Title: GIER: A Danish computer from 1961 with a role in the modern revolution of astronomy,
Abstract: A Danish computer, GIER, from 1961 played a vital role in the development of
a new method for astrometric measurement. This method, photon counting
astrometry, ultimately led to two satellites with a significant role in the
modern revolution of astronomy. A GIER was installed at the Hamburg Observatory
in 1964 where it was used to implement the entirely new method for the
measurement of stellar positions by means of a meridian circle, then the
fundamental instrument of astrometry. An expedition to Perth in Western
Australia with the instrument and the computer was a success. This method was
also implemented in space in the first ever astrometric satellite Hipparcos
launched by ESA in 1989. The Hipparcos results published in 1997 revolutionized
astrometry with an impact in all branches of astronomy from the solar system
and stellar structure to cosmic distances and the dynamics of the Milky Way. In
turn, the results paved the way for a successor, the one million times more
powerful Gaia astrometry satellite launched by ESA in 2013. Preparations for a
Gaia successor in twenty years are making progress.