In 1895 James Keeler used the Doppler shift to investigate the motions of Saturn's rings. If you look at the rings edge on, you observe a:

Question

Quizplus · Accepted Answer

If you observe the rings edge-on, you will see that the Doppler shift decreases as one moves outward from Saturn. This is because the outer portions of the rings are moving more slowly than the inner portions. Therefore, the wavelength of light emitted by the outer portions is less affected by the motion of the rings than the wavelength of light emitted by the inner portions. As a result, the Doppler shift decreases as you move outward from Saturn.

In 1895 James Keeler Used the Doppler Shift to Investigate