A Moon sign is the zodiac sign containing the Moon’s calculated position at a stated time. It is not the same as the Moon’s illuminated phase.
Because the Moon moves noticeably against the zodiac, date-only tables can be ambiguous on a day when it crosses a sign boundary. A reliable result uses a time, timezone, coordinate convention, and declared zodiac. If the birth time is unknown, a calculator can test the possible daily range and report one sign only when the result is unambiguous.
Astrological traditions associate the Moon sign with habits, familiar needs, and emotional rhythm. That is a reflective vocabulary, not a scientific measurement of temperament.
The Moon sign is one of the “Big Three,” alongside the Sun sign and Ascendant. The Sun, Moon, and Rising guide compares their calculation needs and symbolic roles.
Sources and further reading
The project methodology and official NASA/JPL Horizons documentation cover the calculated-position and independent-check layers. They do not establish the Moon sign’s traditional temperament meanings.