Backyard Astronomy: A Piker's Guide to Jupiter (2004-02-21)

By Cheryl Haimann

A dozen things to know about the king of the planets, Jupiter:

This spring, Jupiter is nearing opposition (on the opposite side of Earth, relative to the Sun) and so is visible all night long. It is located at the feet of Leo, the Lion. After dusk, look to the east for a question mark or sickle lying on its back. That is the head and mane of the lion. Below it is a triangle that represents the hindquarters. Jupiter is about midway and slightly to the right of a line between the bottom of the question mark and the far end of the tail.
Jupiter is the largest planet in the solar system, but just how big is it? Suppose you are at a cosmic county fair, and come across a game of "guess how many marbles are in the jar." The marbles are the size of earth, and the jar is the size of Jupiter. If you want to win the prize, write down that 1200 Earth marbles are in the Jupiter jar.
Jupiter is large enough to be seen as a disk in standard binoculars. For the a steady view, mount the binos on a camera tripod or brace your elbows against a railing or a car roof.
Not only is the disk visible in binoculars, but so are Jupiter's four largest moons. Callisto, Ganymede, Europa, and Io are known as the Galilean satellites because, when they were discovered by Galileo in 1610, they were the first satellites discovered other than our own moon. The moons are named for consorts of Zeus (the Greek equivalent of the Roman Jupiter), that rascal.
Jupiter has Earth's back, as a good friend should. Its size and distance from Earth let it sweep up asteroids and other space junk that might instead strike our planet.
Jupiter has the shortest day of any planet in the solar system. The planet completes a rotation in slightly less than ten earth hours. What's more, the planet does not rotate at the same speed. The equator's rotation is about six minutes faster than the rotation at the poles. The Galilean moons also have relatively short orbits - about 17 days for most-distant Callisto, down to less than two days for nearby Io.
You can seen some of the bands that circle Jupiter, as well as the best-known feature, the Great Red Spot, in a modest telescope. The colorful bands are clouds made up of complex chemicals in the atmosphere. Because of Jupiter's rapid rotation, movement of the features can be detected in as little as ten minutes.
Its large, colorful, and rapidly changing features make Jupiter an ideal object to sketch. If you have binoculars, you can plot the location of the moons over a period of time, just as Galileo did almost 400 years ago. With a telescope, you can look for details on the planet surface.
However bad you think your weather is, Jupiter's is worse. The atmosphere is thousands of miles thick and produces ammonia snow, among other things. The Great Red Spot is a storm more than twice as wide as Earth that has been raging for over 300 years. On the other hand, those clouds are probably spectacular to look at.
It's unlikely that humans will ever look at the clouds up close, though. Jupiter has a magnetic field that extends beyond the orbit of Saturn. It also traps solar radiation, making a close approach to the planet dicey, at best.
Gertrude Stein famously remarked about Oakland, "When you get there, there isn't any there there." The same can be said for Jupiter. Don't look for any rovers to land on the surface as they have recently on Mars, because there is no surface. Below the thousands of miles of clouds, rain falls into oceans of compressed hydrogen, known a liquid metallic hydrogen. At the center of the planet is highly compressed molten core. The extreme difference between the temperature of this inferno-like core and the cold outer atmosphere contributes to the massive storms.
Early on the morning of March 28, three of Jupiter's moons will be casting shadows on the planet's surface at the same time. Shadows should be visible in moderate-sized telescopes. See the March 2004 issue of Astronomy magazine for details on what will be visible.