If you look to the south-southeast just after dark, about halfway up from the horizon you will see what looks like a very bright star.
It is on the border between the constellations of Pisces and Aries. But this is not a star; it is the planet Jupiter. Each night at the same time it will be a little farther to the west. Because the Earth circles the Sun faster than Jupiter, we are passing it over the next few months. This means that we have several weeks that we can view Jupiter.
Jupiter is a good object to view with a small telescope. With a magnification of 100 to 150 times on a clear dark night, you should be able to see some features, such as bands of light and dark regions. But Jupiter is not all you will see. Even with an average pair of binoculars, you can see up to four pinpoints of light just to the east and west of Jupiter. These are the four large moons of Jupiter, which were first seen by Galileo Galilei in January 1610.
They were named for the lovers of Zeus -- Io, Europa, Ganymede and Callisto -- by a contemporary of Galileo named Simon Marius, who claimed to have observed them around the same time. You can remember their order from the planet by the phrase "I Eat Green Carrots."
Each of the moons has its own distinct characteristics. Io is the most geologically active object known in the solar system, with more than 400 active volcanoes. It is the psychedelic moon, covered with colorful sulfur and lava compounds and composed of silicate rock with an iron core.
Europa, on the other hand, is covered with ice and may have a liquid ocean of water just under the ice. Astronomers believe that Europa has twice as much water as does Earth.
Ganymede is the largest moon in the entire solar system. It is larger than the planet Mercury. It is also an ice world and may have liquid oceans under the ice. It is the only known moon to have a magnetic field, which could indicate a liquid iron core.
Callisto is composed of approximately equal amounts of ice and silicate rock. It has more surface craters than most other solar system moons. Callisto may also have a thin atmosphere of carbon dioxide and some oxygen. For more information about these four moons, go to ubne.ws/uXkwIS.
Galileo first believed that these pinpoints of light were fixed stars. However, he continued to observe them from January to March 1610. What he discovered was that they were moving back and forth from side to side, which indicated that they were orbiting Jupiter. This was a revolutionary idea; the accepted view at the time was that everything orbited the Earth, which was thought to be the center of the universe.
So, over the next few weeks you can follow in the footsteps of Galileo by watching these four moons orbit Jupiter. On clear nights, as you observe Jupiter and its moons, draw the positions of the moons in relation to Jupiter. Some nights you will see four moons, other nights you will see fewer, but each night they will have moved from the previous night. See if you can figure out which pinpoint is which moon and write the names on your drawing. Here's a hint: Io orbits Jupiter in 1.77 Earth days, Europa in 3.55 days, Ganymede in 7.16 days and Callisto in 16.69. If you need more help, the following website will show you the positions for a given date: ubne.ws/urVpNW.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.