Our learned ancient Astrologers first classified the effects of signs on the life of the native and classified the zodiac in 12 parts consisting of 30 each. They further divided the zodiac into 27 parts each consisting of 1 3°20 (360/27) and called it Nakshatra or constellation. The difference between Rashi and Nakshatra is that a Rashi is of 30 degrees duration each whereas a Nakshatra consists of 13 degrees 20 minutes each.
It means Nakshatra is only a part of the Rashi. Each Rashi is owned by a planet. The nature and effects of signs or Rashis are expressed in accordance with the nature and effects of planets owning these Rashis. Similarly, constellations were also allotted their lords in accordance with their nature and effects. It means that a particular constellation has the characteristics of its lord (planet).
Each constellation (Nakshatra) was named after the name of the star nearer to that constellation/Nakshatra (point of division of zodiac in 27 parts).
Each constellation consists of 13 degrees 20 minutes of arc and each one has been further divided into four quarters (Pada or Charan) of 03 degrees 20 minutes each.
Thus there are 108 quarters of 3 degrees and 20 minutes each (27*4) of all the 27 Nakshatras and group of 9 quarters have been allotted to each sign (108/12).
Hence, 2.25 Nakshatra comprises a Rashi.
The Rashi and Nakshatra are both reckoned from the same point i.e. the zero degree of Aries which is the first sign of the zodiac and the first constellation Ashwini also starts from this zero degree.
The shadowy planets, Rahu and Ketu, also have been given ownership of some constellations (The twenty seven constellations and their lords are given below)
Out of 3O degrees of a sign, 13 degrees and 20 minutes of a sign is owned by one Nakshatra and next 13 degrees and 20 minutes is owned by next Nakshatra in cyclic order as given in the table in next page and the left over balance of 3°20 is Owned by following Nakshatra.
The other three quarters of this Nakshatra will go to the next Rashi and so the cycle goes on.