Different studies of the global pitch angle of the Milky Way's spiral
  arms
  There are many published values for the pitch angle of individual spiral
arms, and their wide distribution (from -3 to -28 degrees) begs for various
attempts for a single value. Each of the four statistical methods used here
yields a mean pitch angle in a small range, between -12 and -14 degrees (table
7, figure 2). The final result of our meta-analysis yields a mean global pitch
angle in the Milky Way's spiral arms of -13.1 degrees, plus or minus 0.6
degree.
