Gaia astrometric accuracy in the past

Gaia is due for launch in the third quarter of 2013 and will open a new page in fundamental astronomy. The astrometric accuracy achievable of about 20 muas/yr at G=15 to below 10 muas/yr at the bright end will permit to know the positions of millions stars with mas accuracy almost 100 years back in time. I will present the current expectations regarding the accuracy of Gaia astrometry at the end of the mission and explain how to propagate the stellar positions and their covariance matrix several decades away from the Gaia epoch without introducing modeling error. I will show also diagrams illustrating the positional accuracy down to 1900 as a function of magnitude and sky position.