The Rotation of the Rings of Saturn

Two spectra show that the two handles of Saturn rotate. Maybe Saturn is surrounded by a disk or rings.
We took one spectrum at the left side and one at the right.

Instrument: The Gimp
The two circles show the projected fiber.

One spectrum is shifted about 2 Ångstroms against the other. This shift is called Doppler shift and allows us to calculate the speed of the rings.

Click the buttons to move the blue spectrum about Å until the larger absorption lines do coincite.
Now we may do two exercises:
  1. Calculate the speed of the ring. Because we did compare the wavelength of the left side of the ring with the wavelength of the right side and not the ring with the body of Saturn, we must use the half value of our shift. The light from the rings is reflected sunlight. We must divide your result once more by 2.
  2. The mass of Saturn is MSaturn5.7×1026kg. With Newton's "Law of Gravitation" we can calculate the radius of the path of a body the Saturn at the calculated speed from above. Does this radius lie within 80,000 km to 140,000 km the inner and outer radius of Saturns bright ring system?


    • Fgrav = G × MSaturn × m × r-2
    • Frad = m × v2 × r-1
    • G = 6.67×10-11 m3 kg-1 s-3

Identification of the five lines:

The solar spectrum (black line) shows:
CaI at 6102.7 FeI at 6102.2 and 6103. We can not resolve these lines.
CaI at 6122.2
FeI at 6136.6, 6137.0 and 6137.7
CaI at 6162.2
CaI at 6169.0, 6169.6 FeI at 6170.5. Not resolved here.

The identifications are taken from Moore, Minnaert, Houtgast: The Solar Spectrum.