A Tale of Three Zodiacs

The zodiac is perhaps the most significant and most confusing aspect of astrology. Not only do most people interested in astrology not really understand its significance or creation, but even contemporary astronomers and NASA seem to be confused about what exactly it is and how astrologers use it. Complicating matters further is the existence of alternative zodiacs that confuse newcomers and embolden skeptics.

​A zodiac is a celestial measurement system used to locate and track the positions of the planets as they move across the sky. Because it is a mathematical measurement system, there can be multiple methods used to calculate it much in the same way we can use the Imperial system (inches, feet, cups, etc) and the Metric system (centimeters, meters, liters, etc) to measure the same thing and get results that appear contradictory. There are three different zodiacs; each used to measure and track the planets in much the same way, but have differing characteristics usually related to starting position or the size or number of the subdivided segments.

The Tropical Zodiac: A Measurement of Seasons

The most common zodiac utilized in western astrology. The tropical zodiac defines its starting point with the Vernal Point. This is the astronomical location when the ecliptic (the path of the Sun) and the celestial equator intersect in the East. When the Sun is at this point it is the Vernal Equinox or the first day of Spring in the Northern Hemisphere. Day and night are equal and daylight hours begin to overtake the nighttime hours as the Sun rises higher in the sky each day.

Because it adheres to the solstices and equinoxes, it easily divides the 360° circle into four 90° seasonal quadrants. The Sun moving into Aries starts spring, into Cancer begins summer, into Libra initiates autumn, and into Capricorn commences winter. These quarters are then subdivided into three producing 12 30° segments of the ecliptic commonly referred to as signs. These 12 segments are named after the 12 dominant constellations that the ecliptic passes through.

The tropical zodiac measures the positions of planets East of the Vernal Point. The Sun in the above diagram is positioned 130° from the Vernal Point. Astrologers would call this specific degree 10° Leo because astrology characterizes the segments of the ecliptic into signs and refers to them with those characterized names. Both "130° from the Vernal Point" and "10° Leo" refer to the same position in the sky. When the Sun is here it is also always summer (more specifically, August 1st or 2nd) because of the anchoring of the signs of the zodiac to the equinoxes and solstices of the year.

The Sidereal Zodiac: A Measurement of Stars

The zodiac most commonly employed in Jyotish or Indian astrology. The sidereal zodiac measures the Sun's relationship with the stars as opposed to the Sun's relationship with the Earth like the tropical system.

This system marks the beginning of the zodiac from the star Spica (Alpha Virginis). It uses Spica as the 0° Libra point and projects to the opposite side of the zodiac calling that point in space 0° Aries. Just like the tropical zodiac, the sidereal circle is sub-divided into 12 equal, 30° segments and named after the same 12 dominant constellations along the ecliptic.

Unlike the tropical zodiac, the sidereal zodiac is not fixed, but moves back 1° every 72 years due to precession of the equinoxes. The sidereal and tropical systems are out of sync by about 24°.

This means that the sidereal system is not measuring from the vernal point like the tropical system, but from its own starting point measured opposite from the star Spica. In the previous example we used the Sun at 10° Leo to show that this was just another way to say that the planet was located 130° east of the Vernal Point. For a planet to be 130° east of the Vernal Point in the sidereal system it would be at roughly 16° Cancer.

The two zodiacs are placed on top of one another in the image above to compare them more clearly. The tropical zodiac in green and the sidereal in blue. Notice how the Vernal Point (which serves as the 0° Aries point in the tropical zodiac) is now in early Pisces (roughly 6°) in the sidereal zodiac while Spica (the 0° Libra point in the sidereal zodiac) is now at roughly 24° Libra in the tropical zodiac.

Illustrating the differences further is the Sun, placed back 130° distant from the Vernal Point is in 10° Leo (the 130th degree of the tropical zodiac) in the tropical zodiac, but is in 16° Cancer (106° distant from the beginning of the sidereal zodiac) in the sidereal system.

The Constellational Zodiac: A Measurement of Sight

​The constellational zodiacal system measures where planets are along the zodiacal constellations themselves. This system reflects what you would witness if you went outside and watched the night sky.

The constellational system utilizes the Vernal Point as its starting point, calling this 0° but doesn’t call special attention to it and uses the uneven constellational figures to subdivide the circle (size of each constellation listed around the circle). Constellation figures cover a rather small portion of the night sky so a lot of the ecliptic runs through the empty space between the constellations. This system utilizes the constellational boundaries implemented by the International Astronomical Union (IAU) in 1930.

Perhaps most striking, the constellational zodiac is divided into 13 constellations and they vary wildly in size from one another. This is the perfect way to illustrate the difference between constellations and zodiac signs. Signs are equal mathematical divisions of the ecliptic. Constellations are visible groups of stars. The signs were named after the dominant constellational figures along the ecliptic, but they were never thought of as or intended to be interchangeable with one another.

The constellational system also measures planetary positions east from the Vernal Point like the tropical zodiac does. Unfortunately, none of the constellations measure exactly 30° which makes the characterization of the segments of the ecliptic more difficult and harder to determine. As a result, this system is not very popular among astrologers but it does have its advocates. This system would also have to include the thirteenth constellation Ophiuchus since it does take up space along the ecliptic. Historically, Ophiuchus was overlooked as only one star of the constellation fell on the ecliptic, but the boundaries drawn by the IAU have given the constellation more space along the Sun's path than it was historically considered to have.

Proponents of the sidereal zodiac either imply or outright state that the sidereal system is accurate to the stars while the tropical zodiac lost its connection with them a long time ago. This is not exactly accurate as we shall see below. The sidereal system measures signs as equal 30° divisions of the ecliptic which is not an accurate reflection of the constellations and their relationships with one another. The sidereal zodiac also does not use Ophiuchus, further separating itself from the stars proponents claim it seeks to reflect.

The above compares all three zodiacal systems; tropical in green, sidereal in blue, and constellational in purple. Each system measures the same circle in different ways by prioritizing different points. The vernal point falls at 0° Aries in the tropical zodiac, 6° Pisces in the sidereal zodiac, and 9° Pisces in the constellational zodiac. Similarly, the fixed star Spica is 24° Libra in the tropical system, 0° Libra in the sidereal system, and 34° Virgo (the constellation to whom the star belongs) in the constellational system.

Because each system measures the same circle differently, it is up to the astrologer to determine which system is more meaningful and makes the most sense by whatever metric they would judge that by. Each of the three zodiacs has its own merits, but each one also sacrifices something in order to have those merits.