Early experiments in optics suggested an elusive quantity existed in light. Measurements of variables such energy and wavelength were possible with calorimeters and prisms. The speed of light was slowly (over a period of centuries) measured with increasing accuracy, and with various...
As with anything galactic, catching the sense of the rotation of the Milky Way takes time and repeated observations, reinterpretation and so on. It takes over 600,000 years just to go around it one degree. Observe, and then a few months later, and so on for several years.
A good way to...