The Doppler Theory is the change in the apparent time interval between two events which arises from the motion of an observer together with the finite velocity of transmission of information about the events.
 
 

Through much devotion to mathematics, mechanics, and astronomy, Christian Doppler was able to articulate the Doppler Theory.   Due to this theory, many advances in science and technology have occured.  Meteorologists can determine upper atmospheric temperatures which allow them to predict weather.  Astronomers can begin to understand "the expansion of the Universe" although the objects or body of masses are remote and inaccessible.  Scientists are able to track rockets and satelllites while navigators in military vehicles can maneuver around requiring no contact with the world outside their vessel.  Radars are used for both civil and military advances.  The Doppler Effect even led to advances in the medicinal field allowing doctors to manipulate an Ultrasonic Doppler Flow Detector, and vetinarians use the Doppler Effect to do case studies in small animal cardiovascular systems.  Doppler's work in mathematics, mechanics, and astronomy has an endless list of contributions.