Same twelve names, two starting points
Both systems divide the ecliptic, the Sun’s apparent yearly path, into twelve equal signs of 30°. They differ only in where the first sign, Aries, begins.
- ✦The tropical zodiac starts at the March equinox: the point where the Sun crosses the celestial equator heading north. Every year the Sun reaches 0° Aries on the day of the March equinox, 0° Cancer at the June solstice, 0° Libra at the September equinox and 0° Capricorn at the December solstice. The signs are tied to the seasons.
- ✦The sidereal zodiac starts from a fixed point among the stars. The signs are tied to the background sky, and the exact anchor is set by a convention called an ayanamsa.
Neither zodiac follows the constellations’ actual outlines, which are unequal in size. Both use equal 30° signs.
Western astrology, and Aurae, use the tropical zodiac. Vedic astrology (Jyotish) uses the sidereal one. When an Indian astrologer, a Vedic app or a family horoscope gives you a different sign from the one you know, this is usually a large part of the reason: the same sky, measured from a different starting line. The other common reason is that Vedic astrology leads with the Moon sign rather than the Sun sign.
The Sun, Moon and Rising calculator gives your Big Three in the tropical zodiac, so you can compare it with any Vedic chart you already have.
Precession: why the gap keeps growing
The Earth’s axis wobbles slowly, like a spinning top, completing one turn in about 25,800 years. As it does, the equinox point slides westward against the stars, by about 50.3 seconds of arc a year. That is roughly 1° every 72 years.
Because the tropical zodiac moves with the equinox and the sidereal zodiac stays with the stars, the two drift apart at that same rate. You can watch it happen with a single star. By Aurae’s own calculation from its catalogue position, the bright star Spica sat at about 203.1° of tropical longitude at the start of 1950 and sits at about 204.2° in October 2026. The star has barely moved; the zero point of the tropical zodiac has moved away from it, by a little over a degree in 77 years.
Run that drift backwards and the two zodiacs meet. Under the Lahiri convention, the one most used in India, they coincided around the third century CE. Other conventions place that meeting some decades or a century or more earlier or later, which is one reason the exact gap is disputed. Today it is about 24°, and it will keep growing by about 1° per lifetime.
How to estimate your sidereal sign
You can get a good estimate from your Western chart with simple subtraction. Aurae does not compute sidereal charts, so treat this as an estimate and use a Jyotish calculation for anything exact.
- ✦Subtract about 24° from each tropical position (for a birth around 2000, about 23.8°; for 1950, about 23.1°).
- ✦If the planet sits in roughly the first 24° of its tropical sign, it moves back one sign.
- ✦If it sits in the last 6° or so, it stays in the same sign.
Worked examples (hypothetical chart, born around 2000)
- ✦Sun at 10° Leo, tropical: 10° minus 24° takes it back into Cancer, at about 16° Cancer. Western Leo, Vedic Cancer.
- ✦Moon at 28° Virgo, tropical: 28° minus 24° leaves about 4° Virgo. The same sign in both systems.
- ✦Rising at 23° Scorpio, tropical: this lands within a degree of the Libra–Scorpio line in sidereal terms. Here the choice of ayanamsa decides the sign, and only an exact Vedic calculation settles it.
The same arithmetic shifts the Sun-sign calendar. The Sun moves about 1° a day, so sidereal Sun-sign dates run roughly 24 days later than tropical ones. As an estimate, the Sun enters sidereal Aries in mid-April rather than around the March equinox.
The Moon moves about 13° a day, so its sidereal sign can also depend on your birth time. That matters in Jyotish, where the Moon sign is central.
Ayanamsas: whose star reference?
The ayanamsa is the angle between the two zodiacs’ starting points. There is more than one, and the choice is a real disagreement among sidereal astrologers.
- ✦Lahiri (also called Chitrapaksha) is the standard in India. It was recommended by India’s Calendar Reform Committee in the 1950s and is built to keep the star Spica, called Chitra in Sanskrit, close to 0° Libra in the sidereal zodiac. Estimated from Spica’s position, the Lahiri gap is about 24°13′ in late 2026.
