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International audienceThe astrometric monitoring of the natural planetary satellites is an important step to assess the formation and the evolution of these systems. However, in order to quantify relevant gravitational effects such as tidal forces, it is necessary to have very accurate observations over a long time interval. Unfortunately, the accuracy is decreasing as one considers older observations. To solve this issue, digitizing of old photographic plates is an attractive method, but a high accuracy in the measurement and the reduction of those plates is absolutely necessary.We have developed methods and algorithms adapted to specific plates provided by USNO, using the DAMIAN digitizer of ROB. From a set of 35 plates taken in 1974, we have been able to produce measurements with an accuracy better than 0.08 μm and after reduction using the UCAC2 catalogue, rms residuals of 35 mas (1.7 μm) for intersatellite positions (when the original reduction provided 100 mas) and of 65 mas for equatorial RA and Dec. positions (which were not possible to get with the original reduction). First results on the dynamics of the satellites and of the planet Jupiter are provided

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Because sociology is seeking mechanisms for explaining social phenomena, we discuss in this paper whether and how the metaphor of a chemical reaction network can be employed as a formal mechanism to describe social and political systems. A reaction network is a quite general concept that allows one to model a variety of dynamical systems. Furthermore, a set of powerful tools can infer potential dynamical properties from the network structure. Using a toy model of the political system inspired by Luhmann, we demonstrate how chemical organization theory can be applied and can give insight into the structure and dynamics of the resulting model. We show how chemical organization theory allows one to identify an overlapping hierarchy of important subsystems in these networks. Simulations reveal how this hierarchy constrains the potential dynamical behavior of the model.

Optical positions on the 50 mas level in the Hipparcos system have been obtained for 327 extragalactic, radio reference frame sources for both hemispheres from a 2-step procedure. Positions of secondary reference stars in the 10 ¹ V ¹ 14 magnitude range were obtained from photographic plates taken at the Hamburg astrograph (Northern Hemisphere) and the US Naval Observatory astrograph at Black Birch (Southern Hemisphere) using Hipparcos stars for astrometric plate solutions. Positions of the optical counterparts of compact, extragalactic, radio reference frame sources were then obtained from CCD direct imaging at the KPNO and CTIO 0.9 m telescopes, using the secondary reference star positions and correcting for Ðeld distortions. Several previously unknown counterparts could be identiÐed, and several previously suspected identiÐcations turned out to be empty Ðelds. CCD images are available on the World Wide Web to serve as Ðnding charts. A comparison between the optical and radio positions reveals systematic o †sets of 10 to 40 mas per coordinate as a function of the observing run. Corrections for Galactic rotation and solar motion have been attempted because of a lack of individual proper motions for the secondary reference stars. Neither 0.9 m telescope is an astrometric instrument, and limitations are clearly seen in remaining systematic errors. Orientation angles between the radio system and our optical data are zero within an accuracy of 4 mas (standard error). A signiÐcant reduction of the systematic errors will be achieved with new wide-Ðeld CCD imaging currently been taken in parallel to new 0.9 m observations, to provide more secondary reference star positions with higher precision at a common epoch, which will allow extended modeling of the mapping properties of 0.9 m observations.

The StarScan machine at the U.S. Naval Observatory (USNO) completed measuring photographic astrograph plates to allow determination of proper motions for the USNO CCD Astrograph Catalog (UCAC) program. All applicable 1940 AGK2 plates, about 2200 Hamburg Zone Astrograph plates, 900 Black Birch (USNO Twin Astrograph) plates, and 300 Lick Astrograph plates have been measured. StarScan comprises of a CCD camera, telecentric lens, air-bearing granite table, stepper motor screws, and Heidenhain scales to operate in a step-stare mode. The repeatability of StarScan measures is about 0.2 µm. The CCD mapping as well as the global table coordinate system has been calibrated using a special dot calibration plate and the overall accuracy of StarScan x, y data is derived to be 0.5 µm. Application to real photographic plate data shows that position information of at least 0.65 µm accuracy can be extracted from course grain 103a-type emulsion astrometric plates. Transformations between "direct" and "reverse" measures of fine grain emulsion plate measures are obtained on the 0.3 µm level per well exposed stellar image and coordinate, which is at the limit of the StarScan machine.

In this pilot investigation, precise optical positions in the FK5 system are presented for a set of 16 compact extragalactic radio sources, which will be part of the future radio-optical reference frame. The 0.9 m KPNO and CTIO telescopes equipped with 2K CCD's have been used for this project. The