Zodiac

From Wikipedia, the free encyclopedia.

The term zodiac (from Greek zodiakos [kyklos], "circle of little animals", from zodiaion, the diminutive of zoon "animal") denotes several places where a circle of twelve animals occurs. Indo-European cultures developed a zodiac of twelve signs associated with a yearly cycle and with constellations of stars that lie along the apparent path of the Sun across the heavens (the ecliptic). Likewise in Chinese astrology, months and years pass through a cycle of twelve animals that imply certain fortunes or misfortunes related to events occurring within those signs.

Below are the Roman names of the signs of the zodiac (with the ecliptic longitudes of their first points):

Aries (0°)
Taurus (30°)
Gemini (60°)
Cancer (90°)
Leo (120°)
Virgo (150°)
Libra (180°)
Scorpio (210°)
Sagittarius (240°)
Capricorn (270°)
Aquarius (300°)
Pisces (330°)

Indo-European zodiac
Indo-European cultures developed a zodiac based on the ecliptic which is the path of the Sun independent of Earth's rotation, so that it is visible as the position of the Sun as seen at the same time each day. Eventually, "ancient astronomers" (Powell 2004) divided those constellations into twelve equal zones of celestial longitude to create the first known celestial coordinate system: a coordinate system that boasts some advantages over modern systems (such as equatorial coordinate system or ecliptic coordinate system). The zodiac is also understood as a region of the celestial sphere that includes a band of eight arc degrees above and below the ecliptic, and therefore encompasses the paths of the Moon and the naked eye planets (Mercury, Venus, Mars, Jupiter, and Saturn). The classical astronomers called these planets wandering stars to differentiate them from the fixed stars of the celestial sphere (Ptolemy).
Strictly speaking, the eventual culmination of the Indo-European zodiac (originating with Babylonian, Greek, and Indian astronomers) includes signs (also constellations) that are not all represented by animals (e.g., Libra, Virgo, Gemini). However, the term probably derives from earlier conceptions of the constellations along the ecliptic as sculpted animals. As with the Chinese zodiac, Indo-European astrologers understand the movement of the planets and the Sun through the zodiac as a method to explain and predict events on Earth.

Zodiac celestial coordinate systems
For any spherical celestial coordinate system one needs to define an equatorial plane and designate an origin for longitude. From these definitions longitudinal meridians perpendicular to the equatorial plane meet at the north and south poles of the celestial sphere and one can precisely specify a unique position on the sphere.
First, the zodiac coordinate system designates the ecliptic as the equatorial plane. One should not confuse the zodiac's equatorial plane with the Earth's equator which is so named because it serves as the equatorial plane for our terrestrial coordinate system. Instead the ecliptic is aligned with the Earth's orbital plane with the Sun. The Earth tilts at an angle of approximately 23° with respect to the orbital plane. This tilt is partly due to the Earth's precession as it gyrates and rotates on its axis. It contributes to the divergence between a tropical year and a sidereal year and thus contributes to the precession of the equinoxes which marks the difference between tropical and sidereal celestial coordinate systems.
Second, a system of coordinates must normally specify an origin (0° point) for longitude. However, the zodiac system names twelve equal regions (called signs) and technically any sign can serve as the beginning or the end of the sphere's precession. However, the system further divides each sign into thirty degree units. One reason this system of coordinates was particularly useful in antiquity is that writing large numbers was difficult before the introduction of arabic numbers. The zodiac system of coordinates kept every number below ninety (the highest value for zodiac latitudes). Since the constellations along the ecliptic varied in size, defining 12 equal signs of 30° each did require an arbitrary assignment of boundaries roughly corresponding to the ecliptic constellations[citation needed].
It is believed many of the classical astronomers specified zodiac signs using two bright stars near the ecliptic and opposite each other to serve as equatorial nodes or poles (not longitudinal poles): Aldebaran and Antares in the constellations Taurus and Scorpius respectively. These stars served rather well because not only were they on opposites sides of the ecliptic, but they also fell very near the center of their constellations and were therefore designated as Taurus 15 and Scorpius 15, meaning the middle 15° points within those signs. From these two stars then the remaining equatorial boundaries of the 12 signs of the zodiac follow (see Powell 2004).

Zodiac in astrology
Astrologers use astronomical observations of the movements of the night sky for divinatory purposes. The zodiac remains in use in modern astrology, though the issue of tropical versus sidereal coordinate systems is even more fundamental in (see sidereal versus tropical above, tropical astrology and sidereal astrology). At issue in the debate is whether the signs should be defined in terms of signs roughly aligned with the constellations of the same name (for sidereal astrologers) or whether the signs should be defined in terms of zones derived from nodal points defined by Earth's motion during a tropical year. The selection of two different astronomical coordinate systems implies the Sun moves through each sign at different times of the year, leading to different mapping of the signs to the days of the year for each of these systems.
Regardless of whether the tropical or sidereal definition of the zodiac is chosen, astrologers chart the positions of the Sun and planets in relation to the zodiacal signs. The horoscope is used to associate properties of a sign with the properties of planets positioned in that sign. The planet is said to influence events on Earth but the way in which it does this is modified by the influence of the sign in which it is positioned.

All text (on this page only) is available under the terms of the GNU Free Documentation License
(see Copyrights for details).