Jupiter is rising late this month. I recently got out my telescope for the first time in a year and trained it on this massive planet. What intrigues me, more than the big red spot on Jupiter, are its moons.

My telescope does not have a very powerful lens, yet I can easily see the four Galilean moons, named for Galileo, the Italian scientist who first saw them in a telescope he built in 1609, nearly 400 years ago.

The moons of Jupiter look like tiny stars near the planet. Individually, they are called Io, Europa, Ganymede and Callisto, names from mythology.

The four moons we can easily see appear stationary in the telescope. Over the course of several days or weeks, however, they do a dance, back and forth along a line.

If we watch them, and take notes of their position each night, it is easy to estimate the time it takes them to make one orbit around Jupiter.

While we see them apparently move back and forth along a line, they in fact are traveling in an ellipse, and we see the elliptical orbit edge-on.

We also know that they are traveling around Jupiter for other reasons: Sometimes they disappear because they are passing behind the planet. Sometimes they appear as black dots on the face of Jupiter when they pass in front of it.

Galileo worked this all out in just a few weeks.

Because the moons of Jupiter are traveling in an apparent straight back-and-forth motion, they are all circling Jupiter in the same plane -- they form a disk around the planet. They do not circle in all directions like our old model of electrons circling an atomic nucleus.

We also know that the planets in our solar system form a disk. All of them appear to travel in a near-straight line across the sky, the "plane of the ecliptic."

The rings and moons of Saturn form a disk. This disklike rotary motion is a direct result of how our solar system was formed by gravitational attraction.

As gas and dust are drawn together to form a planetary system, they will naturally form a whirling disk. Even huge galaxies develop a disklike rotation.

Our own Milky Way galaxy is a large disklike wheel of stars, slowly revolving, with our sun near the perimeter of the disk.

This revolving and rotating is a fundamental condition of our universe.

If we look at the big spot on Jupiter, we can calculate Jupiter's period of rotation as just under 10 hours.

Whether we look at Jupiter, its moons, Venus' changing phases, or our own moon, we are struck by the constant motion in an apparently quiet and motionless starry sky.

Virginia's science Standards of Learning cover our solar system by starting with the Earth's rotation (section 1.6) and moving to phases of the moon (3.8) and Earth-sun-moon motion (4.7). By 6.8, study moves to other planets and their moons, and other solar system objects.