Saturn's Spectacular Sights Stun Skywatchers, On Earth, In Orbit And At Saturn Itself

Once per orbit, as the Earth completes its annual revolution around the Sun, it also passes and laps the outer worlds, who move so slowly they take decades -- or, in the case of Neptune and Pluto, centuries -- to complete even one orbit around our central star. It's for this very reason that Saturn, the farthest naked eye planet and slowest moving world relative to the backdrop of stars, has been known since antiquity as the "old man of the skies."

Yet to those who observe Saturn year-after-year, a few important things become not only evident, but also astounding to behold:

• once a year, Saturn appears brighter and larger than at any other time,
• the shadow of the planet, which normally falls on the rear of the rings, becomes invisible during this time, and
• if you observe the position of the rings year-after-year during this time, you'll find that their inclination (or how "edge-on" versus how "face-on" they appear) changes on an annual basis.

This moment in time happened this past Saturday -- May 23rd -- for Saturn, which will continue to be at its brightest, largest and (apparently) shadowless for the next couple of weeks. If you want to glimpse it for yourself, simply look to the southern skies (from the northern hemisphere) after sunset, near the bright red giant Antares, and to the east of the bright blue star Spica.

Saturn will appear yellowish compared to the other stars, and won't twinkle: a telltale sign of its planethood. And if you can locate it and look at it through a telescope, its rings will be widely separated from it. In fact, its rings will next appear edge-on in 2024 and again at regular 14-15 year intervals, due to its 29.5 year orbital period.

But if the eyes of our telescopes on Earth were especially keen, the exact moments of opposition -- where the Sun is directly opposite to Saturn in the skies -- would have brought on a spectacular phenomenon: an intense brightening at one particular point, known as the opposition effect. Normally, when sunlight falls on an object in space, some of it is absorbed, some of it is reflected, and the absorbed sunlight leaves "shadows" (or regions of darkness) where that light then doesn't fall. In addition, the light that is reflected has electric and magnetic fields that sometimes add coherently, sometimes cancel, and sometimes add only partially; for the most part, it's random.

But, as you'll notice from these Cassini images (above) of Saturn that showcase the opposition effect, there can be a very bright optical phenomenon that looks like a white sphere, radiating outwards along the Saturnian rings. This is light from the Sun, but unlike the normal reflected light that illuminates the planet (or whatever object is behind it), this light has two extra optical properties:

1. 100% of the "shadows" fall directly opposite to the point-of-view of the observer (in this case, Cassini), meaning that there's an incredible increase in reflected light as there are effectively no shadows.
2. The reflected light, due to the perfect optical alignment, scatters back coherently, meaning that the electric and magnetic fields of the reflected light all add up, further intensifying the light.

As the Sun (and the observer) continue to move relative to the planet and its rings, the opposition effect appears to migrate across the planet's surface.

Note how the ring shadows appear to move at a different speed from the opposition effect; this is due to the relative motion of the spacecraft, the Sun and Saturn, while the spacecraft's position doesn't matter for the ring shadows. Right now, this phenomenon is only visible on another world if we're orbiting it; our resolution using telescopes from Earth -- even from Hubble in space -- is simply not good enough to see it. But thanks to Cassini, we even have our very first color image of the opposition effect on Saturn. If you look closely, you might even see a rainbow-colored effect.

But this is not what you'd see at all if you were there! Rather, this is a consequence of Cassini's inferior real-time imagining technology. While our human eyes are capable of taking in red, green, and blue light all at once through our cones, Cassini's camera -- much like Hubble's -- can image only one color at a time. So by time it's done imaging the red light and has moved on to green and then blue, the relative motion of the spacecraft has caused the opposition effect to appear in a different location, and hence the colorful rainbow effect you see here.

The opposition effect is one of the most spectacular optical phenomena visible on another planet, and thus far, only Cassini has been able to see it on Saturn. Perhaps, someday in the future, our telescope technology will be advanced enough that we'll be able to enjoy these sights each and every year from the comfort of our own home world. Until then, we'll have to visit the distant worlds ourselves to catch this spectacular sight!