Quasar

Stars

Calibration

Galaxy

Computer science

Astronomy

Geography

Astrophysics

Quantum mechanics

Parallax

Context (archaeology)

Telecommunications

Geodesy

Proper motion

Satellite

Archaeology

Magnitude (astronomy)

Astrometry

Reference frame

Epoch (astronomy)

Physics

Frame (networking)

Astronomy and Astrophysics

Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these results performed within the astrometry task. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G&lt;14 mag)...

<i>Gaia</i> Data Release 2