If 10 human beings are born , by normal distribution , at least one person would be of inquisitive nature , whom we may call as scientist. They happen in each and every period and in every culture . Such persons generally sleep little as they are thinking all the time . Imagine such person being awake under open sky . Now , that can happen in warm countries of Asia and Africa and not in Europe so astrology started in tropical countries . Such people being awake , facing sky in the night would see clear stars . And entire astronomy and astrology owe their existence to such insomniacs because this study can happen in night only .
Next thing which will attract anyone in night would be Moon . A poet would write a poem looking Moon and stars . A spiritual person would consider them gods and so on . But a scientific temperament person would observe that Moon follows a fixed path in sky . One would find that Sun starts at same horizon point and ends at same horizon point every day . And Moon does the same on New Moon and Full Moon day .
That gave rise to knowledge of a circular highway in space , within which Sun and Moon travel . So one did not need to focus everywhere in sky . The stargazer focussed his time and energy to this Space highway of say 16 degree width round the sky . That is called Zodiac Belt . It is a geocentric view , earth being at centre as observer is on Earth .
Within this Space highway , if one continuously observed , then it will be found that there are lumps of distant stars at regular distance in zodiac belt like cities on highways. These sign posts were named as constellation or as zodiac signs for identifying them like cities , as they shifted every month because of Earth’s revolution around Sun and came back to original position after a year .
Next observation was that some heavenly bodies shifted their place regularly within zodiac road , while distant stars bunch called constellation or zodiac sign did not make any change in their shape . So, moving bodies got more attention , which were called as planets . These too came back to their original place after some period .
Therefore , astronomers identified fixed distant stars bunch as background for the moving planets just like there are hands in clock against its dial background with 1 to 12 numbers .
After long observations , astronomers found that there are not many planets which move within zodiac belt. Apart from Sun and Moon , they found only Mercury , Venus , Mars , Jupiter and Saturn followed the 16 degree wide zodiac road like a 7 lane express high way . That made 7 planets from Sun to Saturn on which weekdays are named . They follow a fixed 16 degree path of zodiac and have different speed of revolution ( from 1 month to 30 years ) and are visible to naked eye .
Indian astronomers divided zodiac circular, clock like road into 27 equal parts based on Moon’s stay for a day under every star bunch ( Nakshatra or constellation) while Babylonians preferred 12 divisions ( Zodiac Signs) in which Sun stays for a Month .
For centuries , only 7 planets were used . Astronomers tried to co-relate planets motion in different signs / constellation and their effect on Earthy events like war , epidemics , crops yield , flood , earth quake , storms , rise and fall of empires, birth and death of humans etc and thus developed an applied science of Astrology . Babylonians and Indians were forerunners of this .
Long after , a need was felt , because of eclipse phenomenon happening at least 2 times in a year , that something is missing in 7 planet predictive model . They visualised that there must be something in sky that casts shadows on Sun and Moon during eclipse . And then they came up with theory of Rahu and Ketu or dragon’s head and tail which are shadows of Moon and Earth . Rahu and Ketu are nothing but a virtual way of including Earth in scheme of astrological model . And that made total 9 planets of astrology . Yash Shukla’s answer to Vedic Astrology: Why in astrology charts , Earth is not considered as planet and also not worshipped in Navgrah scheme ?
So, Rahu and Ketu are like decimal and zero added to number system later on for creating perfect model and that works accurately in astrology .
Later day astrologers added the features of planets and signs based on persons characteristics and visible characteristics of planets and thus whole astrology developed .
Mind it that Indian astrology does not consider Uranus , Neptune and Pluto as part of standard 9 navgraha , because their path of revolution falls out of zodiac . It is just like vehicles moving parallel but below express highway , which cannot affect vehicles moving on zodiac highway . Also, it is based on Geocentric model , where Earth is centre because we need to assess sum of effects on Earthy life and not on Sun .
That is the theory side of Navgraha but practice side has added lot of commercial crap like color , images , idols , stories etc to these Navgraha , which has no proof , though mathematical base does have its applications .
( This answer first written by Quora in 2015 and republished here on 29 June 2020)