Johann Franz Encke

Born: 23 September 1791 in Hamburg, Germany
Died: 26 August 1865 in Spandau, Germany

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Johann Franz Encke’s father was Johann Michael Encke (1749-95), a Lutheran clergyman at the Jacobikirche (St. James church) in Hamburg. His grandfather, Georg Friedrich Encke, had also been a pastor, in Altluneberg, near Bremen. His mother Marie (1755-1811), was the daughter of Johann Gottfried Misler, a civil servant, and Maria Schramm. Marie and Johann Michael had nine children together, three daughters and six sons (though one son died soon after birth). Encke was the second youngest. His youngest brother, August (1794-1860), became Lieutenant General and Inspector of Artillery for the Prussian military.

Encke’s father died when he was still young, shortly after having been made an archdeacon. However, he managed to continue his studies thanks to a schoolteacher named Hipp, who supported him without accepting any payment. Thanks to him, Encke attended secondary school in the prestigious Academic School of the Johanneum in 1808. He completed the course two years later, standing out as one of the best students in the school. Encke’s mother Marie was sick, so he initially wanted to become a doctor. Tragically, his mother died before he could enter university, so, under Hipp’s advice, Encke decided to pursue further studies in mathematics.

On 6 October 1811, he enrolled in Göttingen University to learn under Carl Friedrich Gauss. Gauss was impressed with Encke’s talents in astronomical observation and tried to have him work as an assistant in a small observatory in Ofen. However, war interrupted Encke’s studies. In March 1813, he enlisted to fight against Napoleon. He became a gunner, and eventually reached the rank of sergeant major, in the Hanseatic Legion. He fought in the battle of Gohrde forest (16 September 1813), which resulted in a defeat for the French forces. Encke was discharged in July 1814 and returned to his studies, but he was again interrupted in March 1816, when Napoleon escaped from the island of Elba. This time as a lieutenant of artillery in the Prussian army, he was dispatched to the Netherlands but his unit never participated in Waterloo.

When he came back to Göttingen, the position at Ofen was no longer available, so he instead went to work as an assistant at Seeberg Observatory, near Gotha, in July 1816. Not only did Gauss support his application for the position, but Encke had met the director of the observatory, von Lindenau, during his time in military service. In February 1817, when von Lindeau returned to politics, Encke became director of the observatory in anything but name. He obtained the formal title in 1822. Encke dedicated his time to astronomical observations and the computation of orbits of comets and minor planets. This earned him a prize, but also the recognition of both Gauss and Friedrich Wilhelm Bessel.

During his time in Seeberg, Encke used the records of the transits of Venus (from 1761 and 1769) to derive the solar parallax with surprising accuracy (at 8´´.57). He is most famous for giving his name to Encke’s comet. The comet was not discovered by himself, but by Jean-Louis Pons in 1818. In fact, Encke always referred to it as Pons’ comet. Encke observed it in December 1818 and computed its orbit in Jan 1819. He found it to be elliptical with an extraordinarily short period of about 3.6 years. Encke identified the comet with the ones observed by Pierre François-André Méchain (1786), Caroline Herschel (1795) and Pons (1805). He successfully predicted its return in 1822. For his work on the comet he received his first Gold Medal from the Royal Astronomical Society in 1824 (Pons also received a silver medal in the same year). Because of the comet’s short period, Encke was able to observe it a number of times. He noticed the comet’s period suffered a steady decrease of about three hours each time. To explain this phenomenon, he theorised about the existence of a resisting medium, an ether, which filled interplanetary space.

In 1823, he married Amalie (1787-1879), the daughter of a bookseller, writer and publisher in Gotha named Rudolf Zacharias Becker (1759-1822). They had three sons and two daughters. One of the sons would later become a clergyman, like Encke’s father and grandfather had been.

On 11 October 1825, he became the director of the Berlin observatory under the recommendation of Bessel. Along with the directorship, he was elected a member of the Prussian Academy of Science and received an honorary doctorate from the University of Berlin. This was significant since the war had stopped him from graduating at Göttingen. Considering the observatory in Berlin to be inadequate, Encke decided to move to a new building, equipped with better instruments (including a large Fraunhofer refractor). Though Encke had been promised a new observatory when he took office, the promise would have not been realized so soon were it not for Alexander von Humboldt, who in 1828 appealed for aid to King Frederick William III. The construction started in 1832 and finished three years later. The building would be in use until two generations later, when Hermann Struve moved the observatory to Babelsberg in 1913, further away from the city centre.

Encke was in charge of the Astronomischs Jahrbuch (Astronomical Yearbook), founded by his predecessor, Johann Elert Bode, who edited it for fifty years. Under Bode, it had contained mostly ephemerides, but also articles and announcements of discoveries. Encke worked to improve the publication: he dropped the announcements, kept the articles (writing many of them himself) and radically improved the ephemerides. The Astronomischs Jahrbuch was, under Encke’s direction, considered to have the best ephemerides in Europe, and was the cause of many reforms in similar publications like the British Nautical Almanac. The Royal Astronomical Society recognised Encke’s efforts and gave him a second Gold Medal in 1830. In the ceremony, the President of the Royal Astronomical Society, Sir James South, said:

“[The Nautucal Almanac should], rising, like a phoenix, from its ashes, be as much superior to Encke’s [Astromischs Jahrbuch] as Encke’s is now superior to it.”

Encke was involved in the discovery of the planet Neptune. He initially opposed the use of the telescope for the search of Neptune, but he eventually gave permission to Johann Gottfried Galle and Heinrich d’Arrest, two of his students, to carry it out, using a chart of the night sky printed by Encke between 1830 and 1859. Neptune was found just before midnight on Encke’s 55th birthday, in 1846. Encke was taken out of his birthday party to confirm the discovery and became the third person to see Neptune, at least knowing what he was looking at. As Director, Encke communicated to Astronomische Nachrichten (Astronomical News) the discovery. Encke did not mention d’Arrest in this notice, something he would come to regret later in life.

Through observations, in 1837, he found Encke’s division (or Encke’s gap) in the outer ring of Saturn. In 1838, he became a member of the Board of Studies of the Military College, and in 1844, he earned the title of Professor of Astronomy at the University of Berlin. In 1846, he became a member of a Calendar Comission. Over the years, he was elected to many foreign academies. Encke also derived a method to calculate perturbations on the orbits of minor planets in 1851, which was more convenient than Gauss’ previous method.

In November 1859, he suffered a stroke. It was minor, but after a second, more serious, one in February 1863 he decided to retire. He moved to Spandau, near Berlin, with one of his daughters and his wife. Wilhelm Julius Foerster succeeded him as director of the Observatory in 1865. Encke died that same year, at the age of seventy-four.

Article by: J. G. Mena

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