Image of ring-plane crossing at Saturn taken by NASA’s Hubble Space Telescope in May 1995, with a similar ring-plane crossing event scheduled to occur in March 2025. (Credit: Reta Beebe (New Mexico State University), D. Gilmore, L. Bergeron (STScI), NASA/ESA, Amanda S. Bosh (Lowell Observatory), Andrew S. Rivkin (Univ. of Arizona/LPL), the HST High Speed Photometer Instrument Team (R.C. Bless, PI), and NASA/ESA)
The rings of Saturn are some of the most well-known and captivating spectacles in the night sky, which are so large they can easily be observed with amateur telescopes or even a pair of high-powered binoculars. However, from time to time, Saturn’s rings “disappear” from view, a phenomenon known ring-plane crossing, with the rings being observed as a flat line running straight through the massive gas giant. Ring-plane crossing occurs approximately every 15 years and is slated to happen next in March 2025, with the rings slowly getting “larger” in the months afterwards before “disappearing” again in November 2025. But what causes ring-plane crossing?
The tell-tale white notch of a new storm system emerging on Saturn on April 1st. Image credit and copyright: Damian Peach.
The tell-tale white notch of a new storm system emerging on Saturn on April 1st. Image credit and copyright: Damian Peach.
Are you following the planets this season? The planetary action is about to heat up, as Jupiter, Saturn and Mars all head towards fine oppositions over the next few months.
Spying the Storms of Saturn
Astrophotographer Damian Peach raised the alarm on Twitter this past week of a possible bright storm emerging of the planet Saturn. The spot was noticeable even with the naked eye and in the raw video Peach captured, a sure sign that the storm was a biggie.
Though outbursts of clusters of white spots on the surface of Saturn aren’t uncommon, it’s rare to see one emerge at such a high latitude. The storm had faded considerably the next observing session Peach performed on April 5th, though observers should remain vigilant.
A storm subsiding? The followup view a few days later on April 5th. Image credit and copyright: Damian Peach.
It’s sad to think: Cassini and our eyes in the outer solar system are no more… and the situation will probably remain this way for some years to come. Juno also wraps up its mission at Jupiter (pending extension) this year, and New Horizons visits its final destination Ultima Thule (neé2014 MU69) on New Year’s Day 2019, though it’ll likely continue to chronicle its journey through the outer realms of the solar system, much like the Voyager 1, 2 and Pioneer 10, 11 missions, also bound to orbit the galaxy, mute testaments to human civilization. But even though proposals for Europa Clipper, a nuclear-powered quad-copter for Saturn’s moon Titan, and a Uranus and/or Neptune Orbiter are all on the drawing board, the “gap decade” of outer solar system exploration will indeed come to pass and soon.
Catching a storm on Saturn, Cassini style. Credit: NASA/JPL-Caltech/SSI
But dedicated amateur astronomers continue to monitor the outer solar system for changes. This month sees Saturn rising around 1:30 AM local and transiting highest to the south for northern hemisphere observers at 6:00 AM local, just before sunrise. Saturn crosses the constellation Sagittarius in 2018, bottoming out at its most southerly point this year for its 29 year path around the Sun. Saturn currently shines at +0.4 magnitude, extending 40” across (including rings) as it heads towards a fine opposition on June 27th. After opposition, Saturn formally crosses into the dusk sky. The amazing rings are an automatic draw, but last week’s storm admonishes us not to forget to check out the saffron-colored disk of Saturn itself as well. For example, I’ve always wondered: why didn’t we see the hexagon before? It’s right there festooning the northern hemisphere cap, plain as day in modern amateur images… to be sure, we’re in a modern renaissance of planetary astrophotography today, what with image stacking and processing, but surely eagle-eyed observers of yore could’ve easily picked this feature out.
And the view is changing as well, as Saturn’s rings reached a maximum tilt in respect to our line of sight of 27 degrees in 2017, and now head back towards edge-on again in 2025. And be sure to check out Saturn’s retinue of moons, half a dozen of which are easily visible in a telescope at even low power.
Finally, here’s another elemental mystery poised by Saturn related to the current storm, one that Cassini sought to solve in its final days: how fast does Saturn rotate, exactly? The usual rough guesstimate quoted is usually around 10.5 hours, but we’ve yet to pin down this fundamental value with any degree of precession.
One thing’s definitely for sure: we need to go back. In the meantime, we can enjoy the early morning views of the most glorious of the planets in our Solar System.
Saturn in early May 2016. Image credit: Efrain Morales.
