Christian Andreas Doppler - He is celebrated for his "Doppler effect"
According GhanaSky.com analyst, Christian Andreas Doppler was born on 29th November 1803 in Salzburg city, Austria. He is celebrated for his principle - known as the "Doppler effect" - that the observed frequency of a wave depends on the relative speed of the source and the observer

Doppler was an Austrian mathematician and physicist

Doppler is known by the named after him "Doppler effect"

The Doppler effect is the temporal compression or elongation of a signal with changes in the distance between transmitter and receiver during the duration of the signal. The cause is the change in the running time . This purely kinematic effect occurs in all signals that propagate at a certain speed, usually the speed of light or the speed of sound . If the signal propagates in a medium , its state of motion must be considered.
Doppler effect in action

Doppler was born the son of a stonemason family in Salzburg. Because of his weak physical constitution, however, he was not up to the requirements of the stonemason profession and studied at the Polytechnic Institute Vienna mathematics, physics and philosophy in Salzburg . From 1829 he worked as an assistant at the Polytechnic Institute Vienna and wrote his first publications.  From 1835 he taught at a secondary school in Prague and later at the Prague Polytechnic Institute (about the rank of today's University of Applied Sciences) Mathematics and Physics. In 1836 he married Mathilde Sturm. In 1840 he became an associate member of the Royal Bohemian Society of Sciences. He became the 1841Full professor of mathematics and physics appointed to the Prague Polytechnic Institute.

In his time as a professor, he published more than 50 scientific papers on physics, mathematics and astronomy in Prague . His main work, On the Colored Light of the Double Stars and some other stars of the sky , he read on 25 May 1842 before the Royal Bohemian Society of Sciences, His hypothesis that the color of the stars is based on the change in distance during the emission of light was already untenable according to the astronomers' knowledge at the time - the star movement is far too slow for that. His efforts to gain recognition, however, led to the early experimental proof of the acoustic Doppler effect - the steam locomotives were just fast enough - so that his name remained connected with the effect.

In the course of the turmoil of the March Revolution of 1848, he moved with his family to Vienna and was appointed in 1850 by Emperor Franz Josef as director of the Physics Institute of the University of Vienna . He was the first professor of experimental physics in the fast-growing metropolis . The family man Doppler was always supported by his highly intelligent wife.

In 1853 Doppler died in Venice , in whose mild climate he had avoided in 1852 because of his lung disease, as a result of pneumoconiosis . At the cemetery of San Michele there is a plaque right at the entrance, but his actual grave in this cemetery has not yet been found.

The Doppler effect was made famous by Christian Doppler , who in 1842 tried to convince astronomers that this effect was the reason why color differences could be seen in double stars between the two partner stars . In his opinion, these stars circle each other so fast that the color of the star just moving away from the observer is perceived with a redshift , while the color of the incoming star is shifted into the blue region of the spectrum. This effect could actually be done after the death of Doppler by the measurement of spectral linesbe detected. But it is too low to explain perceptible color differences. The actual cause of color differences between stars that can be recognized by the eye are their temperature differences.

To explain the effect Doppler set up a thought experiment with the duration of water waves, which are generated every minute from a moving boat. From this he also derived a mathematical description. A merit of Doppler is the realization that the finiteness of the speed of light must also cause a change in the wavelength of the incoming light from moving sources. In French-speaking countries, this is often attributed to Armand Fizeau (1848).

The finiteness of light propagation had already been interpreted 180 years ago by Ole Rømer . Rømer was interested in the suitability of the Jupiter moons as a timer for solving the long-term problem . The eclipses of Jupiter's moon Io were known with a frequency of 1 / 1.8d, which would be well suited as a timer. However, Rømer found that this frequency decreases as the earth moves in its orbit around the Sun just from Jupiter. With {\ displaystyle v _ {\ text {Orbit, Earth}} = 30 \, \ mathrm {km / s}} v_ \ text {orbit, earth} = 30 \, \ mathrm {km / s} is this {\ displaystyle v / c = 1/10 \, 000} {\ displaystyle v / c = 1/10 \, 000}and extends the time from Io-Eclipse to Io-Eclipse just by 1.8d / 10 000 , ie about 1/4 minute. This delay totaled 10 minutes after Joupiter's 40 orbits around Jupiter, which Rømer predicted for November 9, 1676. Although Rømer was actually interested in changing the frequency of the Io-eclipses, he interpreted these 10 minutes much easier than the delay that the light needed for the longer distance.

For the sound waves, the naturalist Christoph Buys Ballot proved in 1845 the Doppler effect. He posed to several trumpeters both on a moving railway train as well as next to the railway line. When driving by one of them should play one G and the others should determine the pitch heard. There was a shift of one semitone, corresponding to a speed of 70 km / h.

Only twenty years later did William Huggins find the predicted spectroscopic Doppler shift in the light of stars. He showed that Sirius is steadily moving away from us.

Another century later, the accuracy was radar measurements between Earth and Venus Astronomical unit of 10 -4 (from the horizontal parallax of Eros ) improved to next 10 -6 based on distance measurements in the lower conjunctions of 1959 and 1961 (eg. In JPL  by amplitude modulation up to 32 Hz), then 10 -8by Doppler measurements on the carrier frequencies over several months before and after the lower conjunctions of the years 1964 and 1966. The results were given as 300 years earlier as running time, since the value of the speed of light was then known only at six places.

For the proof of the peri-rotation of Mercury Doppler measurements of the years 1964 to 1966 were sufficient  - with optical methods one and a half centuries were necessary.

General Doppler Law for Sound Sources

The formulas were derived on the assumption that source and observer move directly toward each other. In real cases z. B. the ambulance at a certain minimum distance from the observer over. Therefore, the distance between the source and the observer does not change evenly, and therefore a continuous transition of the pitch from higher to lower is heard, especially immediately before and after passing.

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