Johann Franz Encke
The original choice for Bode's successor in 1826 was Bessel but he preferred to stay in Königsberg. The job was given to a former pupil of Gauss, Johann Franz Encke.
An important event in this period had been the establishment in 1809 of Berlin University by Alexander von Humboldt (latterly the Humboldt University in East Berlin). Humboldt himself persuaded the King both to purchase three large telescopes (including the Fraunhofer-Refraktor used to discover Neptune) and to finance the building of a urgently-needed new observatory :- in the area between Lindenstrasse and Friedrichstraße, Kreuzberg. There is nothing to mark the exact spot where the observatory was actually situated, but the existence of an Enckestraße in the area provides a certain legacy. The basic plans were drawn up by Encke and carried out by the famous architect Schinkel, who was responsible for many of Berlin's public buildings, opening in 1835. The former observatory in Dorotheenstraße served for a time as a station on the optical telegraph connected to the Rheinland and was finally demolished in the early years of this century to make way for the building of the State Library on Unter den Linden.
| The "new" Observatory, where Neptune was discovered. |
The main instrument was a 9'' Fraunhofer refractor. It had an 8 meter rotatable dome, and the basement was split between living quarters and a working observatory section.
Encke was to establish Berlin as a leader in the field of minor planets. He concentrated on the calculation of the orbits of asteroids, and the large planets' influence on them.
For approximately 30 years up to 1859 he was engaged in the drawing up of the new star charts which had been the task of a commission set up by Bessel but which were largely carried out under the leadership of Encke. These charts were soon improved upon by charts produced by Argelander, the director of the Bonn Observatory (Bonn Durchmusterung), but nevertheless were invaluable in Encke's work on minor planets and were also an aid in the discovery of Neptune on the 26. September 1846.
He published 37 volumes of the 'Berliner Astronomisches Jahrbuch' (1830--66), along with his assistants, J.P.Wolfers and Bremiker.
Encke's name is well-known today because of Encke's comet. This was actually discoverd for what it was in 1818 by Jean Louis Pons of Marseille (and had, as it turned out, also been seen by Pierre Mechain in 1786 and Caroline Herschel in 1795, as well as Pons himself in 1805). In early 1819, Encke calculated its orbit, and its period - 3,3 years, the shortest period of any known comet. It was particularly sensational at the time because no comets were known with a period of less than 70 years. Encke's Comet is one of the few comets which are not named after their discoverer.
It's return in 1822 was only observable in the Southern Hemisphere, but was observed by Gauss in 1825, from the Seeberg Observatory, near Gotha, where he was working at the time.
On previous observations, the orbit was seen to be slowing by 2-3 hours, which Encke attempted to explain by proposing an interstellar ether. It is now known that the orbit is affected by the loss of material as the comet approaches the Sun. The comet is the source of the Taurid meteor shower.
In 1823, Encke had used information from the Transits of Venus of 1761 and 1769 to calculate a distance to the Sun of 153.303 million kilometers ( cf. modern value of 149.598 million kilometers ). In 1837 he discovered Encke's Gap, in Saturn's ring A - this Division is now known to be 270 km. wide. Under good seeing conditions, this ring can be seen at either end of the rings, but not all the way around.
| rings and satellites of Saturn |
Encke's gap is now known to be caused by a satellite - Pan. This satellite has a diameter of 20 km, and its gravitational effect maintains the gap at 270 kms wide, and causes wavelike disturbances in the ring either side of the gap. This satellite was only discovered in 1990, after a computer search of Voyager images taken a decade earlier, although its existence had been predicted earlier.
Encke had been appointed Professor in Astronomy at Berlin University in 1844, and remained in this post until 1863. He was still Observatory Director at the time of his death in 1865.
Although the observatory was later demolished, the square adjacent to its site is still known as Enckeplatz (since 1844).