Parameters characterizing properties of our galaxy
The Oort constants (discovered by Jan Oort) formula_1 and formula_2 are empirically derived parameters that characterize the local rotational properties of our galaxy, the Milky Way, in the following manner:
formula_3
where formula_4 and formula_5 are the rotational velocity and distance to the Galactic Center, respectively, measured at the position of the Sun, and v and r are the velocities and distances at other positions in our part of the galaxy. As derived below, A and B depend only on the motions and positions of stars in the solar neighborhood. As of 2018, the most accurate values of these constants are formula_1 
 15.3 ± 0.4 km s−1 kpc−1, formula_2 
 −11.9 ± 0.4 km s−1 kpc−1. From the Oort constants, it is possible to determine the orbital properties of the Sun, such as the orbital velocity and period, and infer local properties of the Galactic disk, such as the mass density and how the rotational velocity changes as a function of radius from the Galactic Center.
Historical significance and background.
By the 1920s, a large fraction of the astronomical community had recognized that some of the diffuse, cloud-like objects, or nebulae, seen in the night sky were collections of stars located beyond our own, local collection of star clusters. These "galaxies" had diverse morphologies, ranging from ellipsoids to disks. The concentrated band of starlight that is the visible signature of the Milky Way was indicative of a disk structure for our galaxy; however, our location within our galaxy made structural determinations from observations difficult.
Classical mechanics predicted that a collection of stars could be supported against gravitational collapse by either random velocities of the stars or their rotation about its center of mass. For a disk-shaped collection, the support should be mainly rotational. Depending on the mass density, or distribution of the mass in the disk, the rotation velocity may be different at each radius from the center of the disk to the outer edge. A plot of these rotational velocities against the radii at which they are measured is called a rotation curve. For external disk galaxies, one can measure the rotation curve by observing the Doppler shifts of spectral features measured along different galactic radii, since one side of the galaxy will be moving towards our line of sight and one side away. However, our position in the Galactic midplane of the Milky Way, where dust in molecular clouds obscures most optical light in many directions, made obtaining our own rotation curve technically difficult until the discovery of the 21 cm hydrogen line in the 1930s.
To confirm the rotation of our galaxy prior to this, in 1927 Jan Oort derived a way to measure the Galactic rotation from just a small fraction of stars in the local neighborhood. As described below, the values he found for formula_1 and formula_2 proved not only that the Galaxy was rotating but also that it rotates differentially, or as a fluid rather than a solid body.
Derivation.
Consider a star in the midplane of the Galactic disk with Galactic longitude formula_10 at a distance formula_11 from the Sun. Assume that both the star and the Sun have circular orbits around the center of the Galaxy at radii of formula_12 and formula_13 from the Galactic Center and rotational velocities of formula_14 and formula_15, respectively. The motion of the star along our line of sight, or "radial velocity", and motion of the star across the plane of the sky, or "transverse velocity", as observed from the position of the Sun are then:
formula_16
With the assumption of circular motion, the rotational velocity is related to the angular velocity by formula_17 and we can substitute this into the velocity expressions:
formula_18
From the geometry in Figure 1, one can see that the triangles formed between the Galactic Center, the Sun, and the star share a side or portions of sides, so the following relationships hold and substitutions can be made:
formula_19
and with these we get
formula_20
To put these expressions only in terms of the known quantities formula_10 and formula_11, we take a Taylor expansion of formula_23 about formula_13.
formula_25
Additionally, we take advantage of the assumption that the stars used for this analysis are "local", i.e. formula_26 is small, and the distance d to the star is smaller than formula_12 or formula_13, and we take:
formula_29.
So:
formula_30
Using the sine and cosine half angle formulae, these velocities may be rewritten as:
formula_31
Writing the velocities in terms of our known quantities and two coefficients formula_1 and formula_2 yields:
formula_34
where
formula_35
At this stage, the observable velocities are related to these coefficients and the position of the star. It is now possible to relate these coefficients to the rotation properties of the galaxy. For a star in a circular orbit, we can express the derivative of the angular velocity with respect to radius in terms of the rotation velocity and radius and evaluate this at the location of the Sun:
formula_36
so
formula_37
formula_1 is the Oort constant describing the shearing motion and formula_2 is the Oort constant describing the rotation of the Galaxy. As described below, one can measure formula_1 and formula_2 from plotting these velocities, measured for many stars, against the galactic longitudes of these stars.
Measurements.
As mentioned in an intermediate step in the derivation above:
formula_42
Therefore, we can write the Oort constants formula_1 and formula_2 as:
formula_45
Thus, the Oort constants can be expressed in terms of the radial and transverse velocities, distances, and galactic longitudes of objects in our Galaxy - all of which are, in principle, observable quantities.
However, there are a number of complications. The simple derivation above assumed that both the Sun and the object in question are traveling on circular orbits about the Galactic center. This is not true for the Sun (the Sun's velocity relative to the local standard of rest is approximately 13.4 km/s), and not necessarily true for other objects in the Milky Way either. The derivation also implicitly assumes that the gravitational potential of the Milky Way is axisymmetric and always directed towards the center. This ignores the effects of spiral arms and the Galaxy's bar. Finally, both transverse velocity and distance are notoriously difficult to measure for objects which are not relatively nearby.
