extended career. His last paper, about the Perseid meteors, was published in the Sidereal Messenger in April 1893, 2 years before his death. His body was returned to Bloomington a week after he died and was buried at Rose Hill Cemetery on 17 June 1895, next to the graves of his wife and daughter. Kirkwood’s funeral was an imposing event. Every business in town was closed for that period. The text of the funeral sermon read: “The heavens declare the Glory of God and the firmament showeth his handiwork.” The minister said: “Dr. Kirkwood knew far more of the heavens than the writer of the eighth psalm.”