Saturn at Opposition: Radiant Rings

If you look at Saturn through a telescope this Thursday morning, you’ll notice something very special about the view.  Saturn’s spectacular ring system—which is presently tilted towards us near its maximum amount—will look unusually bright and white compared with the ball of the planet.  Even though you may have looked at Saturn numerous times before and noticed that the rings are brighter and whiter than the disk of the planet (as well they should be, being composed almost entirely of water ice), you will be wholly unprepared for the view that awaits you this Thursday morning.  The rings will be positively radiant!  Why haven’t you noticed this before?

Saturn, you see, reaches opposition at 5:18 a.m. CDT on Thursday, June 15.  At the moment of opposition in Iowa County, Wisconsin, Saturn will only be 2° above the SW horizon and the Sun just three minutes before sunrise in the NE.  Best to look around 3:03 a.m. at the beginning of astronomical twilight when Saturn will be a respectable 19° above the SSW horizon.

When a superior planet (like Saturn) is at opposition, the Sun, the Earth, and the planet (in that order) form very nearly a straight line.  When we look at Saturn when it is at opposition, we see sunlight reflected off of the icy ring particles pretty much along the path the sunlight took.  Put another way, when light is shining normal (perpendicular) to a reflective surface, more light is reflected back along the normal than is scattered in other directions, so the reflection seems bright.  What’s more, since our line of sight to Saturn’s rings at opposition most closely aligns with the line of incident sunlight, we “see” no shadows from the ring particles, and the rings appear even brighter because of that.  Of course, we can’t see individual ring particles, but when sunlight strikes the rings more from the side, innumerable tiny shadows are cast by ring particles on other ring particles, and the total amount of light reflected back to us is diminished.  This phenomenon is called the opposition effect.

Saturn’s equator and ring plane are tilted 26.7° relative to its orbit around the Sun, so that means we see the rings at different angles throughout Saturn’s 29.5 year orbital period.  For example, the rings were seen edge-on in 1995.  They were tipped 26.7° to the north (meaning we had the best view of the south side of the rings and the southern hemisphere of the planet) in 2003.  The rings were again seen edge-on in 2009, and this year they are tipped 26.7° to the south, meaning we presently have the best view of the north side of the rings and the northern hemisphere of the planet.

Saturn goes through seasons just like the Earth, and the tilt of the rings reflects this.  Just as the northern hemisphere of the Earth is tilted most towards the Sun at summer solstice, Saturn’s northern hemisphere and the north side of its rings are tilted most towards the Sun when Saturn is at its summer solstice.  At the autumnal equinox on Earth, the Earth’s equator lines up with the Sun, and when Saturn is at its autumnal equinox, Saturn’s equator and its rings line up with the Sun.  At winter solstice, the southern hemisphere of the Earth is tilted most towards the Sun.  Likewise, when Saturn is at winter solstice, the south side of Saturn’s rings and its southern hemisphere are tilted most towards the Sun.  At the vernal equinox on Earth, the Earth’s equator once again lines up with the Sun.  And, when Saturn reaches its vernal equinox, Saturn’s equator and its rings once again line up with the Sun.  And so on.  The only difference is our four seasons take a year, but the four seasons on Saturn take nearly 30 years.

The tilt of Saturn’s rings would thus progress peacefully and sinusoidally with two maxima and two minima every 29.5 years—if we were observing from the Sun.  But, of course, we are observing Saturn from spaceship Earth, so the tilt of the rings that we see changes due to our position relative to Saturn in addition to Saturn’s position relative to the Sun.  This causes more rapid—albeit smaller—variations in the observed tilt angle as we orbit much faster around the Sun than Saturn does.  For example, right now we see Saturn’s rings tilted 26.6°.  On July 15, it will be 26.7°.  On August 15, 26.8°.  On September 15, 26.9°.  On October 15, 27.0°.  On November 15, back to 26.9° again.

The best time, then, to see Saturn’s rings at their most radiant is when Saturn is at opposition and when Saturn’s rings are near maximum tilt. Here on Spaceship Earth, that will next occur on Thursday, June 15, 2017.  Fortunately, Saturn’s phase angle will be just 0.1° during all of Wednesday night and Thursday morning.  Saturn rises at 8:27 p.m. Wednesday evening, June 14, and reaches 10° above the SE horizon at 9:40 p.m.  Enjoy!

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