Astronomy as Relevant to Astrology
The Basic Concepts
The Branches of Vedic Astrology
Jyotish is considered to be one of the Vedangas (part of Vedas) propounded by lord Brahma by the scientific study of which human beings can accomplish virtue. Jyotish shastra or the science of Vedic astrology, is a compilation of 4,00,000 verses (vide Narada Purana, II.50.2). Vedic astrology has mainly three branches – Siddhanta (the principle), Jataka or Hora (astrology for individuals) and Samhita (astrology for masses).
The Geocentric System
It is a human tendency to refer to other things in relation to oneself. Sitting in a moving train, we see things passing by the train – trees, farms, hutments, etc. A common question arises in our mind – which is the station coming next? At the back of our mind we do know that it is not the station which is going to come, it is the train which will reach the next station. Similarly we refer to the rising and setting of the Sun. But we do know that it is not the Sun which is rising or setting, it is the spin of the earth which makes it appear so.
Because we feel stationary on the solid earth, the sky seems to spin around us in complicated ways. In our quest to understand what we see, our ancients had evolved a most innovative and powerful tool.
As nothing is stationary in the universe, whether it is a satellite or a planet or even a star, it is convenient to imagine our position in the universe – the earth – as its centre and the whole of the universe moving around us in constant motion. Thus considering the relative positions and movements of all heavenly bodies with respect to the earth is the Geo-centric system. On the other hand, when we consider the relative position of planets (including the earth) in respect of the Sun, it forms the basis of the Helio-centric system. Vedic astronomy and astrology are essentially geo-centric in their concept.
The earth is spherical and rotates from west to east around its axis. The axis of the earth is an imaginary line which, passing through its centre, connects its two poles, the north pole and the south pole. Another imaginary line running across the largest circumference of the earth, equidistant from its poles and running in an east-west direction, is called the equator.
The Celestial Sphere
Think of the sky as a great, hollow, crystalline sphere surrounding the earth. Imagine the stars to be attached to the inside of the sphere like thumbnails stuck in the ceiling. The sphere takes one day to rotate, carrying the Sun, the Moon, the planets and the stars from east to west. We know that the sky is not a great, hollow, crystalline sphere. The stars are scattered through space at different distances, and it isn't the sky that rotates once a day. It is rather the earth that rotates once in a day around its axis. It is convenient as a model of the sky. This model of the sky, the Celestial sphere, is an imaginary hollow sphere of very large radius (infinity) surrounding the earth and to which the stars seem to be attached. On this imaginary sphere the celestial equator, the celestial poles, and other reference points are marked as they are done on the earth; these represent the extensions of the equator and the poles, etc., of the earth into infinity.
The earth takes one year to complete its rotation around the Sun. From the earth, it appears that the Sun moves around the earth. This apparent path of the Sun is known as ecliptic. An imaginary belt of 18 degrees width with ecliptic in its centre is known as the zodiac. Many groups of stars appear to have been studded on this imaginary belt. Vedic astrology recognizes 27 such groups of stars called nakshatras.
The zodiac encircles the earth like a circle consisting of 360 degrees. If this circle is divided into 27 equal parts, each part will be of 13 degrees and 20 minutes arc, known as a nakshatra. Each nakshatra is further divided into 4 quarters (padas or charanas), of 3 degrees and 20 minutes arc each.
Twelve divisions of the zodiac will have an arc of 30 degrees each, known as rashis (or signs).
The above figure shows rising of the Sun in the eastern horizon. The line passing through the centre of the Sun is the ecliptic, the apparent path of the Sun created by its 'revolution' around the earth during its annual journey. The group of stars, referred to as the nakshatras, are the fixed reference points in the zodiac used to locate the position of the Sun, the Moon and other heavenly bodies. All the planets considered in Vedic astrology for the purpose of interpretation, do not decline beyond the belt of the zodiac. They may be on the ecliptic or towards the north or sourth of the ecliptic depending on their latitude with reference to the ecliptic.
For example, the orbit of the Moon is inclined at an angle of 5 degrees to the ecliptic. The Moon does not go beyond 5 degrees on either side of the ecliptic. The orbit of the Moon cuts the ecliptic at two point. In its orbit, when the Moon is on the ecliptic while moving from south of ecliptic to north, this point is known as Rahu or the ascending node of the Moon and when the Moon is on the ecliptic while moving from north of ecliptic to south of ecliptic, this point of intersection is known as Ketu or the descending node of the Moon.
The point of sunrise with respect to the observer keeps changing during the year. If A is the point of sunrise when the Sun is at vernal equinox (around March 21 every year), the point of sunrise will appear to move northwards till it reaches the summer solstice (B) on or around June 21. from this point it will start its southernly journey (Dakshinayana) during which it reaches the autumnal equinox (again A) around September 23 and further until it reaches winter solstice (C) around December 22. At this stage it starts its northward journey (Uttarayana).
The most crucial point in the division of a circle is to know the starting point of the circle. The point where the ecliptic cuts the celestial equator is known as equinox. There are two such equinoxes – the vernal equinox and the autumnal equinox. When the Sun is passing from the southern hemisphere to the northern hemisphere, it cuts the equator at vernal equinox. When the division of the circle of the zodiac is with reference to vernal equinox as its starting point, the zodiac is referred to as the Sayana (or tropical) zodiac, the divisions of this zodiac into twelve equal parts are the Sayana rashis, and the positions of planets in this zodiac represent the Sayana longitudes of the planets.
The Precession of Equinoxes
If we could watch the sky for a few hundred years, we would discover that the north celestial pole is moving slowly with respect to Dhruva (Polaris) star. The celestial poles and the celestial equator, supposed to be the fixed reference marks, are moving very slowly because of the slow change in the direction of Earth's axis of rotation. This slow top-like motion is called precession. Earth's axis sweeps around in a cone, taking almost 26,000 years for each sweep.
Precession is caused by the gravitational pull of the Sun and the Moon. Because earth is not a perfect sphere – it has a slight bulge around its equator – Sun and Moon pull on it, trying to make it spin upright in its orbit. This forces earth's axis to precess.
The result of this precession is that vernal equinox, the cutting point of the ecliptic and the celestial equator, drifts westward on the ecliptic by an approximate angle of 51 seconds of an arc each year. So we have a new vernal equinox every year and hence a new staring point of the Sayana zodiac. This results in the shifting of the Sayana signs.
The Vedic system does not depend on this shifting zodiac and relies on a fixed point on the zodiac as its starting point. There is no clear cut demarcation of this starting point in the zodiac. Some consider this point to be 180 degrees opposite to the Chitra nakshatra. Some consider it to be slightly to the east of the Revati nakshatra, while still others opine differently.
When the division of the circle of the zodiac is with reference to the Vedic starting point, the zodiac is referred to as the Nirayana (or Sidereal) zodiac, the twelve equal parts are the Nirayana rashis, and the positions of planets in this zodiac represent the Nirayana longitudes of the planets.
The angular difference between the vernal equinox and the Vedic starting point of the zodiac is known as the Ayanamsha. When the Vedic starting point is with reference to Chitra nakshatra, the Ayanamsha is refered to as the Chitrapaksha Ayanamsha. According to this system the first point of Sayana zodiac and Nirayana zodiac coincided in the year 285 A.D. The corresponding value of this Ayanamsha on January 1997 is 23°48'56".
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