Naisargika Bala or Inherent Strength of the planets is one of the strengths in Shadbala of Grahas. This strength is fixed and holds well in all nativities. The strength of Planet in the Birth always leaves impact on the Personality of the native. Thus the question arises when it is constant for all horoscopes, why it should be included? The predictive results have also been given in classics; they shall also be the same for all nativities.
Dr. B.V. Raman pioneer of modern Indian Astrology has given in his book Graha and Bhava Balas that the value assigned to each planet depends upon its luminosity.
Now we shall see the luminosity of the planets as per modern astronomical facts. Before proceeding to it, let us see how the luminosity varies from time to time depending upon their distance from the earth.
Phases of planes:
Phases of inner planets are like moon. In figure above the inner planet appears full when it is a H. It appears more than half at D, half at C, less than half at B and not visible at A. The reason is that at H it is on the other side of the sun and the whole disc reflects the light of the sun and becomes visible while at A the dark side (which is not receiving light of the sun) is towards the earth. Similarly at B, C and D only the portion which is receiving light of the sun is visible.
Phases of superior planets:
The superior planets must always appear either full or gibbous; for its orbit is outside the orbit of the earth. It appears full at conjunction or opposition. Its illuminated disc is always more than half towards the earth. The smallest portion of the planet will be seen when the sun and the planet subtend a right angle at the earth.
Brightness (Magnitude) of the planets:
The amount of reflected light received from a planet at the earth is the brightness of the planet/star. This reflected light is based on two things:-
(a) If the distance of a planet from the earth is more, the light received from it will be less and the planet becomes brighter as it reaches near the earth. Brightness of light is inversely proportional to the square of its distance from the earth. If a source of light goes three times away than its earlier distance, the magnitude of light will be one-ninth of the original one.
(b) It depends also on the portion of the illuminated disc of the planet towards the earth. If more illuminated portion of the disc is towards earth, more light will be received on the earth.
Magnitude of inner planets:
We consider the case of Venus. When it is at superior conjunction, it is at maximum distance from the earth and at inferior conjunction it is at the least distance. Diameter of its disc is 11’’ at superior conjunction and 66’’ at inferior conjunction or 6 times more than superior conjunction. At the time of inferior conjunction, a very small part of its illuminated disc is towards the earth or visible to the observer, while at superior conjunction the whole disc is normally visible. When elongation angle of Venus (the angle subtended by the sun and the Venus at the earth) is 40o the brightness is at maximum.
Magnitude of outer planets:
An outer planet is at the time of opposition appears like a full moon. The whole disc is visible and it is nearest to the earth. While at conjunction, through the whole disc is normally visible but it is at the maximum distance from the earth its disc appears smaller also. In between these two positions, at quadrature (the angle substended by the planet and the sun at the earth is 90o) it appears most gibbous.
Maximum magnitudes of planets are as under:
The greater is the magnitude, lesser is the brightness. Planets with minus magnitude are brighter than the positive ones.
In case we take Saturn as 1, Moon 42.3, Mars 9.3, Mercury 6.3, Jupiter 8.7 and Venus 14.7 times brighter than Saturn. But this brightness varies as per their distance from the earth. Take the case of Mars which look like a star at the time it is farthest from the earth and similarly for Jupiter and Saturn. Regarding inner planets Mercury and Venus it has been discussed earlier.
It is concluded that their luminosity varies. Hence the natural strength needs to be removed from the Shadbala of Grahas or another formula depending on their angle of elongation and distance may be found out.
There are many things in the older Astrology that require changes according to the modern knowledge of Astronomy for which a team of Science and Mathematics experts who are also well versed in astrology is required.