Latitude and Longitude
In order to pinpoint locations on the surface of the earth, a coordinate system which is termed latitude and longitude had been devised. Latitude is an angular measurement made from the earth's center, northward or southward from the equator, along a meridian circle to the location in question. Longitude is an angular measurement also made from the center of the earth, east or west from the Prime Meridian, along the equator to the meridian circle which contains the position. The range of longitude is from 0° to 180° east or west of the Prime Meridian, while the range of latitude is from 0° to 90°, north or south of the equator. In both cases the hemisphere of the position must be stated. For example, Allentown, PA is located at approximately 40° north latitude and 75° west longitude. Allentown is in the northern and western hemispheres.
If you imagine the grid of latitude and longitude superimposed on a transparent sphere, which is surrounded by a larger sphere (the celestial sphere, which is really the sky), you are ready to understand how the equatorial coordinate system is formed. This system is used by astronomers to locate objects in the sky. If a flashcube were placed in the inner transparent sphere and fired, the light of the bulb would radiate outward to the celestial sphere, casting the shadows of latitude and longitude onto it. Where these shadows would fall, a new "equatorial" coordinate system would be formed called right ascension and declination. The circles of latitude would now correspond to declination. Instead of being measured north or south of the equator, they would be measured north or south of the celestial equator as either positive or negative angles respectively. Longitude would become meridians of right ascension measured eastward from the intersection of the celestial equator and the ecliptic. The ecliptic represents the path of the sun in the sky as the earth revolves around this star. The intersection position of the celestial equator and ecliptic is known as the vernal equinox, and it is the location of the sun at the first moment of spring. This is also the origin of the equatorial coordinate system. Right ascension is always measured along the celestial equator eastward from the vernal equinox. There is no westward component as in the terrestrial system of longitude.
Right ascension positions are usually measured in hours, minutes, and seconds. The coordinates of the system form a sidereal (star) clock composed of 24 hours which is equal to the interval of one earth rotation. This time system is called sidereal time. Twenty-four hours of sidereal time (literally star time) is about four minutes less than the solar time measured by clocks throughout the world. In other words, one earth rotation equals 23 hours, 56 minutes of clock time. This 23 hour, 56 minute interval is divided into its own 24-hour system which is called a sidereal day.
We want to know where the individual
objects of the universe are in space relative to each other, i.e., we
want to discover the geometry of the universe, and the equatorial coordinate
system will help us attain that objective. The equatorial coordinate
system provides us with a system for measuring the universe as it is
seen naturally from earth.
The Celestial Equator and Celestial Poles
Parallels of Declination
Lines drawn parallel to the earth's equator are called parallels of latitude, while lines drawn parallel to the celestial equator are called parallels of declination. Parallels are useful when considering displacement between the equator and the poles, i.e., when measuring north and south.
To aid us in specifying positions on the surface of the celestial sphere, in addition to the constellation, we now have the system of lines and points represented by the equatorial coordinate system. The system is simple to use since the apparent celestial position of any star can be specified by only two coordinates, one giving the position of the star north or south of the celestial equator, and another giving the star's position east of the zero right ascension circle. The star Vega in the constellation Lyra, for example, has a declination of +38°, and a right ascension of 18 hours 40 minutes. The diagram below illustrates the method of measurement on the celestial sphere.
In the horizon system of coordinates known
as altitude and azimuth, we specify the position of a star relative
to the earth's surface by stating the star's place relative to our local
horizon in terms of its altitude (elevation) and azimuth
(direction). The altitude of a star is simply the angle between the
star and the observer's horizon; and its azimuth is simply the angle
between a point due north of the observer and the star, measured eastward
along the horizon. Thus, to find a star whose altitude and azimuth are
30° and 135°, respectively, an observer should look 30°
above the horizon in the southeast.