SkyWatching: Shadow Play on Jupiter

Jupiter’s rapidly moving moons constantly surprise us with their dance around the giant planet. There will be two spectacular shadow plays this week.

Jupiter’s moons are very small, even in a large telescope, but their shadows are slightly larger, and can often be seen crossing Jupiter’s face with a good amateur telescope. I’ve seen the shadows with telescopes as small as 90mm aperture, but a telescope with 6-inch or larger aperture will show them much more clearly. Steady atmospheric seeing is also essential.

If you live on the eastern seaboard, look for Jupiter just after sunset on Wednesday, May 20 around 8:10 p.m. EDT. The first thing you will notice is that only two of Jupiter’s usual four moons are visible. That’s because two of the Moons, Io and Callisto, are in front of Jupiter’s disk, and are said to be “in transit.” You probably won’t see Io, because its color and brightness blend in so well with the cloud tops behind it. You may be able to see Callisto because its dark surface stands out against Jupiter’s bright clouds. I usually see it as a tiny greyish spot. Look more closely, and you’ll see two small dark shadows on Jupiter’s face. One of these is Io’s shadow, but the other is not the shadow of Callisto. Instead it is the shadow of Ganymede, off to Jupiter’s right. That’s because of the angle at which the sun is illuminating the tableau. 

On Wednesday night, May 20, the shadows of Jupiter’s moons Ganymede and Io will cross Jupiter’s face. This shows the shadows at 8:10 p.m. EDT, just after Io’s shadow has started across, and just before Ganymede’s shadow leaves.  Credit: Starry Night software.

Take another look later in the evening, around 9:55 p.m., and you’ll see that Ganymede’s shadow has left the disk and that Io’s shadow is about to be hidden behind Callisto. This will be the first time I have ever seen a moon’s shadow eclipsed by another moon. 

Nearly two hours later at 9:55 p.m., Ganymede’s shadow has left, and Io’s shadow is about to be eclipsed by the moon Callisto. The Great Red Spot is well placed close to Jupiter’s central meridian. Credit: Starry Night software.

Also keep a lookout for the Great Red Spot, though it is not nearly as “great” nor as “red” as it once was. It’s more usually seen as a light notch in the North edge of Jupiter’s South Equatorial Belt. At moments of steady seeing, its salmon pink color may appear briefly.

A week later on May 27, the situation nearly repeats itself, but is about two hours later, making it more easily seen across the whole of North America. Ganymede’s shadow starts across Jupiter’s face at 8:58 p.m. EDT. Io’s shadow follows at 10:01 p.m., and both shadows are present until Io’s shadow leaves at 12:18 a.m., followed by Ganymede’s at 12:34 a.m. 

Exactly a week later, on Wednesday, May 27 at 10:05 p.m., the pattern repeats, except that the shadows are closer together and Callisto is no longer in front of Jupiter. Credit: Starry Night software.

Notice that at around 11:48 p.m., Io’s faster moving shadow actually passes Ganymede’s, and the two shadows merge.

At 11:48 p.m., Io’s faster moving shadow catches up with Ganymede’s, and the two shadows merge. Again, the Great Red Spot is well placed. Credit: Starry Night software.

Once again, the Great Red Spot should be in evidence.

If you live west of the Eastern time zone, be sure to subtract the appropriate corrections from the times given above: 1 hour for CDT, 2 hours for MDT, and 3 hours for PDT.

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Observing Saturn

On Friday, May 22, at 10 p.m. EDT, Saturn will be in opposition to the sun. This means that it will be directly opposite the sun in our sky. It will rise as the sun sets in the evening, shine brightly all night long, and set as the sun rises at dawn.

On May 22, Saturn reaches opposition with the Sun. It will be right on the border between Libra and Scorpius, just above the three stars which form the Scorpion’s claws. Credit: Starry Night software.

If you just look at the sky on a single night, everything seems quite static. But if you watch Saturn over a period of a few weeks and note its position against the background stars, you will see that it is in constant motion.

Currently Saturn is moving with what is called “retrograde motion,” from left to right against the background stars. This is actually an optical illusion caused by the Earth’s much more rapid movement around the sun. Once the Earth is well past Saturn in early August, Saturn will appear to reverse directions and begin moving in its true direction, from right to left.

This retrograde motion puzzled early skywatchers, who though the planets must go around it tiny circles called epicycles. This was because they incorrectly believed that the Earth was fixed in space and everything revolved around it, the “geocentric theory.” Once Copernicus made clear that the sun, not the Earth, was the center of the Solar System, the geometry of the planets’ motion became much simpler.

Saturn, like all the planets, is much smaller in angular size than most people realize. I once tried an experiment to see how much magnification was needed to see Saturn’s rings. With a binocular magnifying 10 times, Saturn looked just like a bright star. With a 15x binocular, I could just see a hint that Saturn was oval rather than round. It took a telescope magnifying 25 times to see Saturn’s true shape, though even then no detail was visible. I generally use magnifications of 150 to 250 times to see the details of Saturn and its ring system.

Saturn really has multiple rings, of which the brightest are the outer A ring and the inner B ring. The A ring is noticeably darker than the B ring, and the two are separated by the dark Cassini Division, named after 17th century Italian astronomer Giovanni Domenico Cassini, who was the first to observe it in 1675. Cassini also discovered four of Saturn’s five brightest moons.

The Cassini Division separates the A and B rings.

Titan, the largest and brightest of Saturn’s moons was discovered in 1655 by Dutch astronomer Christiaan Huygens. It is visible in even the smallest telescopes. It is the second largest moon in the Solar System (after Jupiter's moon Ganymede), the only moon to have a dense atmosphere, and the only moon other than our own to have been landed on by a spacecraft.

