QUEST FOR QUASARS

Gaia’s second data release precisely mapped 556,869 quasars. Astronomers used this information to create a celestial reference frame: a fixed but imaginary grid against which everything else moves, comparable to Earth’s lines of longitude and latitude. Since the planet spins and wobbles, such a frame is important to not only guiding where we point telescopes but also keeping GPS systems working accurately. This image shows the formal position uncertainty of the quasar sources across the whole sky, which averages less than 0.2 milli-arcseconds.