Hi, I understand the idea of red shift and how that explains that galaxies are receding, etc. I just don't understand how the measurement is made. I know that, in labs, we can shine white light at hydrogen and analyze the absorption spectrum. Some black lines are going to show up, due to the fact that specific wavelength corresponds to a frequency, which corresponds to an energy that is equal to one of the change in energy levels of that element. But what do they measure/analyze in order to get the red shifted spectrum? I am terribly confused! Thanks!