Huygens was also the first person to deduce that Saturn’s rings were flat circular objects in the plane of Saturn’s equator. Further study has shown that they are made up of thousands of tiny fragments of rock and ice. I once watched a star pass behind these rings, and the star continued to be visible, since there is more empty space that rock and ice in the rings, making them translucent.

Saturn’s smaller moons are worth looking for if you have a good telescope. The brighter ones are visible in a 90mm telescope. Because they are in constant motion around Saturn, you need a planetarium program like Starry Night to identify which ones are visible on a given night. Most of the bright moons move in the same plane as the rings, so appear to trace ovals around the planet.

In a telescope at about 150 power, Saturn is small but beautiful in its perfection, the jewel of the Solar System. Look around the planet for its brightest moons. Credit: Starry Night software.

Iapetus is a particularly interesting moon. Its orbit lies outside those of the other bright moons, and is tilted at an angle of 15 degrees compared to the other moons and the rings. Like all major moons in the Solar System, Iapetus always keeps one face permanently turned towards its planet. The side of Iapetus which leads it around in its orbit has encountered a large amount of debris, painting that face of the moon dark black. When that blackened side of Iapetus is facing Earth, at the moon’s greatest elongation east, it is almost two magnitudes fainter than when the trailing side of Iapetus is facing us, at greatest western elongation.

Right now Iapetus is close to its western elongation, so is at its brightest, magnitude 10.1. By greatest elongation east on June 27, it will be at its faintest, magnitude 11.9.

The globe of Saturn itself is rather bland when compared to its more active neighbor Jupiter. It shows a system of darker belts and brighter zones, but their contrast is muted compared to Jupiter. From time to time bright spots have been observed in Saturn’s cloud tops, but they have short lives compared to cloud features on Jupiter. In large telescopes, the polar regions of Saturn take on an olive green color.

It is interesting to observe the pattern of shadows on Saturn. The rings cast shadows on the globe of the planet, and the planet in turn casts its shadow on the rings. I have observed these shadows in a telescope as small as 90mm aperture under steady seeing conditions.

Whenever I observe Saturn in a telescope, I always take a few minutes to just sit back and admire its sheer beauty. Saturn was one of the first objects I looked at when I got my first telescope as a teenager, and I still recall the wonder I felt at witnessing this beauty for the first time with my own eyes: “It really has rings!”

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram. 

Telescope Myths

With pleasant spring evenings arriving, many people may be thinking of buying a telescope. There’s a lot of bad advice out there; here are some samples, counterbalanced with the facts.

Myth: Magnification is a good way to judge a telescope

Reality: Any telescope that has interchangeable eyepieces can produce any magnification. The useful magnification on a telescope is much more limited, and depends on the size of the telescope’s aperture (diameter of lens, mirror, or corrector plate). A telescope with 60mm aperture has a range of useful magnifications from about 12x to 120x, no matter what the advertising on the box may say. And a larger telescope, such as an 8” reflector, will have a larger, but still limited range, 40x to 300x. Unless you live in a place with very stable air, such as Florida, 300x will be your upper limit on magnification no matter how big your telescope is.

The best way to compare telescopes is by their aperture. By and large, a telescope with a larger aperture will outperform a telescope with a smaller aperture on every kind of object. The main counterbalancing factors are size, weight and cost. If a telescope is too large to be set up conveniently, it won’t be used as often as a smaller, more convenient scope.

Myth: Refractors are better than reflectors for planetary observation

Reality: Like many myths, there’s a kernel of truth in this one. For a given aperture, a refractor, with its unobstructed aperture, will have better contrast than a reflector, because of the scattering of light caused by having a secondary mirror in the light path. However, this breaks down when faced with the realities of economics and mechanics. An 8” reflector or Schmidt-Cassegrain costs $360 to $2100 and weighs 40 to 75 pounds, complete, and is easily transported in a small car. An 8” refractor costs at least $3500 for the optical tube alone. The tube is eight feet long and would weigh 40 pounds, requiring a mount that costs at least as much as the tube and a permanent observatory to house it in.

Myth: Smaller apertures show more than larger ones when the seeing is poor

Reality: On many occasions I’ve masked my telescopes down under marginal seeing conditions in the hopes of improving their image, but never have I noticed any improvement. All it does is make the image dimmer and reduce the amount of detail visible.

Myth: Smaller apertures work better than large ones under light polluted skies

Reality: I lived for many years in a big city, and never once was I tempted to use anything other than my largest telescope for all kinds of observing. If light pollution reduces what your naked eye sees by three magnitudes, it will also reduce what your telescope sees by exactly the same amount, three magnitudes.

Myth: Faster scopes are better at showing faint objects than slower scopes

Reality: This myth comes from people who are knowledgeable about photography, where “faster” lenses gather more light than “slower” lenses. In telescopes used visually, the focal ratio is irrelevant in terms of how bright the image will appear at a given magnification. Short focal ratios (f/4 to f/6) are generally preferred because they make the scope more compact and allow a wider field of view, while long focal ratios (f/8 to f/15) are preferred because they provide higher magnifications with relatively simple and inexpensive eyepieces. Objects are equally bright in either scope, given identical magnifications.

Myth: I don’t want a Dobsonian: it doesn’t look like a real telescope

Reality: Despite the fact that the Dobsonian design offers the most “bang for the buck” in any telescope size, some people seem to be turned off by its looks. It doesn’t have a lens at the top of the tube, and the mount looks more like a cannon than a precision instrument.

But, take a look at any modern research telescope, such as the Subaru on Mauna Kea:

That sure looks more like my Dob than like my grandfather’s refractor!

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram.