Johann Franz Encke (September 23, 1791 – August 26, 1865) was a German astronomer, was born at Hamburg.
Encke studied mathematics and astronomy from 1811 at the University of Göttingen under Carl Friedrich Gauss; but he enlisted in the Hanseatic Legion for the campaign of 1813–1814, and became lieutenant of artillery in the Prussian service in 1815. Having returned to Göttingen in 1816, he was at once appointed by Benhardt von Lindenau as his assistant in the observatory of Seeberg near Gotha.
There he completed his investigation of the comet of 1680, for which the Cotta prize was awarded to him in 1817; he correctly assigned a period of 71 years to the comet of 1812. That comet is now called 12P/Pons-Brooks.
Following a suggestion by Jean-Louis Pons, who suspected one of the three comets discovered in 1818 to be the same one already discovered by him in 1805, Encke began to calculate the orbital elements of this planet. At this time, all the known comets only had an orbital period of 70 years and more, were the aphelion is far beyond the Uranus orbit. The most famous comet of this family was the Halley comet with its orbitting time of 76 years. Therefore the orbit of the comet discovered by Pons was a sensation, because his orbit was found to have a period 3.3 years, therefore the aphelion had to be within the Jupiter orbit. Encke predicted its return for 1822, but this return was only observable from the southern hemisphere and was seen by K. Ruemker from Australia. The comet was also identified with the one seen by Pierre Méchain in 1786 and by Caroline Herschel in 1795.
Encke sent his calculations as a note to Gauss, Olbers, and Bessel. His former mathematics professor published this notes and Encke became famous as the discoverer of the short periodic comets. The first object of this family, the Encke comet, was named after him and so it is one of the few comets not named after the discoverer, but after the one who calculated the orbit. Later this comet was identified as the origin of the Taurids meteor showers.
The importance of the predicted return based on the calculation by Encke was rewarded by the Astronomical Society in London by presenting their Gold Medal to him in 1824. In this year Encke married Amalie Becker (1787–1879), daughter of a bookseller. They had three sons and two daughters.
Eight masterly treatises on its movements were published by him in the Berlin Abhandlungen (1829–1859). From a fresh discussion of the transits of Venus in 1761 and 1769 he deduced (1822–1824) a solar parallax of 8.57 arcsecond, long accepted as authoritative.
In 1822 he became director of the Seeberg observatory, and in 1825 was promoted to a corresponding position at Berlin, where a new observatory, built under his superintendence and with the support of Alexander von Humboldt and the Prussian king Friedrich Wilhelm III, was inaugurated in 1835. Encke became director of the new observatory.
He directed the preparation of the star-maps of the Berlin academy 1830–1859, edited from 1830 and greatly improved the Astronomisches Jahrbuch, and issued four volumes of the Astronomische Beobachtungen of the Berlin observatory (1840–1857). Within the following time Encke was involved in the discovery and orbital parameter determination of other short periodic comets and asteroids. In 1837 Encke discovered a gap of about 325 kilometer wide within the A ring of Saturn, the so called Encke division.
In 1844, Encke became university professor for astronomy in Berlin. Much labour was bestowed by him upon facilitating the computation of the movements of the asteroids. With this end in view he expounded to the Berlin academy in 1849 a mode of determining an elliptic orbit from three observations, and communicated to that body in 1851 a new method of calculating planetary perturbations by means of rectangular coordinates (republished in W. Ostwald's Klassiker der exacten Wissenschaften, No. 141, 1903).
Encke visited England in 1840. Incipient brain-disease compelled him to withdraw from official life in November 1863. He still was director of the Berlin observatory until his death on the 26th of August 1865 in Spandau. His successor was Wilhelm Julius Foerster (1832–1921).
He contributed extensively to the periodical literature of astronomy, and was twice, in 1824 and 1830, the recipient of the Gold Medal of the Royal Astronomical Society.
See Johann Franz Encke, sein Leben und Wirken, von Dr C. Bruhns (Leipzig, 1869), to which a list of his writings is appended. Also, Month. Notices Roy. Astr. Society, xxvi. 129; V.J.S. Astr. Gesellschaft, iv. 227; Berlin Abkandlungen (1866), LG Hagen; Sitzungsberichte, Munich Acad. (1866), i. p. 395, etc.