This is a great time to observe Jupiter. It reached opposition on July 9, so it’s particularly bright right now. Any planet with an orbit farther from the Sun than Earth’s is said to be “at opposition” when the planet, Earth, and the Sun are lined up so that Earth is in the middle. In other words, the planet is directly opposite the Sun, from our viewpoint. The effect for us is that Jupiter’s face is fully illuminated by the Sun.
In addition, a planet at or near opposition is at its closest to Earth. This closeness gives Jupiter a slightly larger apparent size, the amount of sky it covers from our perspective on Earth. The larger the apparent size, the more illuminated surface to shine light in our direction.
Like all the planets, Jupiter doesn’t have its own light source. It is merely reflecting light from the Sun, and therein lies a little irony. You see, Jupiter is a stellar wannabe. It’s hot and it’s massive, but when the solar system was forming, it wasn’t quite massive enough for nuclear fusion to ignite in its core. Nuclear fusion is the engine that makes a star--like our Sun--go. And Jupiter, although impressive, didn’t have star power.
What Jupiter does have is take-no-prisoners gravity. Its colossal size (1,300 Earths would fit inside) and significant mass (greater than the other seven planets combined) make it the shield of the inner solar system, which includes our little blue planet. If it weren’t for Jupiter, Earth might not have survived the chaos of the early solar system. Countless asteroids and other rogue bodies that might have pulverized Earth were sucked in by Jupiter’s strong gravitational pull and neutralized. The mighty planet protects us still. In the early 1990s, it ripped a comet into 21 fragments and then swallowed them all.
Naked eye, Jupiter looks like a brilliant star. But a sky tourist with a telescope--even a modest one--can see the “star” magnified into a planetary disk and can have fun hunting some of the planet’s features. Dense, colored cloud bands on Jupiter’s surface are exciting to spot in a telescope. Ammonia, methane, and other chemical compounds in its upper atmosphere create the striped look.
Another famous surface feature is the Great Red Spot, a hurricane-like storm that’s been observed swirling high in Jupiter’s atmosphere for at least 300 years. Red Spot Junior, a smaller storm, appeared nearby in 2006. Because Jupiter spins, the Red Spots are not always in view. To calculate when they’ll be visible, visit this web page.
Jupiter and Ganymede
Credit: NASA/JPL/University of Arizona
Like any good kingpin, Jupiter has a large number of lackeys--in this case, more than 60 moons in tow. In a telescope or binoculars, you can easily spot the four largest, the Galilean moons. These are the moons discovered when the 17th century Italian astronomer Galileo first turned a telescope toward the night sky. They’re named Ganymede (GANN-uh-meed), Io (EYE-oh), Callisto (cah-LISS-toe), and Europa (your-ROPE-uh), after characters in Greek mythology. They look like tiny pinpoint stars near the planet. Occasionally, one or more will be hidden behind the planet as they orbit around. To calculate and view their positions at any given time, visit this web page.
Aurora at Jupiter's north pole
Credit: NASA/ESA, John Clarke (University of Michigan)
In the distant future, when interplanetary tourists can book a solar system cruise, Jupiter will be one of the more exotic ports of call. Imagine cruising over the unlit night side of the planet to watch ultra-powerful lightning storms animate the cloud tops. Imagine flyovers of the north and south poles to view immense auroras: pulsating sheets of neon-colored light. Imagine diving through the barely detectable band of microscopic dust that encircles Jupiter, a vast ring system ethereal as smoke. Everything about Jupiter is king-sized.Hidden under the swirling gasses of Jupiter’s atmosphere is a voluminous sea, but no one will ever build a resort or bathhouse there. The air pressure and temperature are so extreme there that Jupiter’s abundant hydrogen--normally a gas--has become liquid. Beneath the sea of liquid hydrogen is liquid metallic hydrogen, a bizarre substance unknown on Earth. The crushing pressure at that layer is three million times that of Earth’s surface.
Jupiter is the first object I ever observed through a telescope. The impact of seeing with my own eyes the surface of another planet--hundreds of millions of miles away, with colored bands and moons to boot--is hard to describe. I can tell you this: the image in my head will stay with me forever. By the way, this was years before I owned my own telescope. Begged and borrowed views are often the best.