They’re back. After a wintertime largely devoid of evening worlds, the planets are once again in the evening sky. First Jupiter, then Mars have crossed opposition over the past few months, and now Saturn is set to take center stage later next week, reaching opposition at 7:00 Universal Time (UT) on the night of June 2/3rd. This places the ringed world in a position of favorable evening viewing, rising in the east as the Sun sets in the west, and riding highest near local midnight across the meridian.
Opposition 2016 sees the planet Saturn looping through the southern realm of the constellation Ophiuchus, making a retrograde run this summer at the Scorpius border before looping back and resuming eastward motion. That’s right: Saturn currently occupies the dreaded ‘13th house,’ of Ophiuchus, for all you Snake-Bearers out there. Saturn is currently at bright as it can be, at magnitude +0.04.
Saturn rising on the night of June 2nd. Image credit: Starry Night Education Software.
Saturn reaches opposition once every 378 days, as it orbits the Sun at a leisurely pace every 29.5 years. 2016 and the next few oppositions sees Saturn ‘bottoming out,’ sitting around -20 degrees south. Saturn won’t peek northward across the celestial equator again until March 2026. This places the 2016 appearance of Saturn high in the sky south of the equator, transiting about 30 degrees above the southern horizon around midnight for folks observing around 40 degrees north latitude. Saturn also begins looping towards the star-rich region of the galactic equator for a crossing it late next year in December 2017. Saturn sits 9 Astronomical Units (AU) or 1.4 billion kilometers distant on June 3rd, a slightly larger distance than usual, owing to the fact that the planet is headed towards aphelion on April 17th, 2018.
The waxing gibbous Moon passes 3.2 degrees north from Saturn on Sunday, June 19th, just a day before reaching Full.
Watch for a sudden brightening of the planet in early June, known as an ‘opposition surge’ due to what is known as the Seeliger effect. This is a coherent back-scattering of light, focusing it similar to highway retro-reflectors shining your headlights back at you at night. In this case, the Sun is the ‘headlight,’ and the millions of snowball moonlets hiding their shadows from view reaching 100% illumination are the highway retro-reflectors. The effect is subtle, to be sure, but serves to raise the brightness of the planet by about half a magnitude. This should be apparent in an animation sequence shot before, during and after over the span of a about a week. Any takers?
Almost there… the widening tilt of Saturn’s rings. image credit and copyright: Andrew Symes (@failedprotostar).
And speaking of the rings, here’s another reason to check out Saturn this opposition 2016 season. The tilt of rings is about 26 degrees wide as seen from our Earthly perspective… about as wide as they can be. Saturn’s rings were last edge on in 2009, and reach a maximum width of 27 degrees on October 16th, 2017 before slowly heading towards edge on again in 2025.
The path of Saturn through the last half of 2016. Image credit: Starry Night Education software.
At the eyepiece, Saturn shows a yellowish disk 18” extended to 43” across if you count the rings. Crank up the magnification to over 100x under good seeing, and the black thread of the Cassini division jumps into view. Saturn has 62 moons in all, with +9th magnitude Titan being the brightest. You’re looking at the most distant surface outpost of humanity, the site of the 2005 landing of the European Space Agency’s Huygens lander. Six moons are readily visible in a small telescope, while the fainter moons Hyperion and Phoebe present a challenge to owners of extreme light buckets. Also, as Saturn heads past opposition and towards eastern quadrature 90 degrees from the Sun on September 2nd, 2016, watch for the shadow of the bulk of the planet, cast back across the rings.
A sampling of the Moons of Saturn. Image credit: Stellarium.
We never miss a chance to observe Saturn if it’s above the horizon. Saturn is a sure-fire crowd-pleaser for any sidewalk astronomy session, and no one forgets their first glimpse of the glorious ringed world. You can just imagine how much consternation this bizarre-looking planet must have caused Galileo. You can tell just how primitive his first telescope was, as his sketches show off Saturn as more of a two-handled ‘coffee cup’ in appearance. Christaan Huygens first deduced something of the true nature of Saturn’s rings in 1655, correctly claiming that they are physically separated from the disk.
Getting closer... Saturn as seen on March 25th, 2015. Image credit: Efrain Morales
The month of May generally means the end of star party season here in Florida, as schools let out in early June, and humid days make for thunderstorm-laden nights. This also meant that we weren’t about to miss the past rare clear weekend at Starkey Park. Jupiter and Venus rode high in the sky, and even fleeting Mercury and a fine pass of the Hubble Space Telescope over central Florida put in an appearance.
But the ‘star’ of the show was the planet Saturn as it appeared at nightfall low to the southeast. Currently rising about 9:00 PM local, Saturn is joining the evening skies as it approaches opposition next week.
