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Declination and Latitude


We will examine the Equatorial coordinate system of right ascension and declination to see how it differs from the Zodiac or Ecliptic system and how it relates to that system. The equator or equatorial system is based, as many of you will know, on the Earth and its equator as the fundamental reference plane. In fact, the equatorial system is nothing more than the complete set of geographic coordinates (circles of longitude and circles of latitudes of the Earth) expanded and projected to cosmic size. This expanded sphere is called the Celestial Sphere.


Declination and Latitude


In other words, the North Pole of the Earth is extended until it reaches the stars and that point becomes the north pole of the equatorial sphere around which the entire heavens appear to turn. The North Star is used as a reference point for the north celestial pole. The same is true for the South Pole and the south equatorial pole. The equator of our Earth is also projected to the heavens and becomes the celestial equator and this equatorial plane is the fundamental plane of reference for this system. This celestial equator also forms a band or circle through a series of constellations (like the band of the zodiac) although this circle of the signs of the equator is, for the most part, different from those in the zodiac. Thus there is another set of signs of which most astrologers are unaware.

As mentioned, the Celestial Sphere is an exact projection of the geographic sphere and this fact allows for some very interesting astrological considerations. Coordinates on the celestial equatorial sphere are measured in right ascension (similar by analogy to zodiac longitude) and declination (similar to zodiac latitude). We shall explain more about right ascension a little later on. For now, we will investigate the relation between the latitude factor (declination) in the equatorial system and geographic latitude. In a word, they are identical.

Each place and city on this Earth is located at a specific latitude, somewhere between the equator and the North and South poles. Ann Arbor, Michigan, where I used to live, is located at some 42° latitude, North of the equator. In fact, there is a circle of cities at 42° latitude that stretch across the U.S.A. and on around the Earth. Thus there are other cities on the globe that also are located at 42° North geographic latitude.

Now the interesting fact about the relation between geographic latitude and declination in the equatorial sphere is that there exists a circle of stars on the celestial sphere located at a declination that matches the geographic latitude of your home. This circle of declination and the stars at 42° of declination are the only stars that ever pass directly (by zenith transit) overhead your town. Thus, each parallel of geographic latitude on the Earth has a matching parallel of declination on the celestial sphere The diagram on this page will illustrate this fact.

Ann Arbor is located at point 'a' on the rotating Earth. Star 'a' is directly overhead at what is called the zenith. As the Earth turns, it will carry Ann Arbor to point 'b', 'c', and on around in a circle until point 'a' is reached once again. There is also another circle of stars that pass exactly under Ann Arbor on the far side of the Earth each day. This circle would be those stars located at a declination circle of -42° (42 degrees south declination). Every city on Earth could be described in terms of the kind of stars and other objects that make up the declination circles that equal the circle of geographic latitude at which they are located.

What we have done for the declination factor on the celestial sphere, we could also do for the right ascension or longitude equivalent in this coordinate system. Right ascension is similar to zodiac longitude in that it is measured from 0° to 360°, but it is measured along the equator and not along the ecliptic or zodiac. We shall return to the difference between these two systems later on. Right now, we will investigate the relationship between right ascension and the geographic meridian that runs from the North Pole on the Earth through your birthplace and on to the South Pole.


© Copyright © 1997 Michael Erlewine