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Originally Posted by Sam5
Call it “pulses” if you want. A 1,000 Hz pulse sent by radio.
Is the nitpicking night on the board?
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Are you kidding? Every night is nitpicking night, here at the BABB. 8)
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Originally Posted by Sam5
And trust me. If I had said "cycles", someone would have complained because I did not say Hz.
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Yes and no. I'd complain, and will complain even now, that you said "cycles" instead of "cycles per second", when the post to which you were responding was clearly and unambiguously speaking of "cycles per second". You're the only one who wants to bring raw, unadorned "cycles" into it, after bringing up "Herz per second" before.
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Originally Posted by Sam5
What causes this type of Doppler shift is because the transmitter on the spacecraft is sending out 1,000 Hz, but the distance between the earth and the spacecraft increases constantly, so the signal has further and further to travel. This stretches out the wavelength of the signal in space, so that the frequency that is received on earth is only 999 Hz per second. If the spacecraft could stop in space, so that the distance between it and the earth does not increase, then the earth would receive the full 1,000 Hz per second.
If the spacecraft is moving toward the earth and is sending out a 1,000 Hz signal, then the radio wavelengths are compressed, because the distance between the radio and the earth is decreasing, and so the radio on earth receives something like a 1,001 Hz per second signal. This is called a “blueshifted” signal.
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Originally Posted by Sam5 then
As proof of this theory, we could also use a reflective mirror on a moving spacecraft, and we can have a light pulse signal sent from earth and reflected back from the moving mirror. This way we know that the 1,000 Hz signal never changes at the source, because the source is at the earth and is not moving.
But when the mirror moves away from the earth, and the signal is reflected back, the signal is redshifted and only a 999 Hz per second signal comes back to the earth. When the mirror starts back toward the earth, the reflected light signal becomes blueshifted and the earth receives 1,001 Hz per second.
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You don't state that these mirror-bearing spacecraft are moving at the same rate as the transmitting spacecraft, but it would be a reasonable assumption for anyone attempting to learn anything here. So, for their benefit, I'd point out that if they were travelling at the same velocity, Earth would receive first approximately 998 Hz (not per second) as it travels away from Earth, then approximately 1002 Hz on the return trip.
Were it otherwise, you could try to argue your way out of your next speeding ticket by telling the judge that the radar gun manufacturers make radar guns that read twice your actual speed. :wink: