Lying at the core of many astronomical calculations is a natural phenomenon first described by the Austrian scientist Christian Doppler in 1842. In Astronomy the Doppler Shift is the change in wavelength/frequency of electromagnetic radiation due to the relative motion of the source and observer. This is a common occurrence heard when the siren of an emergency vehicle like ambulance sounds higher in pitch as it approaches and lower as it moves away. Since the Universe is expanding, our position on Earth will be considered stationary with respect to this recession although there is the rotation of the Earth at 0.46 km/s and the mean orbital velocity of the Earth around the Sun at 29.78 km/s to consider, but these will be set aside as they turn out to be insignificant when dealing with the celestial velocities in this book. As will be seen in Chap. 8 , gravity also affects the wavelength of radiation leaving a massive body, but this too is a very minor effect and will be similarly ignored.