Saturn is gradually climbing out of the Sun's glare in the pre-dawn sky, and by month's end it clears 20° in altitude before civil twilight starts. The ringed planet is both spectacular and subdued in the eyepiece. The rings, even if beginning to close up compared to the last few years, are just an amazing sight. The globe, however, is subtle with its pastel colors and gradually darkening bands as you move north to the darker NPR. For this reason most of us will admire the planet for a minute or two before moving on to another target of interest.
A unique aspect to Saturn is the shadow play of globe and rings. The most obvious is the shadow of the globe cast on the rings as they arc behind the planet. Prior to opposition it is seen on the preceding (western) limb, reaching maximum visibility at western quadrature (which will be May 16th this year). It gradually wanes until it is invisible at opposition and then emerges on the eastern limb, growing until reaching eastern quadrature.
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| Saturn, a month after opposition, shadow on eastern limb |
A little harder to detect at times (and more complicated to predict) is the shadow of the rings falling upon the globe. The geometry is multifactorial in predicting where (or if) we get to see the rings' shadow; the extent to which the rings are tilted and the angle between Earth and the ringed planet can influence it. In general, the shadow of the rings on the planet is obscured when they are "fully open" and then gradually becomes more visible (and thinner) as we head towards Saturnian equinox.
We're entering the period where the rings' shadow is beginning to peek out along the southern border of the rings. Earlier this month there was an interesting observation submitted to ALPO by Clyde Foster, an accomplished planetary imager from South Africa. In it he noted a bright white speck at the intersection of the western limb and southern edge of the rings (below). At first he considered it as an artifact, but then Martin Lewis from England postulated that this actually represented us seeing sunlight passing through the Cassini Division onto the planet.
Mr. Lewis' analysis was greatly aided by the tool known as WinJUPOS, a software program that handles the calculations of Solar System object geometry (among other marvelous functions). The graphical representation of Saturn for early April when Clyde made his observation showed that the ring gap was ever so slightly peeking out from under the rings on the western limb.
Using WinJUPOS to examine the position of the shadow reveals that the "Cassini Gap Sunshine" will continue to emerge from under the rings on the western limb for several more weeks, reaching its maximum exposure as Saturn reaches western quadrature May 16th. It then begins to retreat until it is tucked up under the rings and lost from view by mid-June. The animated GIF below shows the ebb and flow of the CGS.
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| Animation showing the Cassini Gap Sunshine (CGS) visibility |
I suspect that this might be imaged with a telescope as small as 6" in aperture, and possibly even seen visually. Steady seeing will be a huge determinant in your success, but the good news is that at dawn we often find some of the steadiest conditions and Saturn will be near its culmination in the sky. Also be sure to allow enough time for the optics to cool to the ambient temperature, especially with a Newtonian system. I for one am hoping to see this CGS in mid-May with my 10" reflector and will share any success on the HAL Google Group.
And, in full disclosure, seeing the CGS should be easier next spring as the rings close up even further. Sure, it's going to require a little effort to see if you can observe or capture this first opportunity at seeing the sunlight filtering through the Cassini Division on the cream-colored cloud tops below. It is guaranteed to test your planetary observing (or imaging) skills to snag it like Clyde. But then again, isn't testing your skills at the eyepiece a part of what makes this hobby an enduring passion for so many of us?
