 When we observe light from distant galaxies, we are seeing the light from the stars in those galaxies, and that light has absorption lines. The same lines measured in a lab give us the wavelength of the light at the time it was emitted. What we observe is the wavelength stretched over the time it took to get here. We define redshift z as the difference between the two divided by the wavelength emitted. In this hypothetical example, we have an object with a redshift equal to 6. Once a model for the change in the cosmic scale factor over time is specified, redshift gives us a great deal of information. For now, we'll assume a flat matter-dominated Einstein-Desider universe. This will only get us part of the way to the actual numbers, but it helps illustrate the key role redshift plays in cosmology. First, redshift gives us an object's receding velocity. With our model, we have the object moving away at 6 times the speed of light. Redshift also gives us the actual cosmic scale factor at the time the light was emitted. It gives us the age of the universe at the time the light was emitted, and it gives us the amount of time the light was traveling. Redshift gives us the distance to the object at the current time, and it gives us the distance to the object at the time the light was emitted. You can see why astronomers rely so heavily on redshift measurements. Next, we'll use it extensively to count galaxies.