Lab Exercises in Astronomy: The Rotation Of Saturn and Its Rings

Students will learn the concept of the Doppler effect by measuring Doppler shifts on a spectrum of Saturn and its rings taken with a slit spectrograph. Using these measurements, students will calculate rotation speeds of the planet and ring system.

When the slit of a spectrograph is placed across Saturn and its rings, extending east-west, the resulting spectrum shows the Doppler effect of the rotation of Saturn itself. Its spectral lines slope to the red on the planet's western limb, which is moving away from us, and to the violet on the eastern limb (approaching). The rings exhibit a similar effect, but the ring particles nearest the planet revolve faster than those farther out, in accordance with Kepler’s laws, so the spectral lines’ slope from the rings is opposite to that for the planet. These effects show up clearly on the high-dispersion spectrogram. By measuring spectral line tilts for both planet and rings, the student determines the rotation periods and deduces the nature of the rings and the planet's mass.