Zodiac: An Introduction Tropical Zodiac and Sidereal Zodiac

Zodiac: An Introduction
In astronomical terms, Zodiac refers to each of the twelve divisions of the Sun’s elliptical path in course of one year. Each of the division is of thirty degree and twelve divisions complete a circle and one year. The path of other planets as well as the path of the Moon is located in close proximity with the elliptical path of the Sun and extends eight to nine degrees in the northward or the southward direction from the elliptical path, wi
thin the Zodiac belt. However, the actual position of these paths in respect to the elliptical path is subjected to celestial latitude measurements.

The twelve divisions have twelve different Zodiac signs and so Zodiac is often referred to as a system of celestial coordinates. These coordinates are also known as ecliptic coordinate system where the latitude has its origin in the ecliptic and the longitude has its origin in the Sun’s position precisely at Vernal Equinox. 

Tropical Zodiac and Sidereal Zodiac
In astronomical terms a year is defined using two terms— Tropical and Sidereal. In fact, the Sidereal Zodiac and the Tropical Zodiac are two completely different ecliptic coordinate systems largely used in modern astrology. In both the systems the ecliptic is divided into twelve Zodiac Signs which are then named according to the names of the constellations. Previously in astronomical studies both these systems were the same but with the passage of time emergence of different practices and a phenomenon known as the Procession of Equinoxes made these two systems different from each other. The Earth has two primary motions which mainly cause the Procession of Equinoxes. The difference between the two systems is known as Ayanamsa.

The primary motion of the Earth refers to the Revolution, that is the Earth’s movement around the Sun. The orbit along which the Earth moves is called Ecliptic in astronomical terms. The secondary motion, however, refers to the rotation of the Earth on its Axis. The Sidereal Zodiac system is completely based on the primary motion and it is the Earth’s secondary motion that determines the Tropical Zodiac.

Owing to the fact that in its primary motion around the Sun on its Ecliptic, the Earth takes precisely 365 days, 6 hours, 9 minutes and 9.5 seconds to complete one full round when measured in context with the position of a specific star and the apparent movement of the Sun around the Ecliptic takes precisely 365 days, 5 hours, 48 minutes and 46 seconds, the Sidereal Zodiac year is slightly longer than a Tropical Zodiac year.

In Vedic and Hindu astrology the Sidereal Zodiac system, based on the position of the Earth in relation to the Sun and other important starts, is mainly used while in western astrology the Tropical Zodiac system, based on the position of the Earth in relation to only the Sun is used. It is unclear when the Tropical Zodiac and the Sidereal Zodiac overlapped and so different Ayanamsa is used by different Vedic astrologers.