TheCelestial police (German:Himmelspolizey), officially theUnited Astronomical Society (German: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 ofMars andJupiter. It was formed in 1800 at the second European congress of astronomers.[1] At the first such congress, in 1798, the French mathematicianJérôme Lalande had called for a coordinated search, in which each participating observatory would patrol a particular part of the sky.[2] The group confirmed or discovered the four largestminor planets, which would lead to the identification of theasteroid belt. They also initiated the compilation of betterstar catalogues and the investigation ofvariable stars. They pioneered international collaboration and communication inastronomy.[1][3]
In 1798Franz Xaver von Zach had organised and hosted thefirst European congress of astronomers at his observatory inGotha. Zach was also editor of the monthly journalsAllgemeine Geographische Ephemeriden (since 1798) andMonatliche Correspondenz zur Beförderung der Erd- und Himmels-Kunde (since 1800). The second congress in 1800 was held with smaller attendance and more focussed agenda inLilienthal, at the observatory ofJohann Hieronymus Schröter. Schröter had arranged for a visit byPrince Adolph Frederick to coincide with the congress.[1][4]
Foremost on the agenda for the congress was the founding of theVereinigte 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:[1][5][6][7][4]
The main workload for the society was the compilation of more precisestar catalogues and to improve knowledge ofspherical astronomy andcoordinate systems.This was required for two reasons:[1][5]
An area of 15° width centred on theecliptic was to be catalogued. To share the workload, the ecliptic was divided into 24 zones each extending 15° inlongitude and 7° or 8° either side in latitude.[1][6]
The task that the Celestial police is best known for was the search for a smallplanet that was expected to exist between the orbits ofMars andJupiter. The existence of such a body followed from theTitius-Bode law, ageometric series of theorbital radii fromMercury toUranus, which has a gap at 2.8astronomical units. EvenJohannes Kepler had postulated such an undiscovered planet in 1596 in hisMysterium Cosmographicum.[6]
Given the discovery ofUranus in 1781, more planets might also be found beyondSaturn. And new and telescopiccomets might be found.[1]
The question ofstellar 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.[1]
Another new topic of astronomical research in the early 19th century was the surveillance ofvariable stars andnovae.[1]
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'sMonatliche Correspondenz zur Beförderung der Erd- und Himmels-Kunde.[1]
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:[1][6][5]
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 asFriedrich 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.[6][5][1]
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 firstminor planet,(1) Ceres. He announced it as a newcomet, but due to the lack of nebulosity suspected it might be a smallplanet. It was not until September 1801 that his complete observations were published.Gauss then developed his method of determiningorbits fromastrometric observations. This confirmed not only a planetary rather than a cometary orbit, it also enabledvon Zach andOlbers to "recover" the minor planet, i.e. to find it again after its passage behind theSun.[6][1][8]
Because the orbit of Ceres matched the requirement from theTitius-Bode law, theplanet missing betweenMars andJupiter seemed to have been found, but it was disappointingly faint.[8][7]
In March 1802Olbers was working on the star catalogue of his zone, in preparation of Ceres arriving in the area, when he discovered another moving star, the secondminor planet,(2) Pallas.[1][8]
The presence of two minor planets between Mars and Jupiter had several consequences. It cast doubt on theTitius-Bode law, which called for a single, large planet.[1] It promptedWilliam Herschel, discoverer ofUranus, 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.[8][6]
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.[1][6][7]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.[6][1]
Pursuing Olbers' idea,Harding in September 1804 found(3) Juno, and Olbers in March 1807 found(4) Vesta.[1][6]
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, theNapoleonic Wars had disrupted the work of several group members, especially when the war came toLilienthal, 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.[7][1]
The division of labour pioneered by the celestial police, around 1850, lead to the concept ofsurveys, also to the compilation of catalogues of nebulae. The most famous star catalogue of the 19th century is theBonner Durchmusterung with 300,000 stars, which was later extended through the work of more southerly observatories.[1]
