Celestial Location

To locate the ecliptic, planets, or anything else in the sky, you need something to refer to, a referent system. A referent system is easily established by first imagining the sky to be a celestial sphere just as the ancient Greeks did. A coordinated system of lines can be visualized on this celestial sphere just as you think of the coordinate system of latitude and longitude lines on the earth’s surface. Imagine that you could inflate the earth until its surface touched the celestial sphere. If you now transfer the latitude and longitude lines to the celestial sphere, you will have a system of sky coordinates. The line of the equator of the earth on the celestial sphere is called the celestial equator. The North Pole of the earth touches the celestial sphere at a point called the north celestial pole. From the surface of the earth, you can see that the celestial equator is a line on the celestial sphere directly above the earth’s equator, and the north celestial pole is a point directly above the earth’s equator, and the north celestial pole is a point directly above the North Pole of the earth. Likewise, the south celestial pole is a point directly above the South Pole of the earth.

You can only see half of the overall celestial sphere from any one place on the surface of the earth. Imagine a point on the celestial sphere directly above where you are located. An imaginary line that passes through this point, then passes north through the north celestial pole, continuing all the wary around through the south celestial pole and back to the point directly above you makes a big circle called the celestial meridian. Note that the celestial meridian location is determined by where you are on the earth. The celestial equator and the celestial poles, on the other hand, are always in the same place no matter where you are.

Overall, the celestial sphere appears to spin, turning on an axis through the celestial poles. A photograph made by pointing a camera at the north celestial pole and leaving the shutter open for several hours will show the apparent motion of the celestial sphere with star trails. The moderately bright star near the center is the North Stat, Polaris. Polaris is almost, but not exactly, at the north celestial pole. You can locate Polaris by finding the Big Dipper. The two stars on the end of the dipper opposite the handle are called the pointers. Imagine a line moving from the bottom of the dipper upward through the two pointers. The first bright star that this line meets is Polaris. The angle that you see Northern Hemisphere. Shows the geometric relationships between your latitude and the angle of Polaris above the horizon.

If you observe the constellation night after night, you will see that the stars maintain their positions relative to one another as they turn counterclockwise around Polaris. Those near Polaris pivot around it and are called “circumpolar”. Those farther out rise in the east, move in an arc, then set in the west.

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