On Sunday Saturn reaches opposition, when it will be closest to the Earth for the year. The closer something is to us, the brighter it appears in the sky. Not only will the planet be brighter than compared to months ago and months hence, but around opposition a curious phenomenon known as the Seeliger Effect is at play to briefly make Saturn appear even more luminous.
At opposition Saturn is directly opposite the sun in the sky, meaning it rises in the east at sunset and sets in the west at sunrise. Earth is briefly lined up in the middle between the sun and Saturn as shown in the diagram at right.
This is the same arrangement we see during full moon, when the moon is opposite the sun in the sky, rising at sunset. A few days ago I mentioned that at full phase, the moon’s brightness kicks up a notch. Something is making the moon brighter beyond just the increase in the area lit up by the sun compared to say a half moon.
Several factors contribute to the brightening, but one of the key ones is called the opposition effect. When we face opposite the sun – with sunlight coming from directly behind us – objects in front of us are squarely in sunshine. Any shadows cast by rocks, bumps or irregularities are hidden directly behind the objects. Without shadows to ‘darken’ the scene, the view directly in front of us peaks in light intensity.
Something similar happens when we look at Saturn’s rings during the days leading up to and after opposition. The sun shines directly at the rings, shadows hide behind the icy chunks that compose them, and we witness a surge in their brightness.
This brightness enhancementÂ was first studied by German astronomer Hugo von Seeliger, who lived from 1849 to 1924. Seeliger thought the loss of shadows from the particles in the rings was the cause for their rapid brightening and saw it as confirmation that the rings were made of particles rather than being solid.
While that may be part of the explanation, another phenomenon called coherent backscattering is also at play and may be even more important in the case of Saturn’s rings. It’s like this. When you shine a beam of light at a material made of lots of separate particles or pieces like the rings, it’s reflected back with greater intensity from the direction directly opposite the beam. In other words, a brighter reflection comes straight back at you. Backscattering also plays a role in the bright halo effect in the moon photo above.
Now the big question is, can you see really see this through a telescope? Yes! It’s subtle but visible. If you pay attention and notice how Saturn’s rings look now and then go back and re-observe them in several weeks, you should be able to see the difference. Compare them to the planet’s globe or consider rating them on an intensity scale with 1 for pure white and 10 for dull gray. Assign a number to their brightness each time you observe the planet and then compare your results. We’d love to hear what you find out.