{{Short description|Informal name for the German United Astronomical Society}}
The '''Celestial police''' ({{langx|de|Himmelspolizey}}), officially the '''United Astronomical Society''' ({{langx|de|'''Vereinigte Astronomische Gesellschaft'''}}), was a cooperation of numerous European astronomers in the early 19th century. It is mainly known in relation to the search for objects expected between the orbits of Mars and Jupiter. It was formed in 1800 at the second European congress of astronomers.<ref name="gerdes">{{cite book |last=Gerdes |first=Dieter |date=1990 |title=Die Geschichte der Astronomischen Gesellschaft, gegründet in Lilienthal am 20. September 1800 |publisher=Heimatverein Lilienthal |language=German |isbn=3-927723-06-1}}</ref> At the first such congress, in 1798, the French mathematician Jérôme Lalande had called for a coordinated search, in which each participating observatory would patrol a particular part of the sky.<ref name=roth>{{cite book |first=Günter D. |last=Roth |title=Die Planetoiden |date=1960 |language=German |series=Orionbücher |volume=137 |pages=13{{ndash}}15}}</ref> The group confirmed or discovered the four largest minor planets, which would lead to the identification of the asteroid belt. They also initiated the compilation of better star catalogues and the investigation of variable stars. They pioneered international collaboration and communication in astronomy.<ref name="gerdes" /><ref name="wolfschmidt">{{cite book |last=Wolfschmidt |first=Gudrun |date=2002 |chapter=Internationalität von der VAG (1800) bis zur Astronomischen Gesellschaft |editor-last1=Dick |editor-first1=Wolfgang R. |editor-last2=Hamel |editor-first2=Jürgen |title=Astronomie von Olbers bis Schwarzschild, nationale Entwicklungen und internationale Beziehungen im 19. Jahrhundert |series=Acta Historica Astronomiae |volume=14 |issn=1422-8521 |publisher=Verlag Harri Deutsch |location=Frankfurt am Main |language=German |pages=182{{ndash}}203}}</ref>
== Founding == In 1798 Franz Xaver von Zach had organised and hosted the first European congress of astronomers at his observatory in Gotha. Zach was also editor of the monthly journals ''Allgemeine Geographische Ephemeriden'' (since 1798) and ''Monatliche Correspondenz zur Beförderung der Erd- und Himmels-Kunde'' (since 1800). The second congress in 1800 was held with smaller attendance and more focussed agenda in Lilienthal, at the observatory of Johann Hieronymus Schröter. Schröter had arranged for a visit by Prince Adolph Frederick to coincide with the congress.<ref name="gerdes" /><ref name="müller">{{cite web |url=https://href.hypotheses.org/1999 |title=The very first monthly astronomical journal in Germany: The Celestial Police and their structures of communication |last=Müller |first=Janna Katharina |date=7 September 2021 |website=href.hypotheses.org |publisher=German Historical Institute (GHI) |location=Washington, DC |access-date=16 February 2024}}</ref>
Foremost on the agenda for the congress was the founding of the Vereinigte Astronomische Gesellschaft (United Astronomical Society). Six astronomers were present to found the society on 20 September 1800, with Schröter as president and von Zach as director or secretary. The founding members were:<ref name="gerdes" /><ref name="unesco">{{cite web |title=Category of Astronomical Heritage: tangible immovable Lilienthal Observatory, Germany |url=https://www3.astronomicalheritage.net/index.php/show-entity?identity=104&idsubentity=1 |website=Portal to the Heritage of Astronomy |publisher=UNESCO |access-date=11 February 2024}}</ref><ref name="fodera">{{cite book |first1=G. |last1=Foderà Serio |first2=A. |last2=Manara |first3=P. |last3=Sicoli |chapter=Giuseppe Piazzi and the discovery of Ceres |title=Asteroids III |editor-first1=William F. |editor-last1=Bottke |editor-first2=Alberto |editor-last2=Cellino |editor-first3=Paolo |editor-last3=Paolicchi |editor-first4=Richard P. |editor-last4=Binzel |publisher=University of Arizona Press |location=Tucson |isbn=978-0-8165-2281-1 |chapter-url=https://www.researchgate.net/publication/237423344 |date=2002 |access-date=12 February 2024}}</ref><ref name="winterburn">{{cite magazine |last1=Winterburn |first1=Emily |title=Discovering asteroid Vesta: the story of the Celestial Police |url=https://www.skyatnightmagazine.com/space-science/asteroid-vesta-discovery-celestial-police/ |magazine=Sky at Night Magazine |publisher=British Broadcasting Corporation |access-date=11 February 2024 |date=10 March 2021}}</ref><ref name="müller" /> *Johann Hieronymus Schröter (Lilienthal), president *Adolf von Ende (Celle) *Johann Gildemeister (Bremen) *Wilhelm Olbers (Bremen) *Karl Ludwig Harding (Lilienthal) *Franz Xaver von Zach (Gotha), director or secretary
== Tasks ==
=== Star catalogues === The main workload for the society was the compilation of more precise star catalogues and to improve knowledge of spherical astronomy and coordinate systems. This was required for two reasons:<ref name="gerdes" /><ref name="unesco" /> *It was necessary to identify and locate the positions of fainter celestial objects than in the past. *Sound definitions of coordinate systems were needed as basis for the precise determination of the orbits of newly discovered celestial bodies.
