Astronomy as Relevant to Astrology
The Basic Concepts
Rajeev Jhanji
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
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.
Zodiac
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).
Tropical Zodiac
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.
Sidereal Zodiac
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".
<<Back to Article Page>>