= J. R. Herrnstein
{c}
<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 $7.2\pm0.3\,\mathrm{Mpc}$ supplied an independent anchor for the <cosmic distance ladder>.<A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk by J. R. Herrnstein and others>[Abstract: Keplerian maser disk, proper motions, acceleration and geometric distance]{ref}
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