The "Principia" appeared in 1687. So far as the conclusions there reached apply to the subject before us, the labor of adapting the theoretical results to numerical form and applying them to the practical problem of orbit computation, fell to Edmund Halley.
Halley, a contemporary and friend of Newton, occupies a very prominent place in the history of astronomical and physical science. Among other important discoveries and researches, may be mentioned the long inequality of Jupiter and Saturn, the proper motions of the stars, the secular acceleration of the moon's motion, method of determining the solar parallax by observations of transits of Venus, researches on terrestrial magnetism, and his epoch-making work on the motions of comets.
Halley's Comet.
| No. | Observed Perihelion. | Authority. | Calculated Perihelion. |
Authority. | ||
| (1) | 11 | B. C. Oct. 9, | Jul | Hind | — | — |
| (2) | 66 | Jan. 26 | " | " | — | — |
| (3) | 141 | March 29 | " | " | — | — |
| (4) | 218 | April 6 | " | " | — | — |
| (5) | 295 | April | " | " | — | — |
| (6) | 373 | Beg. Nov.? | " | " | — | — |
| 7 | 451 | July 3 | " | Laugier | — | — |
| (8) | 530 | Beg. Nov.? | " | Hind | — | — |
| (9) | 608 | End Oct.? | " | " | — | — |
| (10) | 684 | Oct. | " | " | — | — |
| 11 | 760 | June 11 | " | Laugier | June 15 | Crommelin-Cowell |
| 12 | 837 | March 1 | " | Pingré | Feb. 25 | "" |
| (13) | 912 | Beg. April | " | Hind | July 19 | "" |
| 14 | 989 | Sept. 12 | " | Burckhard | Oct. 9 | "" |
| 15 | 1066 | April 1 | " | Hind | March 27 | "" |
| 16 | 1145 | April 19 | " | " | April 6 | "" |
| 17 | 1222 | Aug. 22 | " | Crommelin-Cowell | Sept. 10 | "" |
| 18 | 1301 | Oct. 23 | " | Hind | Oct. 26 | "" |
| 19 | 1378 | Nov. 9 | " | Laugier | — | — |
| 20 | 1456 | June 8 | " | Pingré-Celoria | — | — |
| 21 | 1531 | Aug. 26 | " | Halley | — | — |
| 22 | 1607 | Oct. 27 | Greg. | " | Oct. 27 | Lehmann |
| 23 | 1682 | Sept. 14 | " | " | Sept. 15 | " |
| 24 | 1759 | March 13 | " | Various | March 13 | Rosenberger |
| 25 | 1835 | Nov. 16 | " | " | Nov. 15 | Pontécoulant |
| 26 | 1910 | April 8 | Crommelin-Cowell | |||
But perhaps Halley's most important work was the part which he took in the publication of the "Principia." It was largely through Halley's influence that Newton was persuaded to prepare this great work for publication, and entirely at his expense that the printing was finally done.
Among the contemporaries of Halley, we find many astronomers whose names have survived in history. We need only mention Flamsteed, the first astronomer royal, Cassini, Hevelius, Koemer and Huyghens. In his sixty-fourth year, Halley succeeded Flamsteed as astronomer royal and, at once, boldly set about the task of observing the moon through a complete revolution of the nodes, viz., a period of
