Astrology has 108 lunar mansions. That is 27 constellations multiplied by 4. Each asterism or Nakshatra is divided into 4 quarters. Nine such quarters tenant a Zodiacal Sign. Beta Arietis ( Aswini ) 1,2,3 and 4, 41 Arietis, 1,2,3 and 4 and the first quarter of Peiades ( Karthika ) tenant the constellation of Aries.
The quarters of Nakshatras are then the lunar mansion and the Sidereal Zodiac has 108 lunar mansions.
The book “Nakshatras, Lunar Mansions of Vedic Astrology” is a great book by Dr Dennis Harness and Dr David Frawley. It deals with the Sidereal Zodiac and contains a wealth of information about Vedic Astrology. A must for the novitiate.
Both are great scholars of Astrology and can enlighten the students about the great stellar science which is Vedic Astrology. Once the concept of Lunar Mansions is understood, the student can easily grasp the structure of the Sidereal Zodiac, upon which Vedic Astrology is based.