Tropical and Sidereal and the Illusive 13th Sign
There are 88 constellations recognised by the International Astronomical Union but only twelve of them are well known, and these are the modern zodiac signs we observe today. A constellation is simply a collection of stars, imaginatively linked together to represent a person, animal or object in the night sky. Zodiac is derived from the Greek word meaning 'circle of little animals'.
In Western astrology there are four elements of physical existence – Fire, Earth, Air and Water. Each contains three signs of the zodiac. Each zodiac sign is also identified by a constellation of stars which roughly form its symbol and corresponds to the different months of the year. They occupy a sector of the sky which makes up 30° of the ecliptic starting at 0° Aries, which is the spring equinox in the northern hemisphere or autumn equinox in the southern hemisphere.
The origin of what was to become this contemporary zodiac evolved slowly over time. Surviving records apparently show that the Greeks borrowed and established the zodiac from many cultures that predated their own, including Mesopotamia, Egypt, Babylon, and Sumer. An observatory in Metsamor, Armenia which predates the Babylonian kingdom by around 2,000 years is claimed to contain the first recorded example of dividing the year into twelve sections.
There are two different zodiac systems. The sidereal zodiac (also known as the fixed zodiac) is where the position of the Sun as it passes in front of the constellations is used to measure time. Hindu or Vedic astrology use the sidereal zodiac. Western astrologers tend to use the tropical zodiac where the position of the Sun is referenced against the Earth’s horizon at a particular location which gives you the seasons as a measure of time. No reference is made to the stars on the tropical zodiac. Both zodiacs are divided into twelve and they both use the same names.
The zodiacal signs are an abstraction from the physical constellations as they have been designed to represent exactly one twelfth of the full circle. The constellation of Ophiuchus, sometimes called the 13th sign, was left out by ancient astrologers even though it is on the plane of the ecliptic between Scorpio and Sagittarius. Western astrology is based on zodiac signs and not the actual constellations. Practitioners of sidereal/Vedic astrology, which is more closely aligned with the constellations, don’t include Ophiuchus either. It is believed that the reason Ophiuchus was not included is that early astrologers wanted to divide the 360° path of the Sun into 12 easy equal parts. This also maintained equilibrium between the seasons.
The zodiac constellations sit behind the path taken by the Sun as seen from Earth. That is, the Sun blocks that constellation from our view in our daytime sky. Of course, the Sun continues to block the same constellation during the night, however, we are unable to see this as our position on Earth is facing away from the Sun.
Due to what astronomers call precession, in the area of the sky that Aries occupied 2,000 years now appears the constellation of Pisces as the stars shift backwards one tropical constellation every 2,160 years, or by approximately 1º every 71.6 years.
The International Astronomical Union defined the edges of the official constellations based on celestial coordinates, and not the pattern of the constellation. This is because star patterns that we interpret from Earth as being linked together may not be grouped together in space.
The boundary dates of each of the constellations represented on the Australian Nature Wheel are credited to Dr Lee T Shapiro, Director Morehead Planetarium Article by Lee Shapiro - 1977 - International Planetarium Society, Inc. (ips-planetarium.org). The orientation of each of the constellations were duplicated using Stellarium planetarium software.
1660 celestial map of the constellations of the Southern Hemisphere
Image credit: Celestial Atlas