= Stellar orbits at the Galactic centre
The useful idea is to weigh an unseen object through its effect on a visible tracer. A curved <stellar orbit> carries more information than a single fast-moving <star>: its changing position and <velocity> constrain a common centre, a length scale and a clock. The <Galactic-centre stellar-orbit experiments> combine <astrometry> with <Doppler spectroscopy>; the <Galactic-centre point-mass orbit model> below shows how the two measurements complement one another.
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