Title:   Measuring the Cosmos

Abstract: 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, 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
results on parallaxes and motions of star forming regions with near
micro-arcsecond accuracy using the VLBA.  These measurements address
fundamental problems in astrophysics, including the spiral structure,
size and mass the Milky Way.