"Measuring the Cosmos"

Mark Reid

Harvard-Smithsonian CfA

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 the Heliocentric cosmology. While these efforts failed, along the way they lead to many discoveries, including atmospheric refraction, the Earth's precession, and aberration of light. It was not until the 19th century that Bessel measured the first 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. These measurements use the Very Long Baseline Array and achieve accuracies approaching 10 micro-arcseconds. They address fundamental problems in astrophysics, including evidence for supermassive black holes and the mass of the dark matter halo of the Milky Way.