Saturn's rings glint brightly thanks to the "opposition effect"
in this June 12 image, taken by the Cassini spacecraft.
NASA/JPL/SSI
While I was out walking Wednesday evening, brilliant Jupiter stood slightly above the gibbous Moon. With the Moon waxing toward its full phase this weekend, I couldn't help but think about how quickly our satellite brightens as Full Moon approaches. As if to drive the point home, NASA has just released an image of Saturn that shows the same effect.
Astronomers first noticed this "opposition effect" with Saturn's rings in 1895. Hugo von Seeliger noticed the rings appeared unexpectedly brighter when Saturn lay directly opposite the Sun in our sky than they did just a few days before or after opposition. The Cassini spacecraft imaged Saturn's A ring June 12, and the brightness surge clearly shows up at the point in the ring exactly opposite the Sun.
The same effect happens with the Moon. Have you ever noticed that the Full Moon (equivalent to the Moon being at opposition) appears far brighter than a quarter Moon? Even though the Moon is half-lit at each of the quarter phases, it's only one-tenth as bright as a Full Moon. And any section of the lunar surface reflects about 40 percent more light at Full Moon than it does just a day before or after Full Moon.
What causes the opposition effect? Surprisingly, it took astronomers a while to figure it out. The first good suggestion involved "shadow hiding." At opposition, any shadows cast by particles in Saturn's rings, or on the Moon's or a planet's surface, lie directly behind those particles — and thus remain out of sight. At all other times, those shadows remain at least partially visible.
Shadow hiding tells only part of the story, however. Another effect, called "coherent backscattering," contributes a roughly equal amount. In this case, the electromagnetic waves that make up light reflected back toward the source interfere constructively with one another. This constructive interference peaks when the observer lies directly between the light's source and the illuminated object.
It will give you something to think about when the Moon waxes full Sunday evening — assuming clouds don't interfere with the view.