The answer is; Transverse (sideways) motion (perpendicular to your line of sight to the object)
Doppler shift effect occurs when an object that is emitting light is either moving closer toward the observer or is moving away. These astronomical objects that are moving towards the observer, he or she will view them in blue wavelength of light. Those that are moving away will seem to emit a red wavelength of light. Doppler shift effect is similar to the high pitch wails of an an ambulance coming towards you and then followed by low pitch wails by the same ambulance when it passes past you and is moving away from you.