J. R. Herrnstein led a 1999 determination of the Euclidean distance to NGC 4258 from maser emission in its nuclear accretion disk. Very-long-baseline interferometry resolved the changing angular positions of emitting regions orbiting the central black hole.
Combining those proper motions with measured acceleration and the orbital geometry gave a geometric Euclidean distance, rather than a distance obtained by assuming a standard candle's luminosity. The reported supplied an independent anchor for the cosmic distance ladder.[ref]
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