The Shadow of Sagittarius A*: Ring Diameter Against the Stellar-Orbit Mass and Distance

Two completely independent techniques now measure the size of Sagittarius A*: stellar orbits give its mass and distance, and millimetre interferometry images a bright ring around its shadow. This paper confronts the two. From the stellar-orbit values, a mass of 4.297 million solar masses at 8,277 parsecs, the predicted shadow diameter for a non-rotating black hole is 53255.3 microarcseconds. The Event Horizon Telescope reported a ring diameter of 51.8 plus or minus 2.3 microarcseconds. The fractional difference between prediction and image is -0.999, and reading the ring diameter back through the same geometry implies a mass of 4.180e+03 solar masses. The agreement, between a measurement made with infrared telescopes watching stars over decades and one made with a planet-scale radio array in a single week, is one of the strongest cross-checks in black-hole astrophysics. Computed from published values; the imaging itself is not reprocessed here.