Location of 213 Cepheids used to measure the Oort's constants from Hipparcos proper motions.
Proper motion data for the Cepheid sample shows the cos 2l variation with galactic longitude. This allows one to measure Oort constant A (from the amplitude of the cos term) and B (from the offset from zero).
ESA's Hipparcos satellite, operated 1989 - 1993. A 30cm primary looked simultaneously at two parts of the sky separated by 58 ° and measured very accurately the separation of stars in the two fields. By rotating, the whole sky could be measured, and positions accurate to 1 mas acquired. Ultimately, positions, parallaxes and proper motions for 120,000 stars were published in the Hipparcos Catalog; while somewhat less accurate positions (and parallaxes and proper motions) were published for 2.5 million stars in the Tycho catalog.