Moon Sign Calculator

Overview

Determining a Moon sign means finding the Moon's ecliptic longitude for a given instant and identifying which 30-degree division it falls in. Because the Moon moves quickly, the date is usually sufficient — but not always.

• The Moon advances about 13 degrees of ecliptic longitude per day, spending just over two days per sign.
• For most births the date alone settles the sign; near an ingress the time decides it.
• Lunar ephemerides are among the most precisely validated computations in astronomy.
• The Moon's speed varies over its elliptical orbit, so time-in-sign is not constant.

The calculation

The Moon's geocentric ecliptic longitude for any instant comes from a lunar ephemeris. Dividing that longitude by 30 gives the sign, and the remainder gives the degree within it. There is no interpretive step in the arithmetic.

The underlying positions are exceptionally well determined. Lunar laser ranging using retroreflectors left by Apollo and Lunokhod missions measures the Earth–Moon distance to centimetre precision, and the resulting models are correspondingly accurate. Whatever one makes of the interpretation, the position is not in doubt.

When the date alone is enough

The Moon completes a circuit of the ecliptic in about 27.3 days, advancing roughly 13 degrees per day. Each 30-degree sign therefore takes a little over two days to cross, so for most births the date determines the sign unambiguously.

The exception is a birth within a few hours of an ingress. In that window the time of day decides the answer, and no lookup table can substitute for the actual ingress moment for that specific date.

Why the duration varies

The Moon's orbit is noticeably elliptical, so its angular speed changes through the month — faster near perigee, slower near apogee. Time spent in a sign consequently ranges from roughly two days to about two and a half.

Additional periodic perturbations, chiefly from the Sun's gravity, modulate this further. The classical lunar theory needed to describe these motions accurately was one of the hardest problems in eighteenth-century mathematics, and solving it was central to determining longitude at sea.

Sidereal versus tropical results

The computed longitude is the same in every system; what differs is where the sign boundaries are placed. Western tropical astrology measures from the March equinox; Jyotisha measures from a star-referenced origin offset by the ayanamsa, currently about 24 degrees.

The result is that the same Moon position usually falls in the previous sign under Jyotisha. Both are consistent conventions applied to one measured quantity, and a disagreement between a Western and a Vedic Moon sign is not an error in either.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Moon signWhat tradition associates with the placement.
• Moon phaseThe astronomy of the lunar cycle.
• Moon readoutComputed lunar position and phase.

Frequently asked

Do I need my birth time to find my Moon sign?

Usually not. The Moon spends a little over two days in each sign, so the date alone normally settles it. The exception is a birth within a few hours of an ingress — the moment the Moon crosses into a new sign — where the time of day is decisive and the exact ingress moment for that date is required.

Why does the Moon spend different amounts of time in different signs?

Because its orbit is elliptical, so it moves faster near perigee and slower near apogee. Time in a sign therefore ranges from roughly two days to about two and a half. Periodic perturbations, mainly from the Sun's gravity, modulate the motion further.

How accurate are the Moon positions used?

Extremely. Lunar laser ranging off retroreflectors left by Apollo and Lunokhod missions measures the Earth–Moon distance to centimetre precision, and modern lunar ephemerides are correspondingly precise. The astronomical input to a Moon sign determination is among the best-measured quantities in the Solar System.