How the 27 Nakshatras Map onto the 12 Signs
Both systems divide the same circle. One cuts it into twelve, the other into twenty-seven — and 27 does not divide evenly into 12, which is where the confusion starts.
Two and a quarter nakshatras to a sign
The ecliptic is 360°. Twelve signs make 30° each. Twenty-seven nakshatras make 13°20' each. Divide 30 by 13°20' and you get 2.25 — two and a quarter nakshatras fit inside one sign.
That quarter is the whole problem. If the number came out even, each sign would hold a clean set of nakshatras and the two systems would line up. Instead the boundaries slide, and nine of the twenty-seven end up straddling two signs.
Quarter it and the arithmetic cleans up
Each nakshatra divides into four padas of 3°20'. Now a sign holds exactly nine padas, and the whole zodiac holds 108 — a number that recurs throughout the tradition.
This is why a Vedic chart reports the Moon down to its pada rather than stopping at the nakshatra. The pada is where the two systems finally agree: it belongs cleanly to one sign, with no straddling.
108 padas, 27 nakshatras, 12 signs, 9 planets. The numbers interlock, which is part of why this structure survived.
Which nine straddle a boundary
Krittika, Mrigashira, Punarvasu, Uttara Phalguni, Chitra, Vishakha, Uttara Ashadha, Purva Bhadrapada and Uttara Bhadrapada each begin in one sign and end in the next. Krittika, for example, starts in the last degrees of Aries and runs into the beginning of Taurus.
If your Moon falls in one of these, the nakshatra alone does not tell you the sign — you need the pada.
What this changes when reading a chart
The nakshatra and the sign say different things. The sign is the broader temperament; the nakshatra is finer, and it also decides which planet your dasha sequence begins from. Two people with the Moon in the same sign but different nakshatras will run entirely different timing.
So when a chart gives you both, they are not redundant. They are two resolutions of the same position.
Which one to remember
If you keep one thing, keep the nakshatra. The sign follows from it in most cases, and where it does not, the pada resolves it. The reverse does not hold: knowing the sign narrows the nakshatra to two or three candidates, never one.