Miller, age 13, of San Diego, California, for her question:

How do constellations form and appear when they do?

This is really two questions in one. The formation of the constellation designs is related to the earth's position among the stars. Their scheduled appearances are related to our planet's various positions around the heavens.

Looking up at the sky is actually looking out into the vast ocean of space. And space is populated with an assortment of widely separated stars. Those we see from the earth are in our section of the cartwheeling Galaxy. All of them    the larger and smaller ones, the local and remote ones    are swirling in the same direction around the Galaxy at different speeds. And our home planet is speeding with them. Because of their different speeds, the stars are changing their positions in relation to each other. But we do not notice this because the changes are  at enormous distances from us.

So our skies are studded with a background of seemingly fixed stars, each in its own dependable position. The human eye tends to see them in groups and recognizable patterns called constellations. Our view of the sky extends from the neighboring planets to beyond the Galaxy. The stars in each constellation occupy one specific section of the sky. Some may be neighbors and others may be at remote distances. The heavens surround the globe on every side like an immense sphere    and all of it is adorned with fixed constellations.

Of course, we see only half the celestial sphere at a time    while the other half is seen from the opposite side of the globe. V?e see it from our planet which is forever turning around to give us different views of the starry heavens. As the earth turns, different constellations appear in the sky overhead. And because their positions are fixed on the celestial sphere, they follow each other in the same order. As the earth rotates eastward, each night the starry parade rises in the east, marches overhead and sets in the west.

Each year, the earth carries us on a 600 million mile orbit around the sun. This distance is not large enough to make much difference to our view of the infinite heavens. But the axis on which the earth rotates is tilted to its orbital path   23 1/2 degrees. This changes the celestial scenery because through the year it tilts to face different directions. Different constellations appear overhead with the changing seasons of spring and summer, fall and winter. Each year the heavenly parade repeats the schedule. The entire starry spLere is always there, but we see only part of it at a time. In the daytime, the stars above are invisible in the dazzling sunlight. The constellations that adorn our winter skies at night are overhead during the summer days. The summer constellations    are present during the winter days. All these repeating schedules are caused by the rotating, revolving earth.

The earth's tilted axis also governs another aspect of our celestial view. Its surface rotates fastest at the equator and dwindles to zero at the poles. These two ends of the axis stare steadily out in the same direction. So Polaris is always above the North Pole. However, as we move farther south over the globe, we see it lower in the sky.