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Mark Reid studies Galactic structure using precise distances and motions of masers. The cited work combines trigonometric parallaxes of high-mass star-forming regions to map the Milky Way and constrain its rotation. This links direct angular measurements with a large-scale model of our Galaxy.^[ref]

The Milky Way is the barred spiral galaxy containing the Solar system. Its galactic disk, central stellar bar and surrounding stellar halo are viewed from inside the disk, so distance and obscuration complicate attempts to reconstruct the whole system. Radio astronomy supplies a geometric route through the obscuring interstellar dust: masers near young massive stars act as compact markers whose parallaxes and proper motions can be measured by very-long-baseline interferometry. A 2019 analysis of about 200 such markers mapped spiral structure and constrained the rotation curve using their three-dimensional motions. These measurements connect the familiar bright band along the galactic plane with a spatial and dynamical model.^[ref]