An area of 15° width centred on the ecliptic was to be catalogued. To share the workload, the ecliptic was divided into 24 zones each extending 15° in longitude and 7° or 8° either side in latitude.<ref name="gerdes" /><ref name="fodera" />
=== New comets and further planets === The task that the Celestial police is best known for was the search for a small planet that was expected to exist between the orbits of Mars and Jupiter. The existence of such a body followed from the Titius-Bode law, a geometric series of the orbital radii from Mercury to Uranus, which has a gap at 2.8 astronomical units. Even Johannes Kepler had postulated such an undiscovered planet in 1596 in his ''Mysterium Cosmographicum''.<ref name="fodera" />
Given the discovery of Uranus in 1781, more planets might also be found beyond Saturn. And new and telescopic comets might be found.<ref name="gerdes" />
=== Further tasks === The question of stellar parallax and the distance of the stars was an important topic at the turn from the 18th to the 19th century. This was hence also on the agenda of the Celestial police.<ref name="gerdes" />
Another new topic of astronomical research in the early 19th century was the surveillance of variable stars and novae.<ref name="gerdes" />
As an international collaboration of astronomers, the Celestial police also noted the need for communication, both among participants and through a publication like von Zach's ''Monatliche Correspondenz zur Beförderung der Erd- und Himmels-Kunde''.<ref name="gerdes" />
== Members == The division of labour into 24 zones of ecliptic longitude required the Celestial police to have 24 members, with one zone allocated to each member. The canonical list of 24 members of the celestial police are:<ref name="gerdes" /><ref name="fodera" /><ref name="unesco" /> *Johann Elert Bode (Berlin) *Johann Tobias Bürg (Vienna) *Thomas Bugge (Copenhagen) *Johann Karl Burckhardt (Paris) *Adolf von Ende (Celle) *Johann Gildemeister (Bremen) *Karl Ludwig Harding (Lilienthal) *William Herschel (Slough) *Johann Sigismund Gottfried Huth (Frankfurt (Oder)) *Georg Simon Klügel (Halle (Saale)) *Julius August Koch (Gdańsk) *Nevil Maskelyne (Greenwich) *Daniel Melanderhjelm (Stockholm) *Pierre Méchain (Paris) *Charles Messier (Paris) *Wilhelm Olbers (Bremen) *Barnaba Oriani (Milan) *Giuseppe Piazzi (Palermo) *Johann Hieronymus Schröter (Lilienthal) *Theodor von Schubert (Saint Petersburg) *Jöns Svanberg (Uppsala); Svanberg replaced Jan Śniadecki (Kraków) when the future of his observatory was in doubt<ref name="gerdes" /> *Joseph Thulis (Marseille) *Johann Friedrich Wurm (Blaubeuren) *Franz Xaver von Zach (Gotha)
Jérôme Lalande had been invited, but declined due to other commitments. Some invitations may have been issued late or may never have arrived. Not every invitee actively participated in the survey of the ecliptic, and others who worked on the tasks, such as Friedrich Bessel, are not included in the group. Carl Friedrich Gauss (Braunschweig) became a member in 1801 and, jointly with Olbers, became foreign correspondent in 1804.<ref name="fodera" /><ref name="unesco" /><ref name="gerdes" />
== Results ==
=== Ceres === On 1 January 1801, apparently by coincidence and independent of the Celestial police, Piazzi was working on a star catalogue and found a moving object, the first minor planet, (1) Ceres. He announced it as a new comet, but due to the lack of nebulosity suspected it might be a small planet. It was not until September 1801 that his complete observations were published. Gauss then developed his method of determining orbits from astrometric observations. This confirmed not only a planetary rather than a cometary orbit, it also enabled von Zach and Olbers to "recover" the minor planet, i.e. to find it again after its passage behind the Sun.<ref name="fodera" /><ref name="gerdes" /><ref name="jpl">{{cite web |url=https://www.jpl.nasa.gov/news/ceres-keeping-well-guarded-secrets-for-215-years |title=Ceres: Keeping Well-Guarded Secrets for 215 Years |date=26 January 2016 |website=jpl.nasa.gov |publisher=Jet Propulsion Laboratory |access-date=18 October 2022}}</ref>
