How nakshatra boundaries are fixed
Nakshatra boundaries are fixed at multiples of 13 degrees 20 minutes of the sidereal Moon longitude. The end time of a nakshatra is the moment the Moon reaches the next boundary.
What it is
The method is the same as for the tithi: the Moon's sidereal longitude is evaluated, and the search for the boundary is iterated to a precise moment. The result depends on the ayanamsa and the Moon model.
How it is worked out or read
Calculators can differ on a nakshatra end time by a few minutes if they use different ayanamsas or Moon models. The difference is usually small, and it matters only when a birth or event falls at the boundary.
Where it appears on this site
The nakshatra end times are in the panchang. The nakshatra and day timing article explains how the times are read.
Equal divisions
The standard modern method divides the sidereal zodiac into twenty-seven equal nakshatras of 13 degrees 20 minutes each, starting from 0 degrees of sidereal Mesha. Ashwini runs from 0 to 13 degrees 20 minutes, Bharani from there to 26 degrees 40 minutes, Krittika from there to 40 degrees, and so on to Revati, which ends at 360 degrees. This method is used by almost all modern panchangs and software, including this site, and it is what makes the Vimshottari dasha arithmetic so regular.
Older unequal systems
Some older texts, including Brahmagupta's, describe a system of unequal nakshatras, in which some span one and a half times the standard length, some half, and the rest the standard length. These lengths reflect older ideas about the Moon's daily motion and the stars that marked each mansion. The unequal system survives mainly in certain muhurta rules and in the treatment of Abhijit. For chart work and the daily panchang, the equal system is now standard.
Abhijit
Abhijit is sometimes counted as a twenty-eighth nakshatra. In the equal system it is carved out of the end of Uttara Ashadha and the beginning of Shravana, usually from 276 degrees 40 minutes to about 280 degrees 53 minutes of the sidereal zodiac. It is used in some muhurta rules and in certain chakras, but not in the Vimshottari dasha or in the naming syllables, which use the standard twenty-seven.
Boundaries depend on the ayanamsa
Because the nakshatras are measured in the sidereal zodiac, their boundaries in the sky depend on the ayanamsa. Changing from Lahiri to Raman, for example, shifts every sidereal longitude by more than a degree, which can move a planet near a boundary into the neighbouring nakshatra. For the Moon this changes the birth nakshatra, the dasha sequence and the naming syllable. A chart with the Moon within a degree or so of a nakshatra boundary should be checked under the ayanamsa the family follows.
Near the boundary
The Moon moves about half a minute of arc per minute of time, so it crosses a boundary quickly in human terms but slowly enough that a birth time error of half an hour can matter. When the Moon is close to a boundary, the reader should state the uncertainty and, if necessary, describe both nakshatras rather than choosing one silently.