John Martin, age 13, of Houston, Texas for his question:
How do they map the sky?
The sky looks like a great dome over the flat earth. This is how the early stargazers saw it. The heavenly bodies and the blue vault overhead look so firm and solid that they called it the firmament. We know now that the sky is really our view of the vast oceans of space. But, when mapping the sky, it is still convenient to regard it as a vaulted. dome. We ignore the stars distance from the a;arth and map them as if they were painted on the inside of a sphere.
Astronomers call the sky dome the celestial sphere. We map the earth by looking down, outside a sphere. We map the heavens by looking up toward the inside of a sphere ‑ or, morn accurately, what seams to be the inside of a sphere.
The earth is mapped with fixed lines of latitude and longitude. There are corresponding lines in the celestial sphere. However, the heavenly bodies do not remain fixed. Once in every 24 hours the sun, the moon and the stars seem to parade completely around the a arth. This, of course, is because the earth rotates. As it turns, it gives us a complete view of the heavens around us.
The celestial sphere, then, is like a big clock. The parading heavenly bodies become the hands of the clock. The landmarks on earth are fixed and constant. Heavenly landmarks tend to move, and mapping the sky is more complicated. There are, however two fixed spots in the celestial sphere. They are the two celestial poles, directly above the poles of the earth.
The celestial north pole is very close to the North Star, Polaris. The celestial south pole is near the Southern Cross. Halfway between these points the; celestial sphere is divided by the celestial equator: This line hoops clear around the heavens, directly above the earthly equator. We now have three points of reference.
It is now simple to draw lines parallel to the celestial equator, some north and soma south of it. Those are called declination circles, and they correspond to the lines of latitude on the face of the earth. The celestial lines of longitude run through the celestial poles at right angles to the equator. Each of these great circles is divided into 360 equal degrees of arc. Each degree of arc is divided into 60 minutes and each. minute into 60 seconds.
The great celestial circle which corresponds to the Prime Meridian is placed through the center of the sun when it stands at the Spring Equinox. This system of circles holds true for the celestial sphere seen from any place on earth, Another system charts the sky from the zenith, the point directly overhead. This is different for every spot on earth, and it changes with every moment.
The celestial circles parallel with the celestial equator give the north and south location in the heavens. The circles through the poles give the east and roost. It is useful to estimate a celestial degree of distance when locating a star in relation to anothoro The Big Dipper pointer stars are about five degrees apart. A side of the square of Pegasus is about 15 degrees. The moon and the sun are about half a degree, or 30 minutes, in sky measuring from the earth, in diameter.