Because the orbit of Ceres matched the requirement from the Titius-Bode law, the planet missing between Mars and Jupiter seemed to have been found, but it was disappointingly faint.<ref name="jpl" /><ref name="winterburn" />
=== Pallas, Juno and Vesta === In March 1802 Olbers was working on the star catalogue of his zone, in preparation of Ceres arriving in the area, when he discovered another moving star, the second minor planet, (2) Pallas.<ref name="gerdes" /><ref name="jpl" />
The presence of two minor planets between Mars and Jupiter had several consequences. It cast doubt on the Titius-Bode law, which called for a single, large planet.<ref name="gerdes" /> It prompted William Herschel, discoverer of Uranus, to propose an alternative term "asteroid" instead of "planet". While the use of "planet" could not continue, "asteroid" was not generally accepted until decades later.<ref name="jpl" /><ref name="fodera" />
Olbers took the presence of two minor planets to suggest that a former planet had been destroyed by a collision with a comet. This could restore the Titius-Bode law and offered hope to find more minor planets, in particular at the crossing points of the orbits of Ceres and Pallas.<ref name="gerdes" /><ref name="fodera" /><ref name="winterburn" /> Huth and von Zach favoured the opposite idea, that the minor planets were just small planets in a region where they failed to form a full-size planet.<ref name="fodera" /><ref name="gerdes" />
Pursuing Olbers' idea, Harding in September 1804 found (3) Juno, and Olbers in March 1807 found (4) Vesta.<ref name="gerdes" /><ref name="fodera" />
=== Further developments === After discovering such a large number of relatively small objects in a similar orbit, it became clear that no planet-sized object likely existed in that region. The group members' interest waned in the search. Additionally, the Napoleonic Wars had disrupted the work of several group members, especially when the war came to Lilienthal, where Schröter's observatory had served as the home for many of the scientists working with the celestial police. Schröter died in 1816; other members of the Celestial police had moved elsewhere or changed the focus of their work. It would be another generation before any further major discoveries of planets (or even large asteroids) occurred.<ref name="winterburn" /><ref name="gerdes" />
The division of labour pioneered by the celestial police, around 1850, lead to the concept of surveys, also to the compilation of catalogues of nebulae. The most famous star catalogue of the 19th century is the Bonner Durchmusterung with 300,000 stars, which was later extended through the work of more southerly observatories.<ref name="gerdes" />
== See also == * ''Astronomische Gesellschaft'' * List of astronomical societies
== References== {{reflist}}
== Further reading == * {{cite book |editor-last1=Dick |editor-first1=Wolfgang R. |editor-last2=Hamel |editor-first2=Jürgen |editor-link2=Jürgen Hamel |date=2002 |title=Astronomie von Olbers bis Schwarzschild {{mdash}} Nationale Entwicklungen und internationale Beziehungen im 19. Jahrhundert |location=Thun, Frankfurt am Main |publisher=Deutsch |isbn=3-8171-1667-5 |issn=1422-8521 |language=de |series=Acta Historica Astronomiae |volume=14}}
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