Johann Franz Encke, a German astronomer, born in Hamburg, Sept. 23, 1791, died in Span-dau, Aug. 26, 1865. He studied under Gauss at the university of Gottingen, served in the Hanseatic legion against Napoleon in 1813-'14, and in 1815 entered the Prussian military service, but afterward accepted a situation in the observatory of Seeberg near Gotha. In 1825 he was appointed director of the royal observatory at Berlin. He wrote many valuable memoirs on astronomical subjects, of which the most important are the treatises published in the Astronomische Nachrichten, at Berlin, in 1831 and 1832, upon the comet then called by the name of Pons, the astronomer of Marseilles, who discovered it in November, 1818, but now known as Encke's comet. After its discovery Encke diligently applied himself to the determination of its orbit. Making use of the methods of Gauss, as explained in his work Theoria Motus Corporum Coelestium, of calculating an orbit assumed to be elliptical, he showed that its period of recurrence must be about 3 1/4 years, and that it was probably the same comet observed by Mechain in 1786, by Caroline Herschel in 1795, and by Pons in 1805. He calculated the effects of the perturbations it would experience from the planetary bodies, especially from Jupiter, and predicted its return in 1822, though it would probably not be visible in Europe. On June 3 of that year it was discovered at the observatory of Sir Thomas Brisbane, governor of New South Wales. Encke predicted its return in 1825, and with each reappearance as predicted more elements were afforded for computing its exact orbit.
It appeared again Oct. 30, 1828, and he was able to fix its orbit as within that of Jupiter, its greatest distance from the sun being four times the earth's distance, and its least distance but one third that of the earth, and its period of revolution 3.29 years. By comparison of the times of its earlier and later apparitions, Encke was afterward led to detect a gradual acceleration of its movement, amounting to about 2 1/2 hours on each revolution. This secular acceleration, never before recognized in the movement of any other celestial body, Encke ascribed to a resisting medium, which sensibly affects a body of the extreme rarity of this comet, which is transparent to its centre, but has no perceptible effect upon the denser planetary bodies. Resistance shortens the time of the revolution by giving greater effect to the attraction of the sun, which then draws the body more forcibly toward itself, lessening the major axis of the ellipse, and thus its orbit of revolution. In investigating the perturbing effects of the planets upon this comet, of Jupiter in its aphelion, and of Mercury in its perihelion, Encke was led to suspect that the mass of the former had been greatly underrated (a fact afterward established by Prof. Airy); and in 1838 he proved that Lagrange had ascribed nearly three times too great a bulk to Mercury. Encke's explanation of the cause of the acceleration is not universally accepted, although the fact itself is not questioned.
Bessel particularly opposed the explanation; by the English astronomers it is more favorably received. Besides these investigations, Encke improved the theory of Vesta, and published a new method of computing perturbations, especially for orbits considerably elliptical. The planet Neptune was discovered at his observatory by Galle, his assistant. From 1830 Encke annually published the Astrono-misches Jahrbuch, and from 1840 Astronomische Beobachtungen auf der Sternwarte zu Berlin. In 1845 he published dissertations Be Formulis Dioptricis ; and in 1846 a treatise Ueber das Verhaltniss der Astronomie zu den anderen Wissenschaften. In the autumn of 1863 he resigned his office.