Your nakshatra is whichever of twenty-seven segments of sky the Moon occupied when you were born. The calculator above finds it from your date, place and, if you have it, your time. Your chandra rashi, the moon sign, comes from the same position read against the twelve signs instead of the twenty-seven stars.
If you do not know your birth time, the page does something most calculators will not: it checks whether the Moon crossed a nakshatra boundary during your birth day and tells you. On most dates the answer is settled and the missing time costs you nothing. On some dates it decides everything.
What a nakshatra actually is
The ecliptic, the band of sky the Sun and Moon travel along, is cut into twenty-seven equal parts of 13°20′ each. Your janma nakshatra is the one holding the Moon at your birth moment.
Twenty-seven divisions rather than twelve is not an arbitrary refinement. The Moon returns to the same point against the stars in about 27.3 days, so one nakshatra is close to one day of lunar travel. The scheme is a way of naming which day of the Moon's circuit you were born into.
Each nakshatra divides again into four padas of 3°20′. A pada is roughly six hours of lunar motion, which is why it is far more sensitive to birth time than the nakshatra itself.
Each also carries a ruling planet, and those nine lords repeat in a fixed order: Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury, then round again. Ashwini is Ketu's, Bharani belongs to Venus, Krittika to the Sun, and the cycle continues through all twenty-seven. That ordering is not decorative. It is the sequence the Vimshottari dasha system runs on, so your birth nakshatra decides which planetary period your life begins in.
The birth-time problem, answered honestly
Here is the arithmetic that matters.
The Moon covers about 13.2° in a day. A nakshatra is 13°20′ wide. Those numbers are close enough that the Moon changes nakshatra roughly once every twenty-four hours.
A date alone gets your nakshatra right most of the time. It fails when the boundary happens to fall inside your birth day, and there is no way to know which case you are in without checking your specific date.
That is exactly what this page checks. It computes the Moon's position at the start and end of your birth day and compares them. If both fall in the same nakshatra, your birth time cannot change the answer and the page says so. If they differ, it names both and tells you the result assumes noon.
Padas are a different matter. A pada is a quarter of a nakshatra, so the Moon crosses one about every six hours. Even on a date where the nakshatra is settled, the pada usually is not. The page reports that separately rather than presenting a pada as though it were as reliable as the star.
Most calculators handle this by demanding a birth time they know you are guessing at, or by silently assuming noon and printing the result as fact. Neither tells you what you actually want to know, which is whether the answer is safe.
Moon sign, sun sign, and why yours may not match the newspaper
Your chandra rashi is the zodiac sign holding the Moon: the same longitude the nakshatra came from, read against twelve divisions of 30° instead of twenty-seven of 13°20′. In Indian astrology this is the sign that matters most, and when someone in India asks your rashi, this is usually what they mean.
Your surya rashi, the sidereal sun sign, will often disagree with the sign you know from a newspaper horoscope. This is not an error in either system. Western astrology uses the tropical zodiac, anchored to the equinox. Indian astrology uses the sidereal zodiac, anchored to the fixed stars. The equinox drifts against the stars by about 50 arcseconds a year, and over roughly two thousand years that drift has accumulated to about 24°, which is nearly a whole sign.
The practical result: if you are a Western Leo you are quite often a sidereal Karka, one sign back. Both are correctly computed within their own frame. They are answering different questions.
How this is computed
The Moon's position comes from astronomy-engine, whose lunar model is the Nautical Almanac Office's Improved Lunar Ephemeris of 1954, derived from E. W. Brown's lunar theory. That gives an apparent geocentric longitude referred to the true equinox of date.
Converting that to a sidereal longitude takes two subtractions. First the nutation in longitude, which brings the frame to the mean equinox of date. Then the ayanamsa. The default here is Lahiri, the Government of India standard fixed by the Calendar Reform Committee, checked against Swiss Ephemeris at eight epochs from 1900 to 2050 and agreeing within 0.05 arcseconds.
Positions are geocentric, not topocentric. This matters more than it sounds. The Moon's parallax reaches about a degree, so computing a topocentric position would shift nakshatra boundaries by hours and disagree with every published panchang, all of which use geocentric positions.
Time zones are resolved for your birth date rather than assumed. India used Madras Mean Time until 1906 and wartime daylight saving between 1942 and 1945. Bombay kept its own local time at UTC+04:51 until 1955, and Calcutta kept UTC+05:53:20 until 1948, neither of which the standard time-zone database can express. Those are handled explicitly. A birth-chart site applying a flat +05:30 to a 1950 Bombay birth is thirty-nine minutes out.
Why another site may give you a different nakshatra
Unlike tithi, which is immune to this, the nakshatra depends directly on the ayanamsa. It is read from an absolute longitude, so shifting the zero point shifts the answer.
