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Johann Franz Encke

Born September 23, 1791 in Hamburg; died August 26, 1863 in Spandau, Germany.

Encke was a German astronomer, mathematician and physicist who is noted for calculating the orbit of the comet that now bears his name (Encke's Comet) and for discovering a faint division in the outermost ring of Saturn, now known as Encke's Division.

Encke began his education in Hamburg. Then in 1811 he enrolled at the University of Göttingen, where he worked under the direction of Carl Friedrich Gauss. In 1816 Encke became assistant at the Seeberg Observatory near Gotha, Germany. There he completed his investigation of the comet of 1680, which was the subject of much interest by many astronomers. Leonhard Euler had calculated a period for the comet of 170 years, Edmond Halley calculated 575 years, and Alexandre Pingré calculated 5,864 years. Encke's calculations, which were based on more observational data, yielded a more accurate period of 8,814 years. For this achievement, he was awarded the Cotta prize in 1817. Encke also studied observations of the comet of 1812 and correctly assigned it a period of 71 years.

In 1818 Jean-Louis Pons discovered what became known as the Comet of 1819. Immediately following its discovery, Encke was able to associate it with comets "discovered" in 1786, 1795 and 1805. He determined that all of these observations were of the same comet and that it had a period of 3.29 years. He then predicted, correctly, that it would return in 1822. For this achievement, the comet became known as Encke's Comet.

Encke was made vice director of Seeberg Observatory in 1820 and director in 1822. From 1822 to 1824, Encke studied observations from Venus' transits of the Sun recorded in 1761 and 1769. From these, he derived a value for the solar parallax of 8." 57, indicating a distance of 153,000,000 kilometers or 95,300,000 miles. This measurement was very close to the presently accepted average distance of 149,600,000 kilometers or 92,980,000 miles. Encke's parallax distance held until more accurate measurements could be taken of transits by minor planets, which have no atmosphere to confuse the points of contact as they cross the Sun.

In 1825 Encke was appointed professor of astronomy and director of the observatory of the University of Berlin. There he planned and supervised the construction of a new observatory, completed in 1835. From 1829 to 1859 Encke published eight treatises on Encke's Comet in the Berlin Abhandlungen. In 1837, while observing at Berlin, Encke discovered a faint division in the outermost ring (also called "Ring A") of the planet Saturn. This became known as Encke's Division. In addition to his other achievements, Encke compiled star charts, established methods for calculating the orbits of minor planets and methods for calculating the orbits of double stars.

Bibliography

_____. Comets: Readings from Scientific American with Introductions by John C. Brandt. W. H. Freeman and Company, San Francisco, 1981.

_____. Encyclopaedia Britannica, fifteenth edition. Encyclopaedia Britannica, Inc., Chicago, 1988.

_____. Encyclopaedia Britannica, fourteenth edition. Encyclopaedia Britannica, Inc., Chicago, 1943.

Famighetti, Robert, editorial director. World Almanac and Book of Facts 1998, The. World Almanac Books, Mahwah, 1997.

Hellemans, Alexander and Bunch, Bryan H. Timetables of Science: A Chronology of the Most Important People and Events in the History of Science, The. Simon & Schuster, New York, 1991.

Moore, Patrick, editor. International Encyclopedia of Astronomy, The. Orion Books, New York, 1987.


James M. Thomas, last updated October 11, 1999.

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