Tropical vs Sidereal Moon Sign: Why They Differ
Tropical and sidereal moon signs differ because the two zodiacs start at different points: the tropical zodiac starts Aries at the March equinox, while the sidereal zodiac is anchored to the fixed stars, and today the two are a little over 24 degrees apart. As a result, your sidereal moon sign is often one sign earlier than your tropical one, and moon sign change times differ by about two days.
Two ways to draw the zodiac
The tropical zodiac, used by most Western astrologers, divides the Sun's path into twelve 30-degree signs starting from the March equinox. The sidereal zodiac, used in Vedic (Jyotish) astrology and by some Western sidereal astrologers, divides the same path into twelve signs measured from the stars. Because Earth's axis slowly wobbles (precession), the equinox point drifts against the stars by about 1 degree every 72 years, and the two zodiacs move apart.
How big the gap is: the ayanamsa
The gap is called the ayanamsa. The Lahiri ayanamsa, the standard in Indian astrology, is a little over 24 degrees in the 2020s. Other ayanamsas (Fagan-Bradley, Raman and others) differ from Lahiri by up to about a degree, so a Moon near a sign boundary can still come out differently between sidereal sources.
What that means for your moon sign
- If the tropical Moon is in the first 24 degrees or so of a sign, the sidereal Moon is in the previous sign. That is about 4 out of 5 cases.
- If it is in the last 6 degrees, both systems give the same sign.
- Example: a Moon at 10 degrees tropical Taurus is at about 16 degrees sidereal Aries. A Moon at 27 degrees tropical Taurus is about 3 degrees sidereal Taurus.
What it means for change times
The Moon covers 24 degrees in a little under two days, so a sidereal ingress into a sign happens almost two days after the tropical ingress into the same sign. If you plan by moon ingress times, use the times that match your system.
Which one should you use?
Neither zodiac is astronomically "wrong"; they answer different questions. Use tropical if you follow most Western astrology books, sites and horoscopes. Use sidereal if you follow Vedic astrology or a sidereal teacher. Mixing them is the most common reason two sources disagree about your moon sign.
Switch in one tap
Signtide lets you switch between tropical and sidereal (Lahiri) at any time in settings. The Today dial, the moon sign calendar, the exact change times and your birth moon sign in My Moon all follow the setting, so you can compare both for the same date.
Moon sign times are astronomical calculations. What a sign is said to mean for your day comes from astrology, a tradition for reflection, not a scientific prediction.
FAQ
Why is my Vedic moon sign different from my Western one?
Vedic astrology uses the sidereal zodiac, which is about 24 degrees behind the tropical zodiac used in most Western astrology. If your Moon sits in the first 24 degrees of a tropical sign, its sidereal sign is the one before.
What is the Lahiri ayanamsa?
The ayanamsa is the gap between the tropical and sidereal zodiacs. Lahiri is the most widely used value in Indian astrology. It is a little over 24 degrees today and grows by about 1 degree every 72 years.
Can I switch between tropical and sidereal in an app?
Signtide lets you switch between the tropical zodiac and the sidereal zodiac (Lahiri) at any time. The dial, the calendar, change times and your birth moon sign all follow the setting.