The third and final part of the course begins, consisting of a series of lectures on cosmology. A brief history of how cosmology developed into a scientific subject is offered. The discovery of dark energy, along with dark matter, played a crucial role in the development of cosmology. The lecture then discusses the discovery of spiral nebulae in 1920, as well as the "Great Debate" over what they were. Hubble’s famous redshift diagram is presented as the basis for Hubble's Constant and Big Bang cosmology. The difficulty of measuring distance of objects in space, and how to do it using the parallax method and the standard candle method, are discussed. Measure brightness using the magnitude scale is explained. Class ends with a review of logarithms.