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.

Commission 8 has regularly published triennial reports in the past and the current OC therefore voted to adopt a traditional format also for this special Legacy issue of the IAU Transactions. The outgoing President is grateful for the support of many Commission members who contributed to this report. Our contribution consists of 3 parts: 1) this introduction, providing a general overview and highlights of recent research in astrometry, 2) a summary of the astrometry business & science meeting at the 2015 IAU General Assembly, and 3) the activity report of our Commisson covering the mid-2012 to mid-2015 period.

The business meeting was opened by the President, Dafydd Evans, who presented the agenda, which was approved. It was agreed that Norbert Zacharias should be the secretary of the meeting and take the minutes. This session was attended by about 20 participants.

The goal of the division is to address the scientific issues that were developed at the 2009 IAU General Assembly in Rio de Janeiro. These are:

• • Astronomical constants

• —Gaussian gravitational constant, Astronomical Unit, GMSun, geodesic precession-nutation

• • Astronomical software

• • Solar System Ephemerides

• —Pulsar research

• —Comparison of dynamical reference frames

• • Future Optical Reference Frame

• • Future Radio Reference Frame

• • Exoplanets

• —Detection

• —Dynamics

• • Predictions of Earth orientation

• • Units of measurements for astronomical quantities in relativistic context

• • Astronomical units in the relativistic framework

• • Time-dependent ecliptic in the GCRS

• • Asteroid masses

• • Review of space missions

• • Detection of gravitational waves

• • VLBI on the Moon

• • Real time electronic access to UT1-UTC

Gaia is continuing well with its development and construction and is expected to achieve parallax accuracies of about 10 to 300 μas for 6 to 20 mag. It is scheduled to launch in 2013.

The ICRF2 was adopted by the IAU in 2009 as the new fundamental celestial reference frame.

The UCAC project concluded with the publication of its 4th data release (all-sky, over 100 million stars).

The JASMINE project will launch the Nano-JASMINE satellite in 2012 and continues with the development of its other satellites which will complement the Gaia project.

The PPMXL (Heidelberg) and XPM (Ukraine) catalogues provide improved astrometric accuracies from new reductions of USNO-B (USNO-A) and 2MASS data.

It is disappointing that the SIM project was cancelled during this triennium. It would have further complemented the above two satellite projects and would have gone even beyond Gaia in astrometric accuracy.

The business meeting was opened by the President, Irina Kumkova. She presented the agenda, which was approved. This session was attended by 42 participants. The meeting approved Dafydd Evans as secretary of minutes.

A continuation of this WG was voted for at the IAU GA 2006 in Prague. The International Celestial Reference Frame (ICRF) is defined by the positions of 212 distant quasars at radio wavelengths. The primary, optical reference frame is the Hipparcos Celestial Reference Frame (HCRF), which is the Hipparcos Catalog without astrometric ‘problem’ stars (in: H. Rickman (ed.) 2001, Proceedings IAU XXIV General Assembly, Transactions IAU XXIVB (San Francisco: ASP), Resolution B1.2). The Tycho-2 catalog with its 2.5 million brightest stars forms the first step in the densification of the optical reference frame. However, the limiting magnitude of about V = 12 of the Tycho-2 catalog is not sufficient for most applications in astronomy and the goal of this IAU Working Group is to further extend the grid of highly accurate positions and motions toward more and fainter stars. The web site of this WG is at <ad.usno.navy.mil/dens_wg/>.

At the IAU XXVI General Assembly in 2006, the Division I decided to create the Working Group on Astrometry by Small Ground-Based Telescopes (WG-ASGBT). Its scientic goals are to foster the follow-up of small bodies detected by the large surveys including the NEOs; to set-up a dedicated observation network for the follow-up of objects which will be detected by Gaia; to contribute to the observation campaigns of the mutual events of natural satellites, stellar occultations, and binary asteroids; and to encourage teaching astrometry for the next generation. The present report gives the main activities carried out in these areas with small telescopes (diameter less than 2m).

Van Leeuwen has completed and published the new reduction of the Hipparcos data. Parallax accuracies have improved by up to a factor five for the brightest stars and correlations effectively removed.

At the 2003 Sydney IAU meeting, Marion Schmitz (Caltech, USA) took over the chair of the Commission 5 Working Group Designations, succeeding Helene Dickel. The Working Group Designations of IAU Commission 5 clarifies existing astronomical nomenclature and helps astronomers avoid potential problems when designating their sources. The most important function of WG Designations during the period 2003-2005 was overseeing the IAU REGISTRY FOR ACRONYMS (for newly discovered astronomical sources of radiation: see the website <http://cdsweb.u-strasbg.fr/cgi-bin/DicForm>) which is sponsored by the WG and operated by the Centre de Données de Strasbourg (CDS). The Clearing House, a subgroup of the WG, screens the submissions for accuracy and conformity to the IAU Recommendations for Nomenclature (<http://cdsweb.u-strasbg.fr/iau-spec.html>). From its beginning in 1997 through August 2006, there have been 132 submissions and 111 acceptances. Attempts to register asterisms, common star names, and suspected variable stars were rejected. The past three years saw 61 acronyms submitted with 50 of them being accepted. (GIRL - yes; WOMEN - no).

Current and near future all-sky astrometric catalogs on the ICRF are reviewed with the emphasis on reference star data at optical wavelengths for user applications.

We present a mission overview and science possibilities of space astrometry with the Milli-Arcsecond Pathfinder Survey (MAPS).

We present a Véron & Véron based optical extragalactic reference frame progress report.