- ✦Raman, named after the astrologer B. V. Raman, gives a smaller gap.
- ✦Krishnamurti, used in the KP system, sits very close to Lahiri.
- ✦Fagan-Bradley, named after the twentieth-century astrologers Cyril Fagan and Donald Bradley, is the usual choice in Western sidereal astrology, a small modern branch of the Western tradition, and gives a larger gap than Lahiri.
The spread between them runs from a few minutes of arc to roughly a degree and a half. That is small for most placements but decisive for any planet or Ascendant within a degree or two of a sign boundary.
This is contested, and astronomy cannot settle it. The tropical zodiac has a physical anchor, the equinox. A sidereal zodiac needs a choice of star or reference point, and each ayanamsa rests on its own historical and textual argument. If two Vedic charts disagree about a planet near a boundary, ask which ayanamsa each one used.
Which zodiac is right?
Both, for the question each one asks. The tropical zodiac describes where the Sun and planets are relative to the seasons and the Earth’s orientation. The sidereal zodiac describes where they are relative to the background stars. One is not a corrected version of the other, and no established test has shown either to be more accurate as astrology.
One caveat about the seasons: the tropical signs’ seasonal imagery, Aries as the start of spring, fits the Northern Hemisphere. In the Southern Hemisphere the Sun enters Aries at the start of autumn. Western astrologers still use the same tropical zodiac everywhere, because it is defined by the equinox point, not by local weather.
The split is historical. Ptolemy, writing in the second century CE, tied the signs to the equinoxes and solstices, and the Western tradition followed him. Indian astrology kept the sidereal reckoning. Both reckonings have long, continuous histories of use.
Aurae uses the tropical zodiac with Whole Sign houses, the Western standard. It does not compute sidereal charts, rashi, nakshatras or dashas. If you also follow a Vedic astrologer, your two charts are not contradicting each other; they are the same sky in two coordinate systems, each read by its own rules. For how the two traditions differ beyond the zodiac, see Vedic and Western astrology: what actually differs.
What stays the same when you switch
Because the whole sidereal chart is the tropical chart rotated by the same amount, a lot does not change:
- ✦Aspects are identical. The angle between any two planets is the same in both zodiacs, so a square stays a square.
- ✦Quadrant houses do not change. In systems such as Placidus, the house cusps rotate by the same amount as the planets, so each planet keeps its house.
- ✦The order of the planets around the chart is identical. Only the sign labels move.
The exception is Whole Sign houses, which Jyotish uses and Aurae uses too. There, each house is a whole sign, so a house can change when the Ascendant and a planet cross sign boundaries differently.
Example (hypothetical chart)
Ascendant 10° Leo and Mars 28° Virgo, tropical. In Whole Sign houses Leo is the 1st house, so Mars in Virgo is in the 2nd. Shift both by about 24°: the Ascendant moves to about 16° Cancer, while Mars stays in Virgo at about 4°. Now Cancer is the 1st house and Virgo the 3rd, so Mars is in the 3rd house. For more on how house systems differ, see Whole Sign or Placidus?.
Tropical and sidereal: quick answers
Do I have two signs?
You have one sky at birth described in two coordinate systems. In Western astrology use your tropical signs; in Vedic astrology use your sidereal ones. Mixing them, such as reading a sidereal sign with Western sign descriptions, gives muddled results.
Why do Indian horoscopes use the Moon sign?
In Jyotish the Moon’s sign, your rashi, is central: it anchors daily horoscopes, transit readings and the start of the dasha periods. So when an Indian horoscope asks for your sign, it usually means your sidereal Moon sign. See Vedic and Western astrology: what actually differs.
When did the two zodiacs line up?
Under the Lahiri ayanamsa, around the third century CE. Other ayanamsas move that date by decades or more, which is part of why the exact gap is disputed.
Is Vedic astrology more accurate because it uses the stars?
There is no established evidence either way. The zodiacs measure different things, seasons and stars, and each tradition is internally consistent. Choose the system whose methods you want to use.
Can Aurae show my sidereal chart?
No. Aurae calculates tropical charts with Whole Sign houses. You can use the subtraction method above for an estimate.