Briefly explain how we can use spectral lines to determine an object's radial motion. Can we also learn the object's tangential motion (across our line of sight) from its spectral lines?
What will be an ideal response?
By comparing the wavelength of the spectral lines in the object's spectrum to the rest wavelengths of the same lines, we measure the Doppler shift. This tells us the object's radial motion: A shift toward shorter wavelength means the object is moving toward us, and a shift to longer wavelength means it is moving away from us. We cannot learn anything about the object's tangential motion from its spectral lines because this does not affect the line positions.
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