Gravity at the Eyepiece: Jupiter's Moons and Beyond

A Week of Observatory Nights

1Every October, the county observatory opens its large telescope to the Ridgecrest Middle School astronomy club. This year the club pointed it at Jupiter. Through the eyepiece, the planet looked like a small striped disk with four bright dots lined up beside it. Those dots are Jupiter's four largest moons: Io, Europa, Ganymede and Callisto. Galileo first saw them through a small telescope in 1610. Jupiter is about 11 times as wide as Earth and has more mass than all the other planets put together.

2Club member Shayla Okafor sketched the moons each clear night for a week. In her notes, each moon switched from one side of Jupiter to the other over a few nights, and on one night Io was missing until it came out from behind the planet. Bertrand Nakamura, an observatory volunteer, gave the club the data in the first table.

3Back at school, the club used a model. Outdoors, in an open area, Shayla swung a soft foam ball on a string in a flat circle above her head. She kept her hand pulling steadily on the string, and the ball kept circling. When she swung the ball faster in the same size circle, she felt that she had to pull harder on the string. When she let the string slip through her fingers, the ball flew off across the grass.

4Mr. Nakamura told the club that gravity also shaped the solar system itself. About 4.6 billion years ago, a huge cloud of gas and dust began to pull together. Most of the mass collected in the center and became the Sun, and smaller clumps that kept orbiting it formed the planets. The Sun is one of billions of stars in the Milky Way galaxy, and it carries the whole solar system along an orbit around the galaxy's center. One trip around takes about 230 million years, and the Sun moves along that path at roughly 230 km/s. On a dark night away from city lights, the club could see the Milky Way as a pale band across the sky, because we look out along the galaxy's flat disk from inside it.