Oort 1932 Weighed the Galactic Disk from Stars' Vertical Motions

… more less Jan Oort (1932) used the vertical speeds and heights of nearby stars to infer the mass density of the Galactic disk near the Sun, finding more than the visible stars supplied; the pull back toward the plane obeys \( \partial^2\Phi/\partial z^2 = 4\pi G\rho \).