Paranal Observatory and the GRAVITY interferometer

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The European Southern Observatory site on Cerro Paranal in the Atacama Desert, whose four 8.2-metre Unit Telescopes can be combined interferometrically. The GRAVITY beam combiner, which links all four, delivers infrared astrometry at the level of tens of microarcseconds - precise enough to follow individual stars around Sagittarius A* through their closest approaches. That capability produced two of the strongest tests of general relativity in a strong field: the detection of the gravitational redshift in the light of the star S2 at its 2018 pericentre passage, and the detection in 2020 of the Schwarzschild precession of S2's orbit, an effect of the same kind as the anomalous precession of Mercury but far larger. The instrument works by measuring the star's position relative to the black hole's own faint infrared emission, so it measures the orbit directly rather than inferring it.