Ecliptic, the great circular path , that appears to be traversed by Sun during the span of one year, is the fundamental and basic reference line, about which the relative positions of other heavenly bodies such as stars and planets are marked, for the study of celestial sphere and the zodiac.
As the axis of earth is inclined at 23027’ the ecliptic shows the same angular twist with the celestial equator; due to which both these great circles meet at two diagonally opposite points called equinoctial points. Day and night happens to be of equal length at both these equinoctial points and the one happens during the Uttarayana period i.e. when Sun crosses the celestial equator from Southern hemisphere to the Northern hemisphere is considered as the Vernal Equinox and named as the ‘first point of Aries’.
Modern astronomy accepted this point as the root of heaven and based upon which we calculate and define all the astronomical parameters such as hour angle, longitude etc. Western astrologers solely rely on this base point as the starting point of the ‘Rasi Chakra’ and divide the circular path into twelve houses each measuring 300 each; thus we have Sun Signs such as Aries, Taurus, Gemini etc.
But due to the unbalanced gravitational pull of both Sun and Moon over the earth, the Vernal Equinox changes its position continuously, approximately at an angle of 50.3” (seconds) per year i.e. one degree in 72 years. So we have to redefine the first point of Aries every moment or otherwise the relative positions of heavenly bodies will be misjudged and the entire astro calculations go wrong. The precession of equinox causes many practical difficulties for locating a celestial point precisely and hence we adopt epoch years for reference, such as 1900 AD, 2000 AD etc....