Saturn and the Seeliger Effect: Seeing is believing

When an outer or superior planet lines up beyond Earth on the same side as the sun, it’s at opposition and closest to Earth for the year. Saturn reached opposition this past weekend. When the same planet lines up with the Earth on the opposite side of the sun, it’s in conjunction and invisible in the solar glare. Illustration: Bob King

This weekend we looked at Saturn from the unique perspective called opposition, when a planet is directly opposite the sun in the sky. Seen from space, Saturn is lined up with Earth on the same side as the sun. Seen from the ground, the planet rises at sunset, remains visible all night and doesn’t set until the sun rises the next morning. Talk about opposites – the two bodies can’t get any farther apart.

Opposition only happens with the outer planets – Mars through Neptune. Since they’re farther from the sun than Earth, they can periodically sneak up behind us.  Mercury and Venus, being closer to the sun, can’t take a walk around our backside and therefore never appear opposite the sun in the sky. Traditionally, the planets that orbit beyond Earth are called “superior” while Mercury and Venus are “inferior” or inner planets.

What a dramatic difference opposition makes! Compare Saturn’s rings on April 28 and back on March 2. The opposition or Seeliger Effect is obvious. Notice that the globe remains unchanged. Click to see more Go’s Saturn photos. Credit: Christopher Go

As described in this Friday’s blog, our opposition perspective on Saturn affords a unique view of its rings. For a few days around this time, the rings become considerably brighter than the body of the planet. The photo illustrates it dramatically, but you can also see it with your own eyeballs as amateur astronomer Richard Keen of Coal Creek Canyon, Colorado did through his 12.5-inch telescope this weekend:

“The rings were markedly brighter than the ball of the planet, while usually they’re about the same brightness. The contrast lines between where ball of the planet hides the far side ring, and between the near side ring and the planet, were sharper, too.”

Several factors contribute to the brightening, but a key one is called the opposition effect. When we face with our backs to the sun, objects in front of us are squarely lit by sunshine streaming over our shoulders.  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 streams past the Earth (our shoulders) and shines directly onto the ring particles. With their shadows temporarily hidden, the rings surge in brightness compared to the planet’s globe.

Hugo von Seeliger

This brightness enhancement  was first studied by German astronomer
Hugo von Seeliger (1849-1924) who proposed that the disappearance of shadows as the cause for the rapid brightening of the rings. Seeliger saw it as confirmation that the rings were made of individual pieces rather than a series of solid disks.

The opposition surge is also the primary reason the full moon is so much brighter than the moon the night before or after.

Saturn comes to opposition once a year, but the full moon is at opposition (opposite the sun in the sky) about once a month. During the hours before full phase, it brightens by more than 40% and fades an equal amount hours after full.

From our perspective on Earth, the full moon is squarely lit by the sun. With all shadows removed, the moon experiences an extra surge in brightness. At other lunar phases, shadows play an important role in dimming the moon. Photos: Bob King

What’s going on? Like the ice crystals in Saturn’s rings, the moon dust and grit hide shadows best when the sun shines down squarely upon them. Also any divots or pits on the moon’s surface that might otherwise be filled with shadow are fully – if briefly – illuminated by sunlight.

While “shadow hiding” is an important reason for these surges, the phenomenon of coherent backscatter also plays a role in the moon, Saturn’s rings and other planets and asteroids with rocky, dusty surfaces. When a light source shines at a very direct angle at material made of a multitude of tiny, dust-like particles, multiple reflections combine to produce a single brighter reflection directly back at the observer.

So now I’m eager for clear skies to see all this for myself. Hopefully, you’ll have a chance to do the same.

Saturn’s rings surge in brightness this week

Philippine amateur astronomer Christopher Go's photos clearly show the dramatic brightening of Saturn's rings around the time of opposition caused by the Seeliger Effect. Compare their brightness on the two dates. Credit: Christopher Go

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.

When the Earth is lined up on the same side of the sun as Saturn, our two worlds are closest for the year. That happens on April 3. Illustration: Bob King

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.

Apollo 17 astronaut Gene Cernan photographs his shadow on the moon surrounded by a bright halo caused by the "opposition effect". Objects like the grains of lunar soil are especially bright directly opposite the sun because their shadows are hidden. Credit: NASA

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.

Astronomer Hugo von Seeliger

This brightness enhancement  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.