The 27 nakshatras
13°20′ each, from 0° Aries

One of twenty-seven equal divisions of the ecliptic, each 13°20′ wide, starting at 0° Aries. Your nakshatra is the one the Moon occupied at your birth. It is the basis of Vimshottari dasha and of most of kundli matching.

From the Moon’s sidereal longitude at your birth, which needs date, time and place. The Moon takes a little over a day to cross a nakshatra and about six hours to cross a pada, so an approximate birth time can land in the wrong nakshatra and will more often land in the wrong pada — nearer a boundary, sooner.

Because twenty-seven arcs of 13°20′ do not line up with twelve of 30°. 9 nakshatras start in one rashi and finish in the next, so a Krittika birth can be Aries or Taurus depending on which pada the Moon was in.

A quarter of a nakshatra, 3°20′ wide, so 108 across the zodiac. Each pada maps to a navamsa sign, which is what the D9 chart is built from — and it is the part an inexact birth time gets wrong first.