Precision geodesy relies on the stability of the International Celestial Reference Frame (ICRF), yet its reference sources, Active Galactic Nuclei (AGN), exhibit intrinsic changes in source structure that can manifest as apparent shifts in their astrometric positions. The high-precision radio measurements used to maintain the ICRF therefore provide a powerful means to investigate the astrophysical mechanisms driving these position changes. In particular, the observed astrometric variability offers a unique opportunity to link positional shifts in AGN to high-energy astrophysical processes. We therefore investigated the relationship between the astrometric positions of ICRF AGN and their $\gamma$-ray emission. We measured the positional offsets of radio cores relative to the third realisation of the ICRF at both S/X (2.3/8.4 GHz) and K (24 GHz) bands and compared them to Fermi-LAT (Large Area Telescope) $\gamma$-ray fluxes within $\pm$30 days of the radio observation. Out of an initial sample of 92 radio sources selected for having extensive radio astrometric observations, we identified 57 that met our selection criteria of having sufficient overlapping $\gamma$-ray data points to allow for regression analysis. We find a high incidence of statistically significant ($p\lt0.05$) power-law correlations, with $\sim$ 90% of sources exhibiting this behaviour. The nature of this correlation is complex: we observe both positive and negative correlations, and the sign of the correlation can differ between the two frequency bands for the same source. To explain the correlations, we tested several scenarios, including variable $\gamma$-ray emission locations, changes in nuclear opacity, and variations in jet position angle. Our analysis reveals no single, universally applicable explanation. Instead, the results suggest that the observed correlation is driven by a complex interplay of multiple physical mechanisms, the dominance of which likely varies between sources. A search for time lags between the radio position offsets and $\gamma$-ray fluxes revealed tentative – and highly caveated – evidence for a time-delay in only five sources, with no evidence in other sources. A statistical comparison with the Optical Characteristics of Astrometric Radio Sources (OCARS) catalogue shows that, although our sample is biased towards optically brighter sources with better-constrained astrometric solutions due to their larger number of radio observations, it remains representative of the broader AGN population in terms of redshift distribution.

The Gaia optical astrometric mission has measured the precise positions of millions of objects in the sky, including extragalactic sources also observed by Very Long Baseline Interferometry (VLBI). In the recent Gaia EDR3 release, an effect of negative parallax with a magnitude of approximately $-17$ $\mu$as was reported, presumably due to technical reasons related to the relativistic delay model. A recent analysis of a 30-yr set of geodetic VLBI data (1993–2023) revealed a similar negative parallax with an amplitude of $-15.8 \pm 0.5$ $\mu$as. Since both astrometric techniques, optical and radio, provide consistent estimates of this negative parallax, it is necessary to investigate the potential origin of this effect.

We developed the extended group relativistic delay model to incorporate the additional parallactic effect for radio sources at distances less than 1 Mpc and found that the apparent annual signal might appear due the non-orthogonality of the fundamental axes, which are defined by the positions of the reference radio sources themselves. Unlike the conventional parallactic ellipse, the apparent annual effect in this case appears as a circular motion for all objects independently of their ecliptic latitude. The measured amplitude of this circular effect is within a range of 10–15 $\mu$as that is consistent with the ICRF3 stability of the fundamental axis. This annual circular effect could also arise if a Gödel-type cosmological metric were applied, suggesting that, in the future, this phenomenon could be used to indicate global cosmic rotation.

This contribution summarizes the reconstruction of historical constellations. It is based on studies of classics, philologies, history of science and history of art. In the given brevity, I can only sketch the strategic, scientific and educational reasons.

