Transcript: If stars near the Sun share the same general motion around the center of our galaxy, and if visible light can only penetrate a kiloparsec or so which is a small fraction of the size of the disk, how do we know the overall motions? Astronomers use the twenty-one centimeter line of neutral hydrogen which reveals the cold gas clouds where stars are forming. They also use the carbon monoxide line at millimeter wavelengths. Both of these techniques use long wavelength emission that can easily penetrate the disk of the galaxy. It can arise that two clouds in the same direction might be at different distances from the Sun, so they will have different radial components of their circular motion. By simply assuming that the motions of all clouds are circular, it’s possible to deconstruct the full map of the motions in the Milky Way galaxy. The technique only fails in the directions very close to the galactic center and the galactic anti-center direction where there is no radial component to the motions.