Mapping the Sky

            A system that uses two sets of intersecting lines to determine position is called a coordinate system. Lines of latitude and longitude are used to determine positions on the earth precisely. Thus, latitude and longitude comprise a coordinate system. Lines of latitude are parallel to the equator, appearing north and south of the equator. Lines of longitude circle the earth, passing through both the North Pole and the South Pole.
            These lines are labeled in degrees. The equator is considered to be 0° latitude and the poles are both considered to be 90°. There is no natural starting point like the equator to help label the longitude. Instead, the longitude line passing through Greenwich, England, is considered to be 0° longitude. Lines of longitude east and west of Greenwich increase to 180° east or west longitude, or halfway around the earth. By giving coordinates for latitude and longitude, any point on the earth can be located.
            A coordinate system similar to latitude and longitude is used to locate objects in space. Think of the stars as if they were fixed on the inside surface of a huge sphere. Such an imaginary sphere is called the celestial sphere. A person standing on the earth can see only one-half of the celestial sphere – the half that is above the horizon.
            There are two main systems used for mapping the heavens. One system, the Celestial Equator System, is based upon the direction of the earth's axis. Imagine that the earth is at the center of the celestial sphere. The point on the celestial sphere pierced by the earth's axis from the North Pole is called the celestial north pole. The celestial south pole is the point on the celestial sphere pierced by the earth's axis from the South Pole. There is also a celestial equator on the celestial sphere that is formed by extending the earth's equator outward until it intersects the celestial sphere.

            Coordinates on the celestial sphere are just like those used on the earth. One of the celestial coordinates, called declination, corresponds to latitude on earth. It is measured in degrees from the celestial equator. The other celestial coordinate, called right ascension, corresponds to longitude on earth. Right ascension is measured in degrees counter-clockwise along the celestial equator looking down from the celestial north pole. Thus it is possible to locate an object in space by using a set of two intersecting coordinates.

Using Parallax to Measure Distances

            Astronomers have used the parallax principle to find the distance to the moon in the following simplified manner. At exactly the same time, two astronomers on the earth observe one point, which we shall call point M, on the moon. One astronomer is at point A, where the moon appears directly overhead in the nighttime sky. The second astronomer is at point B, where the moon appears on the horizon. The distance between points A and B is known. Both astronomers agree to sight the moon against the position of the same star. Remember, although the astronomers are several thousand miles apart, if each sights the same star, each looks in the same direction.
            After the astronomers sight the point on the moon and then the star, they each obtain a set of two angles. The first angle in each set represents the altitude of the moon above the horizon. The moon is directly above the astronomer at A. He says that the moon is at 90° altitude. The moon is on the horizon of the astronomer at B. He says that the moon is at 0° altitude. The second angle in each set describes the altitude of the star above the horizon.
            The distance between points A and B is known, and now the measures of enough angles are known to enable the astronomers to find AM, the distance of the earth to the moon.