Jay Sparrow, age 12, Falmouth, Me., for the question:

What is the zenith?

The zenith is a point in the sky directly over your head. Imagine a line from your feet straight down to the center of the earth. If you carry this straight line way up into the sky, it will point to the zenith.

This imaginary spot in the sky is very necessary if you want to chart a picture of the heavens. To make  a map of the stars you need a plan, somewhat like the grid of lines of longitude and latitude used in a map of the earth. The sky map is charted on the inside of a demo with the zenith at the tip top,

The bottom rim of the dome is the horizon. Vertical lines run at right angles to the horizon up the through the zenith. Horizontal lines run parallel with the horizon. These intersecting; line a give us a grid for charting the positions of the stars.