Since the non-circular component of the Sun's velocity is known, it can be subtracted out from our observations to compensate. We do not know, however, the non-circular components of the velocity of each individual star we observe, so they cannot be compensated for in this way. But, if we plot transverse velocity divided by distance against galactic longitude for a large sample of stars, we know from the equations above that they will follow a sine function. The non-circular velocities will introduce scatter around this line, but with a large enough sample the true function can be fit for and the values of the Oort constants measured, as shown in figure 2. formula_1 is simply the amplitude of the sinusoid and formula_2 is the vertical offset from zero. Measuring transverse velocities and distances accurately and without biases remains challenging, though, and sets of derived values for formula_1 and formula_2 frequently disagree.
Most methods of measuring formula_1 and formula_2 are fundamentally similar, following the above patterns. The major differences usually lie in what sorts of objects are used and details of how distance or proper motion are measured. Oort, in his original 1927 paper deriving the constants, obtained formula_1 
 31.0 ± 3.7 km s−1 kpc−1. He did not explicitly obtain a value for formula_2, but from his conclusion that the Galaxy was nearly in Keplerian rotation (as in example 2 below), we can presume he would have gotten a value of around −10 km s−1 kpc−1. These differ significantly from modern values, which is indicative of the difficulty of measuring these constants. Measurements of formula_1 and formula_2 since that time have varied widely; in 1964 the IAU adopted formula_1 
 15 km s−1 kpc−1 and formula_2 
 −10 km s−1 kpc−1 as standard values. Although more recent measurements continue to vary, they tend to lie near these values.
The Hipparcos satellite, launched in 1989, was the first space-based astrometric mission, and its precise measurements of parallax and proper motion have enabled much better measurements of the Oort constants. In 1997 Hipparcos data were used to derive the values formula_1 
 14.82 ± 0.84 km s−1 kpc−1 and formula_2 
 −12.37 ± 0.64 km s−1 kpc−1. The Gaia spacecraft, launched in 2013, is an updated successor to Hipparcos; which allowed new and improved levels of accuracy in measuring four Oort constants formula_1 
 15.3 ± 0.4 km s−1 kpc−1, formula_2 
 -11.9 ± 0.4 km s−1 kpc−1, formula_62 
 −3.2 ± 0.4 km s−1 kpc−1 and formula_63 
 −3.3 ± 0.6 km s−1 kpc−1.
With the Gaia values, we find
formula_64
This value of Ω corresponds to a period of 226 million years for the sun's present neighborhood to go around the Milky Way. However, the time it takes for the Sun to go around the Milky Way (a galactic year) may be longer because (in a simple model) it is circulating around a point further from the centre of the galaxy where Ω is smaller (see Sun#Orbit in Milky Way).
The values in km s−1 kpc−1 can be converted into milliarcseconds per year by dividing by 4.740. This gives the following values for the average proper motion of stars in our neighborhood at different galactic longitudes, after correction for the effect due to the Sun's velocity with respect to the local standard of rest:
The motion of the sun towards the solar apex in Hercules adds a generally westward component to the observed proper motions of stars around Vela or Centaurus and a generally eastward component for stars around Cygnus or Cassiopeia. This effect falls off with distance, so the values in the table are more representative for stars that are further away. On the other hand, more distant stars or objects will not follow the table, which is for objects in our neighborhood. For example, Sagittarius A*, the radio source at the centre of the galaxy, will have a proper motion of approximately Ω or 5.7 mas/y southwestward (with a small adjustment due to the Sun's motion toward the solar apex) even though it is in Sagittarius. Note that these proper motions cannot be measured against "background stars" (because the background stars will have similar proper motions), but must be measured against more stationary references such as quasars.
Meaning.
The Oort constants can greatly enlighten one as to how the Galaxy rotates. As one can see formula_1 and formula_2 are both functions of the Sun's orbital velocity as well as the first derivative of the Sun's velocity. As a result, formula_1 describes the shearing motion in the disk surrounding the Sun, while formula_2 describes the angular momentum gradient in the solar neighborhood, also referred to as vorticity.
To illuminate this point, one can look at three examples that describe how stars and gas orbit within the Galaxy giving intuition as to the meaning of formula_1 and formula_2. These three examples are solid body rotation, Keplerian rotation and constant rotation over different annuli. These three types of rotation are plotted as a function of radius (formula_12), and are shown in Figure 3 as the green, blue and red curves respectively. The grey curve is approximately the rotation curve of the Milky Way.
Solid body rotation.
To begin, let one assume that the rotation of the Milky Way can be described by solid body rotation, as shown by the green curve in Figure 3. Solid body rotation assumes that the entire system is moving as a rigid body with no differential rotation. This results in a constant angular velocity, formula_72, which is independent of formula_12. Following this we can see that velocity scales linearly with formula_12, formula_75, thus
formula_76
Using the two Oort constant identities, one then can determine what the formula_1 and formula_2 constants would be,
formula_79
This demonstrates that in solid body rotation, there is no shear motion, i.e. formula_80, and the vorticity is just the angular rotation, formula_81. This is what one would expect because there is no difference in orbital velocity as radius increases, thus no stress between the annuli. Also, in solid body rotation, the only rotation is about the center, so it is reasonable that the resulting vorticity in the system is described by the only rotation in the system. One can actually measure and find that is non-zero (formula_82 km s−1 kpc−1.). Thus the galaxy does not