We are all familiar with the Vernal Equinox and the Autumnal Equinox. These are the two times of the year when the Sun shines directly at the equator. Equinox is not a speciality of planet Earth; all planets that are tilted with respect to their orbits will experience equinox, including Saturn. But unlike Earth, where equinox happens every 6 months, Saturn’s equinoxes is about 15 years apart.
Saturn is always famous for its rings. But anyone who points a telescope at it this year and expects to see its magnificent rings is in for a disappointment – the rings have disappeared…
This is due to the equinox; Saturn’s rings are around its equatorial plane, thus during equinox, sunlight will hit the rings edge-on, and since the rings are so thin, it seems that the rings have disappeared. Like this:
Equinox had finally arrived at Saturn last week on August 11. This equinox was different from the past, at least to us earthling, because we had Cassini there to give us spectacular view of the play between sunlight and shadow on the planet. We can clearly see that the rings had narrowed down to a very thin line.
The shallow angle of the Sun on the ring plane as a result of the equinox also means that moons and structures that are not in the same plane as the rings – the lumps and bumps – will cast long shadows across the rings. This is just like the Sun will cast long shadows on trees and you and me when is low at the horizon during sunrise and sunset.
Let’s sit back and enjoy the shadows.
The shadow of the moon Epimetheus is cast onto Saturn’s rings, striking the outer-most part of the A ring and only just nipping the F ring.
Saturn’s moon Pan, orbiting in the Encke Gap, casts a slender shadow onto the A ring.
The shadow of the moon Mimas is seen through the unlit side of the Cassini Division of Saturn’s rings.
An object, seen as a bulge within the bright core of the F ring (left), casts a shadow that is long enough to reach the A ring. The shadow is barely visible stretching across the top right quadrant of the image. The shadow appears very faint here because this view looks toward the unlit side of the rings.
The moon Janus casts a shadow on the F and A rings while the gravity of potato-shaped Prometheus (seen on the left), creates a streamer-channel in the thin F ring.
The shepherding moon Pandora casts a shadow on Saturn’s thin F ring as the planet nears its August 2009 equinox.
More shadows! These are created by the stuffs that made up the rings. We are seeing the shadow of the stuffs that made up the rings! Amazing!
Shadow of Saturn’s Moon Epimetheus Crosses Saturn’s Rings
All images credited to NASA/JPL/Space Science Institute
My Dark Sky 
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