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Midnight Giant

by Perry Pezzolanella

When it comes to observing planetary detail and action, Jupiter delivers. It is the largest of the planets, and even at its smallest when farthest away from Earth it is still larger than Mars can ever appear through a telescope. Since Jupiter rotates in just 9 hours and 55 minutes the changing features provide an excellent opportunity to practice observing and sketching skills. Jupiter is a completely cloudy world so the only features visible are its distinctive clouds patterns.

The most prominent cloud features will be revealed by even the smallest and cheapest telescope. Dark bands, called belts, run parallel to the equator and each other due to strong winds and the rapid rotation stretches them out. They are areas of sinking air that is warming up and drying out as it descends, therefore thinning and clearing away the high, icy white ammonia clouds, which creates a window into the darker depths. These lower, yellowish-tan clouds are composed mainly of sulfur compounds and water droplets. Jupiter’s bright horizontal stripes, called zones, are areas of rising air that condenses into billowing whitish clouds consisting primarily of ammonia ice. The dusky cloud caps at both poles, called hoods, are usually not as prominent as the equatorial belts. Slight clearing near the poles reveals the lower, darker clouds that are somewhat obscured by an ammonia haze. Larger telescopes will reveal additional thinner belts and zones.

The more delicate features include festoons, which appear like dark fingers bridging the entire width of a light zone by connecting two belts. A rift is a bright cloud that bridges across a dark belt and connects two zones. Garlands are another feature which do not connect, but instead appear like dusky hooks or closed loops along the belts. Knots are very common and are darker thickenings within the belts. Ovals are distinct bright spots that are fairly large, often appearing near the belts while spots are smaller and darker than the ovals. The most famous spot is the Great Red Spot, which is a huge storm slightly larger in size than Earth similar to a hurricane with towering clouds. It is reddish probably from phosphine being dredged up from far below. The Great Red Spot has been steadily shrinking through the decades and can appear as vivid as brick red to as dull as pale salmon.

Four large moons can be easily seen orbiting Jupiter: Callisto, Ganymede, Europa, and Io. These are known as the Galilean Moons in honor of the astronomer who discovered them, Galileo Galilei. It is fun to watch them change position from night to night, and sometimes even within hours if they are grouped together or close to Jupiter. A skilled observer can observe them casting shadows on Jupiter on occasion and watch them pass into Jupiter’s shadow and/or emerge from it. It is also fun to watch the moons pass in front of Jupiter and try to see how long it can still be seen before being lost among Jupiter’s bright clouds. The larger size of Callisto and Ganymede can be compared to smaller Io and Europa if seeing is steady enough. Highly skilled observers may also note the subtle duskiness of Callisto to lighter Ganymede, and icy Europa will appear whitish while sulfur-covered Io may show a yellowish tint.

Jupiter offers many projects to work on and much to observe regardless of the skill level of the observer. The rapid rotation can easily be seen within as little as ten minutes if a prominent feature such as the Great Red Spot is in view as a guide. The flattening of Jupiter’s poles is also obvious from the rapid rotation. Advancement in optics and photography now make it possible to image finer detail than the eye can ever see such as detail within the Great Red Spot, eddies and swirls along the belts and zones, tantalizing fuzzy detail on Ganymede, and even a shot at locating a fifth moon, tiny Himalia. No other planet provides so much; there is always something visible, fascinating, inspiring, and even challenging, but above all, Jupiter is always rewarding.