This manuscript provides a theoretical basis for answering questions about motion. What is motion? How can motion reference frames be illustrated? Are motion reference frames mixed-inertial, non-inertial, or inertial? How can the mixed-inertial, non-inertial, and inertial reference frames be illustrated? How can the mixed-inertial reference frame be applied to the physical world? How can Galileo's question of motion be understood? How can physics define and distinguish between motion and relative motion? What kind of motion is measured by one-way light experiments, the Sagnac experiment, Foucault's pendulum, and the Doppler Affect? What motion can be measured by a more accurate Michelson-Morley type experiment? A one-way light experiment is presented to understand the direction light travels. Illustrations of the mixed-inertial, non-inertial, and inertial frames of reference are presented to understand the possible paths of the light in the one-way light experiment and reveal the inertial status of the fig.1 drawings in Michelson-Morley (1887). Galileo's theoretical motion question is used along with a heavy fog analogy to understand that two physical phenomena, each with a different velocity, must be present to measure motion. These concepts are used to define and understand the Present Theory of Motion. The Sagnac experiment and Foucault's pendulum measure the relative motion of the Earth's rotation to the experimental apparatus. The Doppler Shift measures the relative motion between the Earth and the star. These examples of relative motion are used with the one-way light experimental example of motion to define and distinguish between the concepts of motion and relative motion. The inertial reference frame, the definitions of motion and relative motion, and the Present Theory of Motion are applied to the one-way light experiment, Sagnac experiment, Foucault's pendulum, and Doppler Shift to explain the measured experimental results. These theories are used to predict the measureable relative motion of a more accurate Michelson-Morley type experiment.