This also means we’ve got every naked eye planet set for prime time evening viewing this week with the exception of Mars, which reaches solar conjunction on June 14, 2015. Mercury will be the first world to break this streak, as it descends into the twilight glare by mid-May.
The apparent path of Saturn from May to November 2015. Image credit: Starry Night Education software
Saturn reaches opposition for 2015 on May 23rd at 1:00 Universal Time (UT), which equates to 9:00 PM EDT the evening prior on May 22 at nearly 9 astronomical units (AU) distant. Oppositions of Saturn are getting slightly more distant to the tune of 10 million kilometers in 2015 versus last year as Saturn heads towards aphelion in 2018. Saturn crosses eastward from the astronomical constellation of Scorpius in the first week of May, and spends most of the remainder of 2015 in Libra before looping back into the Scorpion in mid-October. The first of June finds Saturn just over a degree southward of the +4th magnitude star Theta Librae. Saturn takes nearly 30 Earth years to complete one orbit, meaning that it was right around the same position in the sky in 1985, and will appear so again in 2045. Relatively speedy Jupiter also overtakes Saturn as seen from the Earth about once every 20 years, as it last did on 2000 and is set to do so again in 2020.
And though series of occultations of Saturn by the Moon wrapped up in 2014 and won’t resume again until December 9, 2018, there’s also a good chance to spy Saturn two degrees away from the daytime Moon with binoculars on June 1st just 24 hours prior to Full:
Looking east on the evening of June 1st just before sunset. Image credit: Stellarium
The tilt of the rings of Saturn is also slowly widening from our Earthbound perspective. At opposition, Saturn’s rings subtend 43” across, and the ochre disk of Saturn itself spans 19”. Incidentally, on a good pass, the International Station has a visual span roughly equivalent to Saturn plus rings. In 2015, the rings are tilted 24 degrees wide and headed for a maximum approaching 27 degrees in 2017. The rings appeared edge on in 2009 and will do so again in 2025.
Getting wider… our evolving view of Saturn’s rings. Image credit and copyright: Andrew Symes
Also, keep an eye out for the Seeliger effect. Also sometimes referred to as the ‘opposition surge,’ this is a retroreflector-style effect that causes an outer planet to brighten up substantially on the days approaching opposition. In the case of Saturn and its rings, this effect can be especially dramatic. Not only is the disk of Saturn and the billions of icy snowballs casting shadows nearly straight back as seen from our vantage point near opposition, but a phenomenon known as coherent backscatter serves to increase the collective brightness of Saturn as well. You see the same effect at work as you drive down the Interstate at night, and highway signs and retroreflector markers down the center of the road bounce your high-beams back at you.
Highway retroreflectors in action. Image credit: Wikimedia Commons/Public Domain
We’ve seen some pretty nifty image comparisons demonstrating the Seeliger effect on Saturn, but as of yet, we haven’t seen an animation of the same. Certainly, such a feat is well within the capacities of amateur astronomers out there… hey, we’re just throwing that possibility out into the universe.
The changing face of Saturn. Image credit: Stellarium
Through a small telescope, the moons of Saturn become readily apparent. The brightest of them all is Titan at magnitude +9, orbiting Saturn once every 16 days. Discovered by Dutch astronomer Christiaan Huygens on March 25, 1655 using a 63 millimeter refractor with an amazing 337 centimeter focal length, Titan would easily be a planet in its own right were it directly orbiting the Sun. Titan also marks the most distant landing of a spacecraft ever carried out by our species, with the descent of the European Space Agency’s Huygens lander on January 14, 2005. Huygens hitched a ride to Saturn aboard NASA’s Cassini spacecraft, which is slated to end its mission with a destructive reentry over the skies of Saturn in 2017. Saturn has 62 known moons in all, and Enceladus, Mimas, Tethys, Dione, Rhea and two-faced Iapetus are all visible from a backyard telescope.
The scale of the orbits of Saturn’s moons. Image credit: Starry Night Education software
You can check out the current position of Saturn’s major moons (excluding Iapetus) here.
And speaking of Iapetus, the outer moon would make a fine Saturn-viewing vantage point, as it is the only major moon with an inclined orbit out of the ring plane of Saturn:
Expect our Saturn observing resort to open there one day soon.
Up for a challenge? Standard features to watch for include: the shadow of the rings on the planet, and the shadow of the planet across the rings, as well as the Cassini division between the A and B ring… but can you see the disk of the planet through the gap? High magnification and steady seeing are your friends in this feat of visual athletics… catching sight of it definitely adds a three dimensional quality to the overall view.
