This dramatic picture of the ring shadows on Saturn was taken by Cassini July 16, 2005, when the rings had been close to their most tilt towards the Solar. The Solar is to the higher proper. The skinny vertical construction at proper is the rings themselves. Two of Saturn’s moons, Mimas and Tethys, are silhouetted towards the planet. Credit score: NASA/JPL-Caltech/House Science Institute/Jason Main
Why don’t Saturn’s rings throw a shadow onto the planet’s floor, like its moons do?
John GrimleyToronto, Ontario
The straightforward reply is that Saturn’s rings do forged shadows on the planet’s floor! NASA’s Cassini spacecraft, which orbited Saturn from 2004 to 2017, took the dramatic picture of the rings’ shadows on Saturn proven above.
The key components of Saturn’s inside large rings are seen within the picture beneath. The rings consist of giant numbers of principally icy particles starting from centimeters to many meters in dimension, plus mud and some bigger embedded moonlets. Their mixed mass is much like that of Saturn’s moon Mimas. All these particles orbit Saturn in Saturn’s equatorial airplane. The particles don’t emit seen mild of their very own however mirror and scatter daylight. In some components of the rings the particles are packed extra densely than in others, which impacts how brilliant they appear due to scattered mild and the way darkish their shadows are. In the identical picture, the extra densely packed components of the rings, just like the A and B rings, are brighter as a result of they backscatter extra daylight, whereas the Cassini division and the C Ring are dimmer. (There are a couple of darkish gaps the place there may be little materials.)
This seen mild picture exhibits Saturn and its most important rings. The outermost brilliant band is the A Ring. The Cassini Division separates the A Ring from the brightest broad band, known as the B Ring. The C Ring is the sparser grayish ring contained in the B Ring. Discover that in seen mild, the rings solely mirror and scatter daylight, so they’re darkish the place they enter Saturn’s shadow on the left. Credit score: NASA/JPL-Caltech/House Science Institute/G. Ugarkovic
For an Earth-based observer, nevertheless, the ring shadows could be troublesome to note for 2 causes. First, the rings themselves can get in the best way of seeing the shadows from our perspective. Second, at instances once we do get a glimpse of the shadows, one wants a good telescope and good seeing to note them.
Saturn is 10 instances farther from the Solar than Earth. So, as seen from Saturn, Earth is all the time practically in the identical course because the Solar, inside about 6°. Consequently, the intense face of the rings will get in the best way of seeing the ring shadows from Earth.
Saturn’s rotation axis is tilted with respect to its orbital airplane across the Solar by about 26.7° — not too completely different from Earth’s 23.5°. Like a spinning high, the orientation of Saturn’s axis and the equatorial rings stay comparatively mounted in area. So, twice per orbit, the rings are edge-on to the Solar and produce solely a slender shadow throughout Saturn’s equator. Between these edge-on occasions, for about half the orbit, the northern face of the rings tilts towards the Solar, casting shadows on Saturn’s southern hemisphere; for the opposite half, the southern ring face is towards the Solar and shadows are forged on Saturn’s northern hemisphere. The extent of the ring shadows on the planet varies fairly a bit relying on the lean of the rings relative to the Solar.
At instances close to most tilt of the rings to the Solar, the shadows produced by the rings cowl a big a part of the daylight aspect of Saturn, as proven within the picture on the high of this web page — however at these instances, the intense face of the rings hides the in depth shadows for an observer on Earth.
This spectacular Cassini picture was taken just some years after the utmost opening of the rings to the Solar that occurred in 2002 (seen from Earth, this most occurs roughly each 15 years). Utilizing gravitational assists from Saturn’s moons, Cassini recurrently modified its orbit round Saturn, leading to all kinds of views on the ring shadows, as illustrated by many photographs you’ll find from the mission.
The second motive, seeing, refers to how a lot small fluctuations in Earth’s environment blur our view of astronomical objects. “Good” seeing refers to when these fluctuations are comparatively gentle, however good seeing is sort of as arduous to seek out today as views free of sunshine air pollution. That’s one motive skilled optical telescopes are on mountaintops or positioned in orbit or on interplanetary spacecraft like Cassini.
So even an observer on Earth with a good telescope can have a tough time really seeing the rings’ shadows. Let’s quantify that. Actually good seeing, which is once more arduous to return by, will blur a star by one second of arc (1″) in angle. Saturn is barely 15″ to twenty″ in diameter as seen from Earth. And the glimpses we get of the ring shadows from Earth throughout instances of minimal ring tilt could be thinner than an arcsecond. (See “A Saturn ring mirage” within the November 2024 situation for extra particulars about making such a difficult remark.)
Richard H. Durisen and Paul R. EstradaProfessor Emeritus of Astronomy, Indiana College, Bloomington, Indiana, and Analysis Scientist, NASA Ames Analysis Heart, Mountain View, California
