Tropical versus sidereal
This site uses the tropical zodiac. Sidereal systems subtract an ayanamsa from the tropical longitude, and there is more than one ayanamsa convention. A difference between systems does not mean the Moon physically moved. Compare like with like.
Timezone and daylight-saving rules
The same wall-clock time corresponds to different UTC instants in different places. Historical daylight-saving rules are easy to overlook. Check the IANA timezone and the actual offset for that date, especially before 1970. If you have documentary evidence, use the advanced UTC-offset override.
A date is sometimes not enough
The Moon may change signs during your birth date. Some tools silently use noon when a time is missing. Our unknown-time mode shows both candidates and the local transition instead. A time rounded to the nearest quarter-hour can also cross a boundary.
Small numerical differences
Astronomy engines and coordinate conventions can differ slightly. A difference of a few arcminutes is rarely relevant far from a sign boundary, but can matter within minutes of a transition. Keep the original birth record and the UTC conversion with any comparison.
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