2 Accuracy of astrometric observations 1 mas 1 µas 10 µas 100 µas 10 mas 100 mas 1 10 100 1000 1 µas 10 µas 100 µas 1 mas 10 mas 100 mas 1 10 100 1000 140015001700190020002100016001800 Ulugh Beg Wilhelm IV Tycho Brahe Hevelius Flamsteed Bradley-Bessel FK5 Hipparcos Gaia SIM ICRF GC naked eye telescopes space 140015001700190020002100016001800 Hipparchus 4.5 orders of magnitude in 2000 years further 4.5 orders in 20 years 1  as is the thickness of a sheet of paper seen from the other side of the Earth
{}