With the launch of Gaia in December 2013, Europe entered a new era of space astrometry following in the footsteps of the very successful Hipparcos mission. A weakness of Gaia is that it only operates at optical wavelengths. However, much of the Galactic centre and the spiral arm regions are obscured by interstellar extinction. An obvious improvement on Gaia is to include the Near-Infra-Red (NIR) which requires the use of new types of detectors. Additionally, to scan the entire sky and measure global absolute parallaxes the spacecraft must have a constant rotation resulting in a moving image that must be compensated for by, for example, operating the detectors in Time Delayed Integration (TDI) mode. If these technical issues can be solved a new Gaia-like mission separated by a 20 year interval would give; 1) NIR all-sky astrometry and photometry to penetrate the obscured regions and to observe intrinsically red objects with almost diffraction limited resolution; 2) improved proper motions with fourteen times smaller errors than from Gaia alone opening up new science cases, such as long period exoplanets and accurate halo measurements; 3) allow the slowly degrading accuracy of the Gaia reference frame, which will be the basis for future astronomical measurements, to be reset.

The Gaia first data release (DR1) already provides an almost error free optical reference frame on the milli-arcsecond (mas) level allowing significantly better calibration of ground-based astrometric data than ever before. Gaia DR1 provides positions, proper motions and trigonometric parallaxes for just over 2 million stars in the Tycho-2 catalog. For over 1.1 billion additional stars DR1 gives positions. Proper motions for these, mainly fainter stars (G ≥ 11.5) are currently provided by several new projects which combine earlier epoch ground-based observations with Gaia DR1 positions. These data are very helpful in the interim period but will become obsolete with the second Gaia data release (DR2) expected in April 2018. The era of traditional, ground-based, wide-field astrometry with the goal to provide accurate reference stars has come to an end. Future ground-based astrometry will fill in some gaps (very bright stars, observations needed at many or specific epochs) and mainly will go fainter than the Gaia limit, like the PanSTARRS and the upcoming LSST surveys.

Since September 2016, the first release (DR1) of the Gaia catalogue was appeared. The optical Gaia positions of sources will be linked to the ICRF (VLBI radio positions of mostly quasars, QSOs). For high accurate link we need to investigate variations of optical flux of QSOs via their magnitude variations using data of ground-based telescopes. To do that, from 2013 we observed 47 QSOs and other sources; nine optical telescopes were used for that monitoring. To increase the total number of objects for the link, after a first set of 70 objects (Bourda et al. 2008), Bourda et al. (2011) established a second set of 47 objects. It is necessary to investigate the photometry and morphology of these objects. We collected ground-based data of QSOs (B, V and R mag) and compared with G mag of Gaia DR1; some results are presented here.

We combined the data from the Gaia DR1 and Two-Micron All Sky Survey (2MASS) catalogues in order to derive the absolute proper motions more than 420 million stars distributed all over the sky in the stellar magnitude range 8 mag < G < 21 mag (Gaia magnitude). To eliminate the systematic zonal errors in position of 2MASS catalogue objects, the 2-dimensional median filter was used. The PMA system of proper motion has been obtained by direct link to 1.6 millions extragalactic sources. The short analysis of the absolute proper motion of the PMA stars Catalogue is presented in this work. From a comparison of this data with same stars from the TGAS, UCAC4 and PPMXL catalogues, the equatorial components of the mutual rotation vector of these coordinate systems are determined.

Gaia DR1 is based on the first 14 months of Gaia's observations. This is not long enough to reliably disentangle the parallax effect from proper motion. For most sources, therefore, only positions and magnitudes are given. Parallaxes and proper motions were nevertheless obtained for about two million of the brighter stars through the Tycho-Gaia astrometric solution (TGAS), combining the Gaia observations with the much earlier Hipparcos and Tycho-2 positions. In this review I focus on some important characteristics and limitations of TGAS, in particular the reference frame, astrometric uncertainties, correlations, and systematic errors.

The aim of this paper is the study of the impact that the consideration of different physical properties as magnitude and spectral type of stars has on the geometric relations between Hipparcos2 and UCAC4. In this sense, the pairs of residuals Δα* and Δδ can be considered as functions of (α, δ, r) and for each fixed r, we can fit a vector field on the sphere from which to obtain its components in the VSH basis. The same can be done by grouping the stars considering their magnitudes, spectral types (or mixing them) and then studying the variations in the mentioned geometry. We must not forget that Δα* and Δδ are numerical random variables whose regression on the magnitude m, for example, can be estimated. The results will be computed taking into account r as well as the physical mentioned properties. So, we avoid the assumption that the harmonic coefficients depend only on m.

