The Celestial Equator

The heavy dark line running horizontally across the center of the Equatorial Sky Chart represents the Celestial Equator. The numbers that you see along the equator line represent celestial longitude, that is, hours of right ascension.

Just as the earth's equator divides the sphere of the earth into two hemispheres - northern and southern -, so the Celestial Equator divides the apparent sphere of the sky, the Celestial Sphere, into a Northern and a Southern Celestial Hemisphere. Like the equator of the earth, the Celestial Equator lies halfway between two poles. In the case of the sky these are the North Celestial Pole and the South Celestial Pole.

One of the continuing themes of the observational section of ASTR 106 is that the apparent geography of the sky reflects the geography of the earth from which we observe the sky. As the earth has poles, hemispheres, and an equator, so we imagine poles, hemispheres, and an equator in the sky.

Figure 1-3 of the Text

As the picture above shows, the sky can be thought of as a sphere concentric with the earth with corresponding features aligned, so that directly above the north pole of the earth we find the North Celestial Pole in the sky. Directly above the equator of the earth we find the Celestial Equator in the sky.

So what you find directly overhead in the sky at the zenith will depend on where you are on the earth. If you are looking at the sky from the north pole of the earth, what will be overhead is the North Celestial Pole at the zenith. But if you are standing at the equator of the earth, what you will see overhead will be the Celestial Equator running through your zenith.

This relationship is a general one: declination that you see directly overhead at your zenith reflects the latitude from which you are looking at the sky. The value of the declination is numerically equal to the latitude. Looking at the sky from St. Cloud at about 45° N. latitude, we see overhead at our zenith a declination of about 45°.