This description suppresses the motion of the fluid perpendicular to the Galactic disk. The small motions of the stellar fluid relative to the Sun are described by the vorticity, the shear in two directions within the plane of the disk, and the divergence of the flow along the Galactic plane. The distances, and therefore the gravitational gradient and the curvature of the orbital paths, are smaller for the subset of stars that have their motions on the sky measured to a reasonable precision. The stellar velocities change direction by a comparable amount over 1 kpc along the direction of rotation.

This means that the stars we see are influenced by a Galactic gravitational field that varies by an order of 15% along the direction to the Galactic center. Our sight within the Galactic disk is limited to a distance of about 1 kpc from the Sun, compared to a distance of 7.6 kpc to the Galactic center.

The shear and vorticity within the Galactic disk do not give large relative velocities to the stars we see. The actual rotation of the disk is somewhere between differential Keplerian rotation and solid body rotation. At the extreme, the gravitational field generated by this extended mass distribution could cause every star to have the same orbital period, and the Galactic disk would rotate like a solid body, with the rotation velocity of the disk increased proportionally with radius. But unlike the gravitational field in the Solar System, which is almost entirely generated by the Sun, the gravitational field in Galactic disk depends on the distribution of mass within the Galaxy. If the stars in the Galactic disk were orbiting the Galactic center in circular Keplerian orbits, the rotation velocity of the disk would fall as the inverses square root of the distance from the Galactic center. A planet's orbital period increases proportionally with the orbital semimajor axis to the 3/2 power. The shear is greatest if the stars obey Kepler's laws, like the planets orbiting the Sun. This form of rotation gives a shear to the stellar fluid. Analogy to other astrophysical disks, such as accretion disks around neutron stars and the planets orbiting the Sun, suggests that the Milky Way's disk is differentially rotating, with the inner regions completing a rotating in less time than the outer regions. Within the stellar fluid the rotation of the disk appears as a vorticity, a property that describes the rotations within a fluid. The very form of the galactic disks in other galaxies suggests that the Milky Way's disk is rotating around the Galactic center. How this fluid moves as it flows in a disk around the Galactic center tells us something about how the gravitational field and the mass generating the field are distributed within the Galaxy. The fluid itself moves in swirls, shears, and divergences with a velocity that is an average of the velocities of its stellar atoms. As with molecules in a gas, the stars in this fluid have random thermal motions that they acquire over billions of years of numerous weak encounters among themselves. The Sun and its neighboring stars, lying in the Galactic disk, constitute a stellar fluid orbiting the Galactic center. Milky Way Galaxy The Local Motion of the Galaxy

Local motion