Saturn at Opposition 2015

Saturn at the 2014 opposition as imaged by Damian Peach (damianpeach.com)

Saturn at the 2014 opposition as imaged by Damian Peach (damianpeach.com)

The planet Saturn reaches opposition on May 22, 2015. It rises as the Sun sets and makes its closest approach to Earth this year. The planet is as bright as it’s been in eight years, with rings dramatically tilted towards our point of view. This is your best chance in many years to see this beautiful planet in a telescope and share a view of it with others.

Saturn lies well south of the celestial equator this observing season near the border between the constellations Libra and Scorpius in the southeastern sky (as seen from the northern hemisphere) and the eastern sky (as seen from the southern hemisphere). At magnitude 0.0 in late May and early June, Saturn is the brightest object in that part of the sky. It outshines even the brilliant red-orange star Antares to the east. By 10 p.m., you can see the planet low over the horizon, nestled near the three bright stars in the head of Scorpius. You can watch the planet’s motion relative to these stars from night to night over the next few months. It’s been moving ‘in retrograde’ a little westward each day, over and above its daily westward motion. It resumes its normal eastward motion relative to the background stars on August 2.

Saturn as seen looking south from the northern hemisphere about 3 hours after sunset in late May 2015.

Saturn as seen looking south from the northern hemisphere about 3 hours after sunset in late May 2015.

As a gas giant and the second largest planet in the solar system, Saturn is much like Jupiter. It’s made of cold hydrogen and helium gas in its outer layers, with no solid surface. Like Jupiter, it likely has a solid rocky core that’s 10x Earth’s mass.  And it has a strong magnetic field and a huge collection of 60 moons, almost as many as Jupiter.

But Saturn differs from Jupiter in ways that make it especially fascinating.

Its rings, for example.  While Jupiter and Uranus have faint rings, Saturn has the brightest and most complex ring system in the solar system.  No one knows for sure how or when the rings formed. They might have assembled with the planet 4.5 billion years ago. Or they may have formed just 100 million years ago when a small moon or comet came too close to the planet and was ripped to pieces by tidal forces. In any case, Saturn’s rings are made mostly from tiny ice particles that extend from 6,600 km to 120,000 km directly above the equator of the planet. The whole set of rings, despite their extent, are just 10-20 meters thick.

(This excellent video of a TED talk by Carolyn Porco highlights views and discoveries by NASA’s Cassini spacecraft to Saturn).

The gravity of the planet and several embedded “shepherd moons” segment the rings into amazingly complex filamentary structures. The rings are organized into four main sections labelled from A, the outermost ring, to D which is closest to the planet.  On a clear night with a small telescope, you can see the A and B rings.  And you can see the narrow gap between these rings called the Cassini division.

While it’s a mighty big planet, Saturn is just 30% as massive as Jupiter.  But it spans 84% of Jupiter’s diameter.  That means it’s far less dense than Jupiter, or any other planet in the solar system.  It has just 68% the density of water, which means it would float in the bathtub (if you had a big enough bathtub).

Saturn also rotates quickly… just once every 10 hours… so it’s flattened at the poles more than any other planet. Since the surface isn’t solid, it’s a little tricky to get a good estimate of the rotational period; astronomers figured out the true rotation period of the planet by measuring the magnetic field.

The face of Saturn shows a few faint bands but few other features.  Though there is the matter of the strange white storm that appears on the surface every 30 years or so…

The “Great White Spot”, as it’s called was observed from Earth in 1876, 1903, 1933, and 1960, and 1990. The spot is presumably a storm that assembles in the atmosphere or bubbles up to the outer layers.  It starts small, then spreads out for several weeks before disappearing.  It seems to correspond to summer solstice in Saturn’s northern hemisphere because it occurs once per revolution of the planet around the sun. The next appearance of this mysterious spot is due around 2020.

Along with the intricate systems of rings around Saturn, stargazers can see up to 8 of Saturn’s 62 moons in amateur telescopes. The brightest and biggest is Titan, a moon which appears as a small orange disk at high magnification. The color of this fascinating moon comes from clouds of frozen organic smog in the thick atmosphere.  Because the major moons of Saturn lie in the same plane as the rings, they will appear over the next few years from our point of view slightly above or below the planet’s disk as they move through their orbits.

Saturn as it might appear in a small telescope at high magnification in excellent seeing (credit: Mark Mathosian on Flickr)

Saturn as it might appear in a small telescope at high magnification in excellent seeing conditions (credit: Mark Mathosian on Flickr)

With the rings tilted at an angle of 24º from edge-on, the view of Saturn will be magnificent this year. To best see the planet, you will need a telescope. Most binoculars don’t have enough aperture or magnification to resolve the rings, although you can see the largest moon Titan with binoculars.

The steep ring tilt is partly why the planet appears so bright this year. The disk of the planet is about 19″ across, while the rings span about 42″ at opposition. That’s still fairly small, and most new observers are surprised how small Saturn appears in a small scope. You will want to use as much magnification as your telescope and seeing conditions can handle. Crank it up to at least 150x if you can… the higher the better. You will find it’s a trade-off between image size and quality. You’re better off with a sharp, bright, small image than a large, faint, blurry image.

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