essary to explain a few terms, and to remember that we are assuming that the earth is in the center of the universewe are using a geocentric model of the universe. The ecliptic measures the annual rotation of the sun around the earth, and is inclined 23deg. from the celestial equator. It is not a representation of the daily rising and setting of the sun. The Greeks divided the ecliptic into twelve sections, and each section was named after the constellation it containedAries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pices respectively. The ecliptic, divided in this fashion, is called the zodiac. The Greeks further divided each of these twelve sections into thirty units, effectively graduating the entire circle for longitudinal measurement (30 multiplied by 12 is equal to 360). The system began at the vernal equinox, the intersection of the ecliptic with the celestial equator in the constellation of Ares, and completed a 360deg. circle around the circumference of the celestial sphere. The Greeks used the ecliptical to measure a star's horizontal, angular displacement from the vernal equinox. Vertical, angular displacement was measured by constructing a graduated circle perpendicular to the ecliptical. If you are completely confused by my written description, take a look at the diagram I have created. Ecliptical coordinates were used by Hipparchos and Ptolemy in their star catalogues, and were the standard of celestial measurement until the Renaissance, when they were replaced by the equatorial coordinate system. The equatorial coordinate system is identical to the ecliptical system, except that it uses the celestial equator for horizontal measurement instead of the ecliptic. Because the celestial equator is simply a projection of the earth's equator, the equatorial coordinate system is analogous to terrestrial longitude and latitude, and provides a more accurate system of measurement. This...
