Planets on Parade: Saturn at Opposition 2018

2018 planets
2018 planets
Saturn, Mars and Jupiter all beckon this summer. Image credit and copyright: Sharin Ahmad (@shahgazer).

We’re in the midst of a parade of planets crossing the evening sky. Jupiter reached opposition on May 9th, and sits high to the east at dusk. Mars heads towards a fine opposition on July 27th, nearly as favorable as the historic opposition of 2003. And Venus rules the dusk sky in the west after the setting Sun for most of 2018.

June is Saturn’s turn, as the planet reaches opposition this year on June 27th, rising opposite to the setting Sun at dusk.

In classical times, right up until just over two short centuries ago, Saturn represented the very outer limit of the solar system, the border lands where the realm of the planets came to an end. Sir William Herschel extended this view, when he spied Uranus—the first planet discovered in the telescopic era—slowly moving through the constellation Gemini just across the border of Taurus the Bull using a 7-foot reflector (in the olden days, telescopes specs were often quoted referring to their focal length versus aperture) while observing from his backyard garden in Bath, England on the night of March 13th, 1781.

vesta
Looking east tonight at sunset… note Vesta to the upper left. Credit: Stellarium.

Orbiting the Sun once every 29.5 years, Saturn is the slowest moving of the naked eye planets, fitting for a planet named after Father Time. Saturn slowly loops from one astronomical constellation along the zodiac to the next eastward, moving through one about every two years.

path of saturn
The path of Saturn through 2018. Image credit: Starry Night Education software.

2018 sees Saturn in the constellation Sagittarius the Archer, just above the ‘lid’ of the Teapot asterism, favoring the southern hemisphere for this apparition. Saturn won’t cross the celestial equator northward again until 2026. Not that that should discourage northern hemisphere viewers from going after this most glorious of planets. A low southerly declination also means that Saturn is also up in the evening in the summertime up north, a conducive time for observing. Taking 29-30 years to complete one lap around the ecliptic as seen from our Earthly vantage point, Saturn also makes a great timekeeper with respect to personal life milestones… where were you back in 1989, when Saturn occupied the same spot along the ecliptic?

Saturn also shows the least variation of all the planets in terms of brightness and size, owing to its immense distance 9.5 AU from the Sun, and consequently 8.5 to 10.5 AU from the Earth. Saturn actually just passed its most distant aphelion since 1959 on April 17th, 2018 at 10.066 AU from the Sun.

Saturn’s in 2018 Dates with Destiny

Saturn sits just 1.6 degrees south of the waning gibbous Moon tonight. The Moon will lap it again one lunation later on June 28th. Note that the brightest of the asteroids, +5.7 magnitude 4 Vesta is nearby in northern Sagittarius, also reaching opposition on June 19th. Can you spy Vesta with the naked eye from a dark sky site? 4 Vesta passes just 4 degrees from Saturn on September 23rd, and both flirt with the galactic plane and some famous deep sky targets, including the Trifid and Lagoon Nebulae.

Saturn reaches quadrature 90 degrees east of the Sun on September 25th, then ends its evening apparition when it reaches solar conjunction on New Year’s Day, 2019.

Saturn is well clear of the Moon’s path for most of this year, but stick around: starting on December 9th, 2018, the slow-moving planet will make a great target for the Moon, which will begin occulting it for every lunation through the end of 2019.

It’s ironic: Saturn mostly hides its beauty to unaided eye. Presenting a slight saffron color in appearance, it never strays much from magnitude -0.2 to +1.4 in brightness. One naked eye observation to watch for is a sudden spurt in brightness known as the opposition surge or Seeliger Effect. This is a retro reflector type effect, caused by all those tiny iceball moonlets in the rings reaching 100% illumination at once. Think of how the Full Moon is actually 3 to 4 times brighter than the 50% illuminated Quarter Moon… all those little peaks, ridges and crater rims no longer casting shadows do indeed add up.

Saturn
Saturn in all its glory (note the moons Enceladus and Tethys, too!). Image credit and copyright: Efrain Morales.

And this effect is more prominent in recent years for another reason: Saturn’s rings passed maximum tilt (26.7 degrees) with respect to our line of sight just last year, and are still relatively wide open in 2018. They’ll start slimming down again over the next few oppositions, reaching edge-on again in 2028.

Even using a pair of 7×50 hunting binoculars on Saturn, you can tell that something is amiss. You’re getting the same view that Galileo had through his spyglass, the pinnacle of early 17th century technology. He could tell that something about the planet was awry, and drew sketches showing an oblong world with coffee cup handles on the side. Crank up the magnification using even a small 60 mm refractor, and the rings easily jump into view. This is what makes Saturn a star party staple, an eye candy feast capable of drawing the aim of all the telescopes down the row.

If seeing and atmospheric conditions allow, crank up the magnification up to 150x or higher, and the dark groove of the Cassini division snaps into view. Can you see the shadow of the disk of Saturn, cast back onto the plane of the rings? The shadow of the planet hides behind it near opposition, then becomes most prominent towards quadrature, when we get to peek around its edge. Can you spy the limb of the planet itself, through the Cassini Gap?

Though the disk of Saturn is often featureless, tiny swirls of white storms do occasionally pop up. Astrophotographer Damian Peach noted just one such short-lived storm on the ringed planet this past April 2018.

Saturn’s retinue of moons are also interesting to follow in there own right. The first one you’ll note is +8.5 magnitude smog-shrouded Titan. Larger in diameter than Mercury, Titan would easily be a planet in its own right, were it liberated from its primary’s domain.

Though Saturn has 62 known moons, only six in addition to Titan are in range of a modest backyard telescope: Enceladus, Rhea, Dione, Mimas, Tethys and Iapetus. Two-faced Iapetus is especially interesting to follow, as it varies two full magnitudes in brightness during its 79 day orbit. Arthur C. Clarke originally placed the final monolith in 2001: A Space Odyssey on this moon, its artificial coating a beacon to astronomers. Today, we know from flybys carried out by NASA’s Cassini spacecraft that the leading hemisphere of Iapetus is coated with dark in-falling material, originating from the dark Phoebe ring around Saturn.

iapetus
Two-faced Iapetus as imaged by Cassini. Image credit: NASA/JPL/Space Science Institute.

Owners of large light bucket telescopes may also want to try from two fainter +15th magnitude moons: Hyperion and Phoebe.

Fun fact: Saturn’s moons can also cast shadows back on the planet itself, much like the Galilean moons do on Jupiter… the catch, however, is that these events only occur around equinox season in the years around when Saturn’s rings are edge-on. This next occurs starting in 2026.

Cassini finished up its thrilling 20 year mission just last year, with a dramatic plunge into Saturn itself. It will be a while before we return again, perhaps in the next decade if NASA selects a nuclear-powered helicopter to explore Titan. Until then, be sure to explore Saturn this summer, from your Earthbound backyard.

Love to observe the planets? Check out our new forthcoming book, The Universe Today Ultimate Guide to Viewing the Cosmos – out on October 23rd, now up for pre-order.

A Guide to Saturn Through Opposition 2015

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.

Image credit: Starry Night Education software
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:

Stellarium
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
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.

Wikimedia Commons
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.

Stellarium
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.

Image credit: Starry Night Education software
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.

Let ‘the season of Saturn 2015’ begin!