Rhonda Von Rueden, age 11, of Hinckley, Minnesota, for her question:

Why don't we see all the planets at night?

From our point of view, the planets are the busiest bodies in the heavens. They wander around, weaving paths against the background of fixed stars. Venus may appear in the western evening sky and set soon after the sun. Mars, Jupiter or Saturn may do the same thing. Sometimes these planets appear in the east at sunset and take all night to ride over the sky. Sometimes they rise dust before dawn and ride over the sky with the sun. Then the dazzling daylight makes them invisible. Sometimes they are out of sight because they are traveling around the far side of the sun.

The reason for this odd behavior is motion. Each planet orbits around at its own speed and at its own distance from the sun. The outer planets travel more slowly around their longer orbits. The faster inner planets are always catching up with the traffic in the slower lanes and moving ahead. Their positions are always changing. Sometimes we can see this one or that one somewhere in the night sky. But as a rule, we do not see them when they are overhead during the day.