Under Tropical zodiac system, following the path in the region of the Sun in its oval course, the Earth also revolves in the region of its North-South bloc. The plane of its path in the region of the Sun is tilted with the plane of Earth’s revolution with respect to its North-South bloc related to Tropical zodiac system.
This is in respect to 23 ½-degrees. The Earth’s revolution in the region itself is starting from West to East (eastward) just the once every 24 hours. As a result, the Sun becomes visible to go up in the East in the morning and set in the West in the evening. As seen through consecutive nights, the stars in the sky will come into view to go from East to West associated with Tropical zodiac system.
The circular space in the region of the Earth (at anytime we see simply a fraction of it) is called the outer space sphere under Tropical zodiac system. If we expand the Earth’s North-South bloc into the space in both ways, the positions it meets the prospect are referred as the Celestial Poles, North and South respectively. Given that the Earth revolves eastward in the region of its bloc, the celestial area becomes visible to move about in the reverse path, the westward. The projection of the Earth’s equator on the celestial area is called the Celestial Equator. The Celestial Equator separates the space into the Northern and the Southern Hemispheres related to Tropical zodiac system.
The lines at right angles to the Celestial Equator going by the Celestial Poles are the circles. These circles are the projections of the Meridians or Meridian circles on the celestial sphere (Meridians are the unreal lines making circles on the Earth’s face drawn at right angles to the Equator going by the North and South poles of the Earth). These projected Meridian circles on the celestial sphere are called Declination circles.
Seeing that the Earth ranges in the region of the Sun in its oval path just the once every year the Sun gradually becomes visible to shift from South of the Celestial Equator towards North in the Northern Hemisphere in the former part of the year (January through March). Since the Sun passes through the Celestial Equator on about March 21, indicating the starting of the Spring in the Northern Hemisphere (and the Fall in the Southern Hemisphere), the day and night happen to be equal in duration (12 hours each) throughout the world. This time the sun remains at the Vernal Equinox, the position where the Sun passes through Celestial Equator in the northward direction related to Tropical zodiac system.
For the duration of three months, the sun shifts to northward and goes up to its north–most position (Aaproximately23 ½ degrees north of the Celestial Equator) in the sky near about June 21. It is the beginning of summer in the Northern Hemisphere (and Winter in the Southern Hemisphere) having the optimum hours of day light on June 21.
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Following June 21, the Sun becomes visible to start its journey southward passing through the Celestial Equator just about September 23. The Sun’s passing point across the Celestial Equator as it shifts southward is called the Autumnal Equinox. The Autumnal Equinox points the starting of the Fall Season in the Northern Hemisphere (and Spring in the Southern Hemisphere), and on this day, the duration of the day and night turns to twelve hours each all around the world.
The Sun maintains its southward journey in the sky until it meets the south-most point on near about December 22. Currently, the night is the best ever in the Northern Hemisphere and it makes the starting of the Winter Season (As Summer starts in the Southern Hemisphere). Later going to south-most point, the Sun turns its path in the sky and continues its northward journey towards the Celestial Equator. As a result in the Northern Hemisphere, days turn out to be longer and nights shorter. Again on March 21 it passes the Vernal Equinox, the Sun’s northward crossing point of the Celestial Equator under Tropical zodiac system.
The Sun’s virtual path in the celestial area as mentioned above is called the Ecliptic. In different words, the Ecliptic is the projection of the Earth’s path in the region of the sun on the celestial area. Meeting points of the Celestial Equator and Ecliptic are termed as Equinoctial points. Hence, the Vernal Equinox takes place on near around March 21 and the Autumnal Equinox on September 23. This happens similar to the sun becomes visible at the Equinoctial points recently. The celestial longitude is the covered path considered in one way (from West to East in the sky and then back to West beneath the Earth in the region of the other side of the Earth) by the side of the Ecliptic from the Vernal Equinox, while the celestial latitude is the path covered and considered at the right angles to the Ecliptic.
Having viewed from the Earth, the passage by the side of the Moon and planets become visible to move in the sky happens to be in a narrow ten-degree celestial latitude zone on each side of the Ecliptic. Accordingly, the group of nearly twenty degrees latitude area in the region of the Ecliptic spreading in the East-West ways on the on the celestial sphere is called as the Zodiac. The zodiac is separated in 12 different parts of thirty degrees each side by side the Ecliptic and the beginning point or the reference of the zodiac happens to be the Vernal Equinox.
The important 12 zodiac signs come in touch with these 12 identical parts of the zodiac. The regular earthly positions used in astrology are determined with the help of Vernal Equinox as zero degrees or the reference point of Aries. No doubt, stars do move, their movement is so small with respect to the earthly motions; they seem to be almost “fixed” in the celestial area. Because of the slow change of orientation, the Earth’s polar bloc regularly shifts.
With the passage of time, it indicates to various pole stars and mentions a circle in space in about 26,000 years associated with Tropical zodiac system. As a result, the conditions of the equinoctial points compared to the “ fixed” stars in the celestial area slowly change by the side of the Ecliptic at an annual rate of about one degree in seventy-two years. Hence, the equinox comes to the same position on the Ecliptic in reference to a fixed star in the process about 26,000 years.
This entire process of cycle is called as the precession of equinox. The duration divided by twelve, gives the period of a precessional age. The present one is the Piscean. The movement of the equinoxes being retrograde, the other age will be the Aquarian.
Consequently, of the moving of the equinoxes forward, the reference point does not remain unchanged compared to a fixed star. It shifts along the Ecliptic. Simply put, at the zero degree of Aries the Sun does not point to the similar unchanged or fixed star. The way it did during the time of the spring equinox centuries and thousands years ago. Around 2,160 years earlier, the position of the zero degree of Aries on the Ecliptic will communicate to the position of zero degree of Taurus (the sign of the other zodiac) currently. Tropical or moving zodiac system is the system, which bases its reference point to the Vernal Equinox.
From the West’s point of view, the tropical or zodiac system has been quite popular and acknowledged. But the case is different in the East particularly in India. The properly accepted zodiac system or sidereal zodiac system has been in practice for a pretty longer period of time. So, the astrological features of a specific area or location in the sidereal zodiac system should always be the same. Since astrological characteristics are defined with respect to the zodiac’s particular direction in the celestial area having the fixed stars.
Though, for the unchanged or the fixed sidereal zodiac system zodiac system, the problem comes in establishing the reference point related to or on the zodiac. No doubt there are many reference points that probably might be in practice today, the top one is based on the star Spica. Basically, the star Spica denotes the 180-degree location of the unchanged sidereal zodiac system. The fixed zodiac or sidereal zodiac system reference point coincided with that of the moving zodiac or the tropical reference point about 1700 years earlier. In different words, at present, the zero degree of Aries of the sidereal zodiac system communicates to nearly 23.75 degrees of Aries of the tropical area.