One of the important challenges that Gaia imposes on the Astrometric Catalogs, is a careful study in everything affected by parallax. A particularly important case is the necessary linkage Gaia - HCRF - ICRF2, which require methods of analysis that are accurate enough so that the provided results are at the same precision level as the work data.

Comparison of the Gaia DR1 auxiliary quasar solution to recent ground based VLBI solutions for ICRF2 sources.

We use methods of differential astrometry to construct a small field inertial reference frame stable at the micro-arcsecond level. Using Gaia measurements of field angles we look at the influence of the number of reference stars and the stars magnitude as well as astrometric systematics on the total error budget with the help of Gaia-like simulations around the Ecliptic Pole in a differential astrometric scenario. We find that the systematic errors are modeled and reliably estimated to the μas level even in fields with a modest number of 37 stars with G <13 mag over a 0.24 sq. degrees field of view for short timescales of the order of a day for a perfect instrument and with high-cadence observations. Accounting for large-scale calibrations by including the geometric instrument model over such short timescales requires fainter stars down to G=14 mag without diminishing the accuracy of the reference frame.

We report the results of a successful 12-hour 22-GHz VLBI experiment using a heterogeneous network that includes radio telescopes of the Long Baseline Array (LBA) in Australia and several VLBI stations that regularly observe in geodetic VLBI campaigns. We have determined positions of three VLBI stations, atca-104, ceduna and mopra, with an accuracy of 4–30 mm using a novel technique of data analysis. These stations have never before participated in geodetic experiments. We observed 105 radio sources, and amongst them 5 objects which have not previously been observed with VLBI. We have determined positions of these new sources with the accuracy of 2–5 mas. We make the conclusion that the LBA network is capable of conducting absolute astrometry VLBI surveys with an accuracy better than 5 mas.

We report the results of a successful 7-hour 1.4 GHz Very Long Baseline Interferometry (VLBI) experiment using two new stations, ASKAP-29 located in Western Australia and WARK12M located on the North Island of New Zealand. This was the first geodetic VLBI observing session with the participation of these new stations. We have determined the positions of ASKAP-29 and WARK12M. Random errors on position estimates are 150–200 mm for the vertical component and 40–50 mm for the horizontal component. Systematic errors caused by the unmodeled ionosphere path delay may reach 1.3 m for the vertical component.

An overview is given over the broad field of Relativity in Fundamental Astronomy. The present status is recalled and deficiencies are pointed out that might lead to future work within IAU Commission 52.

The last version of the planet part of EPM's ephemerides of IAA RAS (EPM2011) is described briefly. At present EPM ephemerides are the basis for the Russian Astronomical and Nautical Astronomical Yearbooks and are used for scientific research.

In this paper we outline several problems related to the realization of the international celestial and terrestrial reference frames — the ICRF and ITRF — at the millimeter level of accuracy, with emphasis on ICRF issues. We consider here the current status of the ICRF, the interrelationship between the ICRF and ITRF, and considerations for future ICRF realizations.

The IERS Conventions (2010) provides the international standard for models for use in the generation of celestial reference systems (CRS), terrestrial reference systems (TRS), and the Earth orientation parameters (EOPs) that relate the associated frames. Significant improvements over the previous IERS Conventions (2003) are outlined, and an overview of the latest adopted models and standards is shown. Finally, future plans for the Conventions are provided.

Expectations of the Gaia astrometry mission regarding the realisation of an optical kinematical reference frame based on extragalactic sources are summarized.

Standards of Fundamental Astronomy (SOFA) is an International Astronomical Union (IAU) service that provides accessible and authoritative algorithms and procedures that implement standard models used in fundamental astronomy. This paper summaries the current status, noting the changes during 2009-2012, and discusses issues that may arise in the future.