We are observing the roughly circular orbit of a star from the side, like watching a record spin on a turn-table, or a flying disk float through the air.

As the star rotates from position 1 to position 2, position 3, position 4, and then back to position 1, it will move toward us and to the right, away from us and to the right, away from us and to the left, and then toward us and to the left. Trace its movement along the orbital path with your finger, and make sure that you are aware of both its velocity from side to side (moving across the sky, neither toward us nor away from us), and its velocity in and out of the screen (toward us when the star is blueshifted, and away from us when the star is redshifted).

If the star's motion is purely across the sky, there will be no shift in the position of emission or absorption features in its spectrum. It will be observed at its rest wavelength (the same wavelength which is measured for this feature in a laboratory at rest).

If the star is moving toward or away from us, however, then any features will shift to shorter or longer wavelengths. If a particular feature is blueshifted toward shorter wavelengths by a certain number of Angstroms (recall that there are one-hundred-million Angstroms per centimeter) in one position, the amplitude of that shift in wavelength will correlate with the speed of the star. If it is then observed later on moving away from us at the same speed, where will the feature be shifted in wavelength?