Lahiri and Raman sit about 1.45° apart. A nakshatra is 13°20′ wide, so that gap is roughly a tenth of a nakshatra. About one birth in ten therefore lands on a different star depending on which system the site uses, and neither site is malfunctioning.
You can test your own case in seconds. Change the ayanamsa selector above from Lahiri to Raman. If your nakshatra stays put, you are comfortably inside it and the disagreement never affects you. If it jumps, you were born near a boundary, and that is worth knowing before you take any single site's answer as settled.
Three other reasons a result can differ:
A different birth time. The most common cause by far, and usually a family disagreement rather than a computational one.
A different place. The position is geocentric, so the place does not change the Moon's longitude at a given instant. What it changes is which instant your stated local time corresponds to, and that is enough to matter near a boundary.
True versus mean lunar node, which affects Rahu and Ketu rather than the nakshatra, but shows up on sites that report both.
The twenty-seven, and their ruling planets
The lords repeat in a nine-step cycle, so every ninth nakshatra shares a ruler. Reading them in order, with the lord in brackets:
Ashwini (Ketu), Bharani (Venus), Krittika (Sun), Rohini (Moon), Mrigashira (Mars), Ardra (Rahu), Punarvasu (Jupiter), Pushya (Saturn), Ashlesha (Mercury).
Then the cycle restarts: Magha (Ketu), Purva Phalguni (Venus), Uttara Phalguni (Sun), Hasta (Moon), Chitra (Mars), Swati (Rahu), Vishakha (Jupiter), Anuradha (Saturn), Jyeshtha (Mercury).
And a third time: Mula (Ketu), Purva Ashadha (Venus), Uttara Ashadha (Sun), Shravana (Moon), Dhanishtha (Mars), Shatabhisha (Rahu), Purva Bhadrapada (Jupiter), Uttara Bhadrapada (Saturn), Revati (Mercury).
Several are named for what is actually there. Krittika is the Pleiades, the cluster most people can find without help. Rohini is Aldebaran, the red eye of Taurus. Chitra is Spica, and it carries more weight than the others because the standard Indian ayanamsa is defined by placing Chitra at exactly 180° sidereal longitude. Jyeshtha is Antares. Mula sits toward the galactic centre.
Why twenty-seven, when there is a twenty-eighth
Older lists give twenty-eight. The extra one is Abhijit, Vega, inserted between Uttara Ashadha and Shravana.
Twenty-seven divides 360° evenly into segments of 13°20′; twenty-eight does not. The tradition kept the arithmetic and dropped Abhijit from the standard scheme, though it survives in muhurta as an auspicious window straddling midday, which is where the name is met most often now.
The Vedic texts themselves are not unanimous about which end the list starts from. Several early enumerations, including the one in the Taittiriya Samhita, begin at Krittika rather than Ashwini. The shift to an Ashwini-first list came later, and it matters only for how the sequence is recited, not for where the Moon is.
That nakshatra lists appear in the Samhita literature at all is worth noting. This is one of the older observational schemes to survive anywhere, and it survived because it was memorised inside a ritual calendar rather than written down and left to rot.
Pada, and the syllables
Each nakshatra splits into four padas of 3°20′, and each pada carries an assigned syllable used in the traditional naming ceremony. Ashwini's four are Chu, Che, Cho and La; Bharani's are Li, Lu, Le and Lo, and so on around the circle, so that a child's name begins with the sound belonging to the quarter of sky the Moon stood in.
Two consequences follow, and they pull against each other. The naming convention is one of the more concrete pieces of practice attached to the nakshatra system, which is why the pada matters at all outside a chart. But a pada is six hours wide, so a name chosen from a guessed birth time is a name chosen from a guess. If the syllable is the reason you are here, it is worth finding the time before deciding.
The padas also map onto the navamsa, the ninth divisional chart, which is why the same 3°20′ unit shows up in chart work that has nothing to do with names.
What this page will not do
It will not tell you what your nakshatra means for your character, your marriage, your career or your health.
The computation here is astronomy: this is where the Moon was, and this is the segment of sky it occupied. What a tradition builds on top of that is a separate matter, and much of what is sold on that foundation is sold by people with something to sell.
The one genuinely practical use, and the reason the ruling planet is shown, is that the birth nakshatra determines the starting point of the Vimshottari dasha cycle. That is arithmetic rather than interpretation, and it is worth having correct.
Using the result
If you are looking up a naming syllable, you need the pada as well as the nakshatra, so a birth time is close to essential. Each pada carries its own assigned syllable.
If you are checking compatibility, most systems use the nakshatra and moon sign rather than the pada, so a date is often enough.
If you are having a chart cast, take the exact time. Everything downstream of the ascendant depends on it, and the ascendant moves through a whole sign every two hours.