What is Zodiac?
Astronomically, the term 'Zodiac' is used to represent a circle of twelve 30 degrees divisions of celestial longitude which is positioned in the centre of the elliptical path which is the apparent path of Sun across the celestial sphere over the course of the year. The path of the moon along with the path of the other planets is situated close to this elliptical path, within the belt of the Zodiac extending 8 to 9 degrees north or south of the elliptical path according to the measurement of celestial latitude.
These twelve signs are referred as the twelve 'Zodiac signs' and hence the Zodiac is a celestial coordinate system also called as ecliptic coordinate system which takes the ecliptic as the origin of the latitude and the position of Sun at the vernal equinox as the origin of the latitude.
Sidereal Zodiac and Tropical Zodiac:
In Astronomy, Sidereal and Tropical are two different terms which are used to define a 'year'. They are two different systems of the ecliptic coordinate used in the Astrology. Both Sidereal and Tropical are used to divide the ecliptic into twelve Zodiac Signs which are named after the name of the constellations. Earlier both the Sidereal zodiac and Tropical Zodiac were identical but over centuries, they drifted apart by a phenomenon called as Procession of Equinoxes which is caused due to the two primary motions of the Earth. The difference between Sidereal Zodiac and Tropic Zodiac is called Ayanamsa.
The Primary motion of the Earth was related to its orbit around the Sun and this plane of orbit is known as Ecliptic. While the secondary motion of the Earth is based on its rotation on its axis. The Sidereal Zodiac was solely based on the primary motion of the Earth while the Tropical Zodiac was solely based on the secondary motion of the Earth.
The Earth's first primary motion in orbit around the Sun on the ECLIPTIC takes 365 days, 6 hours, 9 minutes, and 9.5 seconds in relation to any specific star. While the Sun's apparent movement around the ECLIPTIC takes 365 days, 5 hours, 48 minutes, and 46 seconds which makes a Sidereal Zodiac year longer than the Tropical Zodiac year.
The idereal Zodiac system is used in Hindu astrology of Vedic astrology and is based on the orientation of the Earth relative to the Sun as well as stars while the Tropical Zodiac system is used in western astrology is based on the orientation of Earth relative to the Sun. There is no consensus among Vedic astrologer on the day when Sidereal Zodiac and Tropical Zodiac overlapped and the rate of precession and hence different Vedic astrologers use different ayanamsa or adjustment values.