The Earth, while orbiting around the Sun in its elliptical trajectory, also rotates around its North-South axis.  The plane of Earth's orbit around the Sun is tilted with the plane of Earth's rotation about its North-South axis by about 23 1/2 degrees as shown in Figure 1. The Earth's rotation around itself is from West to East (eastward) once every 24 hours.  As a result, the Sun appears to rise in the East in the morning and set in the West in the evening.  When observed through successive nights, the stars in the sky will appear to move from East to West. 

The spherical space around the Earth (at anytime we see only a part of it) is called the celestial sphere.  If we extend the Earth's North-South axis into the space in the both directions, the points it meets the horizons are referred as the Celestial Poles, North and South respectively.  Since the Earth rotates eastward around its axis the celestial sphere appears to move in the opposite direction, the westward.  The projection of the Earth's equator on the celestial sphere is called the Celestial Equator.  The Celestial Equator divides the space into the Northern and the Southern Hemispheres. 

The lines perpendicular to the Celestial Equator passing through the Celestial Poles are the circles.  These circles are the projections of the Meridians or Meridian circles on the celestial sphere (Meridians are the imaginary lines forming circles on the Earth's surface drawn perpendicular to the Equator passing through the North and South poles of the Earth).  These projected Meridian circles on the celestial sphere are called Declination circles. 

As the Earth orbits around the Sun in its elliptical trajectory once every year, the Sun slowly appears to move from South of the Celestial Equator towards North in the Northern Hemisphere in the earlier part of the year (January through March). As the Sun crosses the Celestial Equator on about March 21, marking the beginning of the Spring in the Northern Hemisphere (and the Fall in the Southern Hemisphere), the day and night become equal in duration (12 hours each) all over the world.  On this day the Sun is said to be at the Vernal Equinox, the point at which the Sun crosses the Celestial Equator in the northward direction.   

For the next three months the Sun moves northward and reaches its northmost point (about 23 1/2 degrees north of the Celestial Equator) in the sky on about June 21.  It is the beginning of Summer in the Northern Hemisphere (and Winter in the Southern Hemisphere) with the maximum hours of daylight on June 21. After June 21, the Sun appears to start its journey southward crossing the Celestial Equator around September 23.  The Sun's crossing point across the Celestial Equator as it moves southward is called the Autumnal Equinox. The Autumnal Equinox marks the beginning of the Fall Season in the Northern Hemisphere (and Spring in the Southern Hemisphere), and again on this day, the length of day and night become twelve hours each all over the world.  

The sun continues its southward journey in the sky until it reaches the southmost point (about 23 1/2 degrees south of the Celestial Equator) on about December 22.  Now, the night is longest in the Northern Hemisphere and it marks the beginning of the Winter Season (while Summer begins in the Southern Hemisphere).  After reaching the southmost point, the Sun reverses its course in the sky resuming its northward journey towards the Celestial Equator.  Consequently in the Northern Hemisphere days become longer and nights shorter.  Once again on March 21 it crosses the Vernal Equinox, the Sun's northward crossing point of the Celestial Equator.  

The Sun's apparent path in the celestial sphere as described above is called the Ecliptic. In other words, the Ecliptic is the projection of the Earth's orbit around the Sun on the celestial sphere.  The crossing points of the Ecliptic and the Celestial Equator are called Equinoctial points. Thus, the Vernal Equinox occurs on about March 21 and the Autumnal Equinox on September 23 as the Sun appears at the Equinoctial points on these days. The celestial longitude is the distance measured in one direction (from West to East in the sky and then back to West underneath the Earth around the other side of the Earth) along the Ecliptic from the Vernal Equinox, while the celestial latitude is the distance measured perpendicular to the Ecliptic. 

When observed from the Earth, the path along which the planets and the Moon appear to travel in the sky is with in a narrow ten-degree celestial latitude zone on either side of the Ecliptic. Thus, the band of about 20 degrees latitude zone about the Ecliptic extending in the East-West directions on the celestial sphere is called the zodiac. The zodiac is divided in twelve equal parts of 30 degrees each along the Ecliptic, and the reference or the starting point of the zodiac is the Vernal Equinox. The popular twelve zodiac signs correspond to these twelve equal parts of the zodiac. The daily planetary positions used in astrology today are calculated with the Vernal Equinox as the reference point or zero degrees of Aries. 

 Although stars do move, their motion is so small compared with the planetary motions, they are considered practically "fixed" in the celestial sphere.  Due to the gradual change of orientation, the earth's polar axis continually wobbles.  In the course of time, it points to different pole stars and describes a circle in space in about 26,000 years.  Consequently, the positions of the equinoctial points (The Vernal and the Autumnal equinoxes) with respect to the "fixed" stars in the celestial sphere gradually change (and move westward) along the Ecliptic at an annual rate of 50 seconds of arc or at about one degree in 72 years. Thus, the equinoxes return to the same point on the Ecliptic with respect to a fixed star after about 26,000 years. This cycle is called the precession of the equinoxes. This period, divided by 12, gives us the duration for a precessional Age, the current one being the Piscean.  The motion of the equinoxes being retrograde, the next Age will be the Aquarian.  

As a result of the precession of the equinoxes, the reference point (zero degrees of Aries) of the zodiac (the Vernal Equinox) doesn't remain fixed with respect to a fixed star, but moves along the Ecliptic.  In other words, now, at the zero degree of Aries (at the Vernal Equinox) the Sun does not point to the same fixed star as it did at the time of the Spring equinox two thousand years ago.  About 2,160 years from now the location of the zero degree of Aries on the Ecliptic will correspond to the location of zero degree of Taurus (the next zodiac sign) today. A zodiac system that bases its reference point to the Vernal Equinox is called the tropical or moving zodiac system. 

In the West, the moving zodiac (tropical) system has been widely popular and accepted.  However, in the East, particularly in India, the fixed zodiac system (also referred as sidereal zodiac system) has been in use since a very long time.  The very basis of the latter system is the eastern premise that the astrological significance of a zodiac sign only depends on its (zodiac sign's) orientation with respect to the "fixed" stars.  In other words, the astrological characteristics of a particular location in the zodiac should always be the same since those are defined in relation to the zodiac's specific orientation in the celestial sphere containing the fixed stars.  

However, for the sidereal or fixed zodiac system, the problem arises in establishing the reference point (or the starting point of Aries) on the zodiac. Although there are several reference points (all within a range of four to five degrees) that might be in use today, the most popular and accepted one is based on the star Spica.  The star Spica corresponds to the 180 degree location of the fixed zodiac system. The sidereal or fixed zodiac reference point coincided with that of the tropical or moving zodiac reference point about 1700 years ago.  In other words, today the zero degree of Aries of the sidereal zodiac corresponds to about 23.75 degrees of Aries of the tropical zodiac