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Moon Sign Calculator

Calculate the Moon's geocentric tropical longitude and 30-degree sign sector for a birth moment, with a documented UTC or UTC-equivalent offset and a visible boundary warning.

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Enter the local birth date, clock time, and documented UTC or UTC-equivalent offset for that moment.
  2. 2.Calculate to see the normalized UTC instant, geocentric true-ecliptic-of-date longitude, and tropical sector.
  3. 3.If the result is near a 30-degree boundary, verify the birth time and historical offset before treating the cultural label as stable.

About Moon Sign Calculator

Moon Sign Calculator converts a supplied local date, clock time, and documented UTC or UTC-equivalent offset into one proleptic UTC-like instant, then calculates the Moon center's geocentric true-ecliptic-of-date longitude. It shows that astronomical longitude, the degree within a Western tropical 30-degree sector, the normalized calculation instant, and the resulting cultural sign label. It does not request a city, coordinates, GPS, or location permission.

Location is not needed for this deliberately narrow quantity. Geocentric means that the observer origin is the center of Earth. A birthplace field on a full natal-chart site is commonly used to resolve a historical time convention and to calculate the Ascendant or houses. This tool asks for a documented UTC or UTC-equivalent offset directly and calculates neither topocentric position nor houses. The page therefore remains private, deterministic, and fully functional offline after its assets load.

The coordinate convention is frozen: true ecliptic of date, Earth-center Moon position, and Western tropical sectors. Longitude is normalized to zero through less than 360 degrees. Aries covers 0 through less than 30 degrees, Taurus 30 through less than 60, continuing in equal half-open sectors through Pisces at 330 through less than 360. This twelve-sector mapping is an astrological and product convention. It is not an IAU constellation boundary or a scientific personality classification.

Date, time, and the documented UTC or UTC-equivalent offset are all required. Before standard time zones, local mean time may not map to a reliable modern offset, so early-date results are proleptic UTC-like estimates. The interface accepts non-whole-hour offsets and displays the resolved calculation instant so a visitor can audit the conversion. It does not silently assume that today's browser offset applied decades or centuries ago. Impossible calendar values, invalid offsets, and resolved calculation instants outside 1700 through 2100 are rejected.

The astronomy result is independently checked. Twelve literal cases lie just before and after tropical sector boundaries and cover 1700, 1800, 1900, 1999, 2020, and 2099. Each case freezes official observer-centered ecliptic longitude from NASA JPL Horizons and an independent IMCCE/INPOP ephemeris. Astronomy Engine must remain within 0.03 degrees of each official oracle and produce the expected half-open sector label. JPL, IMCCE, and other remote services are fixture sources only; no runtime request is made.

A result within 0.05 degrees of a 30-degree boundary carries a warning. An imprecise birth time, incorrect documented offset, or different ephemeris convention can change the cultural label near that edge. The page avoids second-level, professional-ephemeris, navigational, or scientific-personality claims.

For entertainment and cultural interest. Astrology is not a scientific personality or relationship assessment and should not guide medical, legal, financial, safety, employment, or relationship decisions. The calculator reports a reproducible longitude and disclosed display convention; it does not provide compatibility, prediction, diagnosis, or advice.

Time-history caveat: the numeric input is a documented UTC offset or UTC-equivalent offset, not a promise that modern UTC or a standard civil time zone existed on the entered date. Before standard time zones, local mean time may not map to a reliable modern offset. Early-date output is therefore a proleptic UTC-like estimate, and the entered convention remains visible in the result.

Methodology & sources

Convert valid local civil fields and a signed documented UTC or UTC-equivalent offset to a proleptic UTC-like instant; before standard time zones, local mean time may not map to a reliable modern offset. Through a narrow lazy adapter around astronomy-engine 2.1.19, obtain the Moon center's geocentric true-ecliptic-of-date longitude. Normalize the angle and map it into twelve equal half-open Western tropical sectors. Warn within 0.05 degrees of a boundary. Validate twelve literal boundary cases against both JPL Horizons quantity 31 and IMCCE/INPOP with circular longitude error no more than 0.03 degrees.

Frequently asked questions

Why does this Moon sign calculator not need a birthplace?
It calculates an Earth-center, or geocentric, longitude and asks for a documented UTC or UTC-equivalent offset directly. Before standard time zones, local mean time may not map to a reliable modern offset. Local horizon quantities such as houses or the Ascendant are outside this tool.
Is a Moon sign the same as a Moon phase?
No. A Moon sign is the Moon's longitude sector in a disclosed tropical convention. A Moon phase is the Moon-Sun angle and illuminated fraction.
What does the boundary warning mean?
Within 0.05 degrees of a sector edge, a small error in birth time or UTC offset, or a different ephemeris convention, can change the displayed sign label.

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