Saturn's sweet spot
If alien visitors to our solar system ever decide to vote on which of the nine planets is the most photogenic, I have little doubt that Saturn would win by a landslide.
I suspect we humans will never tire of seeing the Ringed Planet.
True, all of the giant planets possess rings, but Saturn's bright, broad, complex system puts the others to shame. Saturn's rings represent of landmark of sorts, a singular feature of the solar system that transforms one planet into the very symbol of otherworldliness.
Despite detailed pictures from observatories and close-ups by spacecraft, Saturn remains the object that makes the deepest immediate impression when seen through a small telescope.
No one ever forgets that first clear glimpse of the Ringed Planet.
My own came when I was fifteen. It was December 1974 and Saturn was in Gemini, just a few degrees from where it lies tonight. Opposition was less than three weeks away, so it was nearly as close and as bright then as it is now.
Late one cold and windy night, I decided to brave the elements and make a quick assault on Saturn. Images in the eyepiece danced as the wind buffeted my little telescope, a 60-millimeter refractor set on a shaky tripod. Soon my eyes were watering from the New England cold.
And then suddenly there it was — a small yellowish oval bouncing around in the field of view. In moments of relative calm I could clearly see the tiny ball of the planet set within an oval formed by the rings.
It looked absolutely unreal, as if I were looking at a tiny Christmas ornament suspended in front of the telescope.
In 2003, as in 1974, Saturn is closer and brighter than it will be in decades. The south side of its rings are tipped toward us at nearly their maximum angle. This is Saturn's " sweet spot," when it lies in the part of its orbit that brings it closest to Earth and tips its rings toward us at their greatest angle.
Saturn is at its best when it lies near the part of its elliptical orbit that brings it closest to the sun — and to Earth. Right now, Saturn is nearly as close as it can get, but each upcoming opposition will find a little farther from Earth until it reaches the most distant part of its orbit in 2018. Then the situation slowly improves until Saturn's next perihelion, in 2032, and the cycle begins again. In fact the Ringed Planet will actually be a bit brighter and closer to us at its 2031 opposition than it is now.
But those icy rings also affect Saturn's brightness. Saturn spins on an axis tilted about twenty-seven degrees to its orbit. This tilt gives us different perspectives on the ring system as the planet circles the sun.
Twice during Saturn's 29-year orbit the rings appear edge-on from Earth. At Saturn's equinoxes, when neither its north nor south pole angles into the sun, we pass through the plane of the rings and they appear edge-on. Naturally, when these reflective rings are edge-on to us, they add nothing to Saturn's brightness and it appears unusually dim.
The southern side of the rings began angling toward us in 1996 and they were fully open this year, giving a boost to Saturn's magnitude. But the rings will close throughout the decade and are edge-on again in September 2009.
So between oppositions occurring ever farther from Earth and the gradual closing of its rings, Saturn fades steadily throughout the decade. By its 2010 opposition, the Ringed Planet will be less than half as bright as it is right now.
After that something curious happens: Saturn brightens even though it continues to move farther from the sun. The reason, of course, is that the northern side of the rings begins tipping our way.
Saturn reaches opposition this year on New Year's Eve. It won't be as close or as bright again until it approaches perihelion in 2031. The southern side of the rings will be tilted toward us then, just as they are tonight — and just as they were in 1974.
I was 15 when Saturn last arrived at its sweet spot. With a little luck, I'll be pushing 72 when the next one comes along.
- The main rings cover an area of just over 15 billion square miles (40 billion square kilometers), or eighty times the total surface area of the Earth.
- They span 174,000 miles (280,000 kilometers), or about 73 percent of the distance separating Earth and the Moon.
- It's little wonder that they vanish when edge-on to Earth — the rings are probably less than 100 feet (30 meters) thick.
- The total amount of material in the rings is surprisingly small, about the same mass as Saturn's moon Mimas (120 miles or 195 kilometers across).
- In 1676 Giovanni Domenico Cassini (1625–1712) observed a gap in the rings known today as the Cassini Division. It's caused by gravitational resonances with Saturn's moon Mimas. Other, smaller gaps are associated with different moons.
- In 1858 physicist James Clerk Maxwell (1831–1879) proved that the rings had to be made of billions of particles orbiting independently.
