Astrology: A rational introduction

Astrology can be defined as “Logical interpretation of movements and relative positions of certain astronomical bodies with a belief that they have an influence on human affairs.”

When it comes to Astrology, entire population on earth fall into two groups; the first group is the ones who do not believe in astrology, while the second group is those who believe in astrology with a question, “People have been believing it for several millennia. How can they all be wrong?”.

True. There are evidence pointing out that kings had referred to astrology to predict the outcome of a war or natural calamities such as flood and drought. Astro bodies at certain positions were considered as a positive omen to start a public project such as building bridges, dams or temples and sometimes, even for the journey of a pilgrimage.

Does the position of certain astronomical bodies such as sun, moon, and planets really have an effect in predicting human behavior? Maybe, maybe not. But that is not what I'm going to write about in this blog post. Rather, I am going to focus on understanding the rationale behind astrology, scientific arguments which have made many generations believe in this field.

Let me first introduce you to the framework of a horoscope. The following picture shows a typical Tamil horoscope (also known as Jathagam). The building blocks of such a horoscope are the twelve boxes, popularly known as houses.

Typical horoscope showing the position of major planets

Before explaining the significance of twelve, let us first look into one of the most profound mathematical concept; 360 degrees. Many things that happen around us can be explained using 360 degrees. For example, earth completes 360 degrees when it revolves around itself once. It completes 360 degrees when it completes one full rotation (orbit) around the sun. Similar to Earth, other planets also complete 360 degrees for one full revolution about itself or one full orbit around the sun. Moon completes 360 degrees to orbit around the earth.

How can we perceive this 360 degrees movement of various astronomical bodies based on observations from a naked eye? There are several ways to do it. One simple way is to divide 360 degrees into 360 1-degree sectors and keep track of the planets when the move from one sector to another. However, certain planets such as Jupiter are farther away from the sun that it is extremely difficult to keep track of their 1-degree movement using naked eye.

We need to divide 360 into larger sectors to make the objects traceable. How large? Let us say, 4 sectors of 90 degrees each? The problem with such a fewer number of segments is that it becomes impossible to track the faster movement of planets that are closer to the sun, such as Mercury. So just like one would conduct a grid independence study, based on the experience people have found that having 12 sectors of 30 degrees each will make this problem tractable. But, on what rational basis did they arrive at twelve?

A long time ago, when there were no telescopes and technological gadgets around, people relied on naked eye observations. With no city lights to pollute, clear sky nights were absolutely dark that they were able to see far more stellar objects that are possible to see today only with the help of powerful telescopes.

One disadvantage of such a dark and starry sky is that it is extremely difficult to keep track of the movement of one particular object over a period of time. Solution? They started looking at the star's background to find its relative position with respect to other stars in its vicinity. In order to remember the position of various stars in the surrounding of the planet under observation, they identified some of the bright stars which do not move (on a relative scale) with respect to the planet. To make the problem easier, they grouped some of those bright stars to form a shape that is easy to remember. In other words, they formed a constellation.

They identified twelve major constellations with the help of twenty seven brightest and easily recognizable stars. The twelve constellations were roughly 30 degrees apart. So the problem of tracing a planet is now reduced to identifying in what background (constellation) does the planet lies at a specified time.

We would love to give names to anything right? Of course, we give names not only to the newborn but also to pets and things around. So it's not a wonder that people named those constellations (houses) based on their patterns such as Aries (Mesha), Taurus (Vrishabha) etc.

Now, all that one needs to do is to find in which house (constellation) does each of the observable planets reside at the time of their birth. In the typical example shown in the picture above, one can see that when viewed from the Earth (that's the reason why you won't find Earth in any of the houses), Mercury would set before sun while Venus sets right after sunset, followed by the Moon. When the sun and moon are in the same house, it is no moon day (or Amavasya) and when the sun and the moon are 180 degrees apart (or 6 houses apart), it is full moon day (or Purnima).

In terms of time, Earth takes 24 hours to complete one full revolution around itself (360 degrees). In other words, it takes 2 hours to rotate by 30 degrees. Since sunrise and sunset is defined with respect to Earth's rotation, the setting time of Mercury (Venus) would not be more than 2 hours before (after) sunset. However, using the same chart, it is possible to fix our frame of reference for time on some other planet as well.

For example, Jupiter takes 11.86 Earth years (4332 Earth days) to complete one orbit around the Sun (360 degrees). Hence, in order that Jupiter moves its position from one house to another (Transit of Jupiter), it will take 361 days (~ 1 Earth year).

Now the main question still remains. Does the location of planets in an arbitrary background at the time of one's birth have any significance in predicting their future? I will leave it up to the readers to find an answer to this question!