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 at a velocity toward or away from us, however, then any features will shift to shorter or longer wavelengths.

By measuring the shifts in wavelength of a certain absorption line as the star moves through its orbit, we can plot its observed velocity over time. Note how the velocity–time curve rises up and then down, appearing symmetric about the line where the velocity is zero (when it is moving neither toward us nor away from us). The up–down pattern repeats with every orbit.

By comparing the velocity of the star at one point with the velocity it will have a little bit later along its orbit, we can match the points along the velocity–time curve to the star's positions along the orbital path. For example, we can distinguish between position 2 and position 4 by examining whether the star shifts from a velocity of zero to a positive, or a negative, velocity next.