Mapping Our Galaxy in 3D

Over 2000 years ago, Hipparcus measured the distance to the Moon by triangulation from two locations across the Mediterranean Sea.  However, determining distances to stars proved much more difficult.  Many of the best scientists of the 16th through 18th centuries attempted to measure stellar parallax, not only to determine the scale of the cosmos but also to test whether the Earth orbited the Sun or vica versa.  While these efforts failed, they lead to many discoveries, including atmospheric refraction, precession of the Earth's spin axis, and aberration of light.  It was not until the 19th century that Friedrich Bessel measured the first credible stellar parallax.

     Distance measurement in astronomy remained a difficult problem even into the early 20th century, when the nature of galaxies ("spiral nebulae") was still debated.  While we now know the distances of galaxies at the edge of the Universe, we have only just begun to measure distances accurately throughout the Milky Way.  I will present new results on parallaxes and motions of star forming regions and the compact object at the center of the Milky Way.  Using the Very Long Baseline Array we now can achieve positional accuracy approaching 10 micro-arcseconds!  These measurements address fundamental problems in Galactic dynamics, evidence for supermassive black holes, and the mass of the dark matter halo of the Milky Way.