Astrometry: GAIA

Astrometry ^? is the measurement of the position and velocity of objects in the sky.

Ancient, or positional astronomy, measured the position of the stars and the planets on the celestial sphere, in relation to each other. Until the appearance of telescopes, the movements detected only concerned the planets, which changed position during the course of the year in relation to the stars, and the sphere of the stars, whose return to the sky defined the passing seasons. The only exceptions to this regularity were comets ^? .

The first estimates of the size and movement of the Moon and the Sun based on observations with the naked eye were approximate.
Photography, and then CCD ^?  were to make it possible to detect and then measure the positions of the stars, but also their motion. In particular, nearby stars change position every six months with respect to their remote counterparts: this “parallax” movement, resulting from the Earth’s movement around the Sun, gives a measurement of their distance.

The first astrometric satellite ^?  Hipparcos, launched in 1989, improved the distance calculations of thousands of stars by a factor of 50.

The GAIA, satellite ^? , the major mapping satellite ^? from the European Space Agency (ESA), was launched in 2013 and has detected numerous planets by measuring the disturbance of the star’s movement caused by the planet, in the reference plane of the sky.

With a maximum precision of 5 arc-microseconds, and by observing the movements of the stars, it will be able to list all Jupiter-type planets orbiting around the 200,000 closest stars, up to several hundred parsecs ^? from the Sun.