CICLOPS: Cassini Imaging Central Laboratory for OPerationS
Trumpeting the Equinox

Joining other moons in heralding the coming of Saturn's August 2009 equinox, the moon Tethys casts its shadow across Saturn's F ring and part of the A ring.

The penumbra, or outer non-opaque part of Tethys' shadow, has intersected the A ring. Only around the time of equinox are the shadows of the moons cast onto Saturn's rings. For an animation of the moon Epimetheus casting a shadow on the A ring, see PIA11651.

The image on the left was taken in visible light with the Cassini spacecraft wide-angle camera on March 20, 2009. The image on the right was taken with the narrow-angle camera at nearly the same time. Tethys itself is not visible in these images, but the small moon Prometheus (86 kilometers, 53 miles across) can be seen between the F and A rings in the top of the wide-angle camera image.

This view looks toward the unilluminated side of the rings from about 37 degrees above the ringplane. The view was obtained at a distance of approximately 880,000 kilometers (547,000 miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 124 degrees. Image scale is 49 kilometers (31 miles) per pixel in the wide-angle image. In the narrow-angle camera image, the scale is 5 kilometers (3 miles) per pixel.

The Cassini Equinox Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini Equinox Mission visit, and

Credit: NASA/JPL/Space Science Institute
Released: May 1, 2009 (PIA 11483)
Image/Caption Information
  Trumpeting the Equinox
PIA 11483

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Alliance Member Comments
CheshireCat (May 4, 2009 at 11:16 AM):

Actually, that's pretty much the shape that we expected. Remember that the shadow is stretched in one direction (north-south) and not in the other, so the spherical moon casts an (approximately) elliptical shadow. You can simulate this yourself with a ball and the Sun early in the morning or late in the evening. (Or, if you prefer, with a table lamp shining obliquely onto the floor.)
Red_dragon (May 4, 2009 at 3:35 AM):
Superb image. A mosaic with both the shadow and the moon that produces it would be great.

As a side note, surely you've thought this much before than me but why not to map the rings looking for shadows that could be caused by moonlets?. Perhaps that would be a good way to locate them.
Dragon_of_Luck_Mah_Jonng1971 (May 3, 2009 at 6:55 PM):
Side note: That's a somewhat strange shadow cast by a spherical body ( Tethys ).
mipsandbips (May 1, 2009 at 6:54 PM):
What a tremendous opportunity to confirm and map the orbit of
Saturn's moons while during the time of the equinox where the shadows
are the most visible on Saturn's rings!