Stephen Hartmann, age 11, of Glenolden, Pennsylvania, for his question:

Do they see our stars at the equator?

The star studded reaches of space surround our planet on every side. Our view of the celestial population depends upon where we are on the curved surface of the globe. Everywhere we stand, the changing celestial scenery is regulated by the earth's daily rotation and its yearly orbit around the sun. The tilt of its axis governs the alternating seasonal changes in the northern and southern hemispheres. The axis also governs the unique view of the stars seen above the equator.

The North and South Poles face opposite points in the starry heavens. In Philadelphia, Polaris, the North Star, is always 40 degrees above the horizon, but the people there never see the Southern Cross that shines above the Southern Hemisphere. At the North Pole, Polaris is directly overhead. Farther south, it appears lower in the sky, matching the earth's degrees of latitude. Its distance above the horizon shows that Philadelphia is close to Latitude 40 degrees north of the equator. Farther south, along Latitude 30 degrees near Jacksonville, Florida, Polaris is 30 degrees above the horizon. Still farther south, at the equator, Polaris is about level with the northern horizon.

The Equator is Latitude 0 degrees, halfway between the two poles. It is the special circle that divides the two hemispheres, midway between their opposite views of the heavens. Thus, people at the equator have a unique view of the complete celestial sphere. The opposite poles lie fixed on the northern and southern horizons. Every calendar day, all the stars in our sky pass overhead or to the side and so do the southern hemisphere stars, which we never see above southern latitudes.

The rotating earth orbits the sun and changes the nightly constellations with the seasons, even at the Equator. The stars that appear on Christmas night pass overhead on Christmas Day. All the stars seen above the two hemispheres pass over the equator every calendar day, but half of them are hidden by dazzling daylight.

In our latitudes, most of the stars trace sloping paths over the sky and the length of the nights varies with the seasons. At the Equator, all the stars rise and set more or less at right angles to the horizon and trace perfect half circles over the sky. Every calendar day, their half circles trace the equator's 12 hour period of night.

This neat pattern is slightly modified by the 23 1/2 degree tilt of the earth's axis and the changing seasons. On the two equinox days, the stars rise and set at perfect right angles to the horizon and the noon sun is exactly overhead. On the opposite solstice days, their paths vary by 23 1/2 degrees.

In our latitudes, the setting sun slopes down and its slanting rays shed a period of twilight. At the equator the daily sun traces a 12 hour half circle over the sky. It sinks straight down and drops suddenly, with no twilight pause. Night arrives in a rush, with all the stars in our sky plus all those appearing above the southern hemisphere.