If a planet (of any size) orbits a star perpendicular to the line of sight, we would observe no Doppler effect at all. ![]() The Doppler effect that we would observe is reduced. By simple geometry, the component of reflex motion in our direction is cos (45º) or 0.7 times smaller at every point in the orbit. Suppose that the plane of the planet orbit was inclined at 45º to the line of sight. If we are lucky enough that the plane of the planet orbit is parallel to our line of sight, then the planet is moving directly toward us and directly away from us as it orbits the star. And to be safe we would want to observe for longer than 12 years to make sure that the phenomenon repeated and was cyclic. We would have to make measurements for 12 years to see a complete cycle of variation. Light emitted from galaxies moving toward you would be squished, making the wavelength shorter and the light bluer. This circular motion gives a sine-wave variation of the Doppler effect. The nearly circular motion of the planet creates a nearly circular reflex motion of the star. By how much would its wing lights be blueshifted? Since 1000 mph is 0.46 kilometers per second and the speed of light is 300,000 kilometers per second, the blueshift would only be 0.46 / 300,000 = 1.5 x 10 -6 or about one part in a million. Imagine a supersonic jet fighter was heading toward us at night at 1000 mph. The Doppler effect for light is generally very small. To make this happen a police car would have to travel at 1/3 the speed of sound, or 110 meters per second (225 mph)! How fast would a police car have to be moving toward you for the pitch of its siren to increase by 50%? (This is called a "fifth" on the musical scale, or a change in the note from C to G, for example.) Increasing the pitch by a factor of 1.5 means decreasing the wavelength by a factor of 1/1.5 = 0.67, so Δ λ / λ = 1/3. In this case, c is not the speed of light but the speed of sound - 330 meters per second. Here is an example of the Doppler effect using sound waves. By repeating the experiment with different train speeds, they deduced the relationship given above. The scientists then measured the change in pitch of the note as the train approached and receded, compared with the note of a stationary musician. Scientists hired a brass band and got the trumpeters to belt out a single sustained note from an open railroad car as it passed by at a known speed. ![]() In Doppler's time, this equation was derived using a sound experiment. The fractional (or percentage) wavelength shift is equal to the velocity of the source as a fraction of the velocity of light. In astronomy, for instance, the detection of extra solar planet became. Here, Δ λ is the small change or shift in wavelength and λ is the normal wavelength when the source is not moving. The Doppler effect has found practical expression in various sections of science. The size of the Doppler effect is given by the simple formula
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