Gaia is orbiting around the L2 Lagrange point. As it traverses its orbit, the stars will appear to wiggle (relative to their previous recorded position) because

Gaia is orbiting around the L2 Lagrange point. As it traverses its orbit, the stars will appear to wiggle (relative to their previous recorded position) because of the stellar parallax effect. This (along with precisely knowing Gaia's orbit) enables scientists to accurately measure the distance to the star - if it wiggles more, it's closer. If it wiggles less, it's further away. There is a limit to this method of measurement, based on the accuracy of the telescope. For Gaia: > The European Space Agency's Gaia mission is expected to measure parallax angles to an accuracy of 10 microarcseconds for all moderately bright stars, thus mapping nearby stars (and potentially planets) up to a distance of tens of thousands of light-years from Earth. The Milky Way is about 100,000 light-years wide. So it is able to accurately measure the distance for a _lot_ of stars around us in our galaxy. This video probably explains it better: https://www.youtube.com/watch?v=Jdy09y0A4t0 https://www.youtube.com/watch?v=Jdy09y0A4t0