What is Rashi in Jyotisha or Hindu astrology?
Is it the same as sun-sign of a person?
Is Rashi determined by the birth details of a person or by the name they have?
Rashi in Jyotisha means in which constellation among the 12 zodiac sign is a planet positioned. For eg. If Sun is positioned in Mithun then the Surya Rashi is Mithun. Similar is the case with all planets. But the term Rashi especially refers to the constellation in which Moon is positioned at the time of birth.
Surya Rashi is the same with the Sun-sign of a person but it is not same for which we generally call Rashi because actually we are meaning to Chandra Rashi.
Surya Rashi is very easy to find. 12 months of a year corresponds with 12 Rashi respectively. Sun travels in each Rashi for a month. The months Baisakh, Jestha, Ashada, Shrawan, Bhadra, ....Chaitra corresponds with the Rashi as Mesha, Vrisha, Mithuna, Karkata, Simha,... Meen respectively in the order.
So, for eg if a person is born on Baisakh then his Surya Rashi is Mesha.
This is the same reason why 1st of Magha is called Makara Sankranti and 1st of Shrawan is called Karkat Sakranti.It is because Magha is 10th Month and Makara is 10th Rashi. Sun enters Makara/Capricorn in 1st of Magha. Similar is the case with Karkat Sankranti.
Rashi is determined by the birth detail of person. From birth detail, we find at which degree moon is situated. From the degree of the moon, we find at which constellation it is situated. This constellation is the Rashi of a person. From constellation, we find at which nakshatra is the moon situated. From Nakshatra, we find in which pada of that nakshrata is the moon situated. Then we name the person on the basis of Pada of Nakshtra it is occupying.
There are 27 nakshtra each divided into 4 padas and a unique letter is ascribed to represent it Thus there are total 27×4=108 unique letters to represent it.
For eg. Padas of Ashwini nakshtra is represented by letters 'chu', 'che', 'cho', 'la'.
If by calculation it is found that Moon is situated in second pada of Ashwini nakshtra, Then that person is named from 'che'such as Chekitan, Chetan, Chetnath, etc..
Thus, if a person is named by such methods then we can know his Rashi easily just by his name.
For eg. If someone says his name is "Chudamani" then from "Chu" we can know it is 1st pada of Ashwini, and as Ashwini falls in Mesha Rashi we can easily know his Rashi as Mesha.
But for this he/she should be named first based on position of the moon on padas of nakshatras. We cannot apply for any other names.