Free Sidereal Birth Chart Calculator

A sidereal astrology chart shows your natal planets in a fixed-star zodiac instead of the tropical zodiac used by most Western astrology. In this calculator, sidereal means Lahiri: twelve equal 30-degree signs shifted by the ayanamsa. It is not a true-sidereal map of uneven constellation boundaries.

That 24 degree gap is called the ayanamsa. It is not a rounding error or a calibration knob. It is the central technical question of sidereal astrology, because picking a different ayanamsa moves every placement by minutes of arc or, in some systems, close to a degree. That occasionally crosses a sign boundary. This calculator uses the Lahiri (Chitrapaksha) ayanamsa, the standard the Indian government adopted in 1955 and the default reference for many English-language Vedic calculators. The applied ayanamsa value is printed in the results, so you can cross-check it against another Lahiri-based calculator.

Planetary longitudes come from the Rust ephemeris service using ANISE and JPL DE440s data where available. The Lahiri ayanamsa is then subtracted from each tropical longitude to derive the sidereal one, and the chart displays placements by degree and minute. Aspects (the angles between two planets) are not shifted, because they measure relative geometry that is invariant across zodiac choice. A trine in your tropical chart is a trine in your sidereal chart. The signs the trine connects may change.

The roots of the zodiac are Babylonian. Star catalogs like MUL.APIN preserved an earlier Moon-path constellation layer, and later Babylonian mathematical astronomy standardized the ecliptic into twelve equal 30-degree signs anchored to a star-referenced frame. When Greek astronomers inherited Babylonian astronomy through trade and conquest in the Hellenistic period, they inherited that sidereal frame. Early Hellenistic astrology, the same tradition Augurine focuses on, was built on top of that star-referenced zodiac.

The split happened around 130 BCE. Hipparchus measured axial precession, the slow westward drift of the equinoxes against the fixed stars, and made the discrepancy mathematically visible. About three centuries later Claudius Ptolemy, in the Tetrabiblos, made the case for anchoring the zodiac to the equinox rather than to the stars. That is the tropical zodiac. The Western tradition gradually followed Ptolemy. The sidereal frame stayed alive in India, where Hellenistic ideas had already arrived through texts like the Yavanajataka and merged with indigenous astronomy to become the sidereal Jyotish tradition still used today.

So the short version: the star-referenced zodiac predates the tropical convention, and Vedic astrology kept that sidereal logic alive. A Lahiri sidereal chart shows your placements in that frame rather than the later tropical convention. That is the calculator on this page. If you want to see Hellenistic technique applied to your full natal chart in the tropical frame the later Western tradition adopted, see the Hellenistic chart calculator.

There is no single “sidereal zodiac.” Common sidereal options can disagree enough to push some placements across a sign boundary. Knowing which one you are looking at matters as much as the sidereal-versus-tropical choice for many charts.

Lahiri (used by this calculator). The Indian government standard, fixed by the Indian Calendar Reform Committee and traditionally associated with Spica/Citra near 0° Libra. Lahiri is the default reference for many Vedic (Jyotish) calculators and for much English-language Jyotish material. Lahiri-based calculators should return the same signs and very similar degrees, usually within minutes of arc. If you are doing Vedic timing work or want a chart compatible with classical Indian techniques, Lahiri is usually the right first input, though some lineages use Raman, KP, Yukteswar, or other ayanamsas.

Fagan-Bradley. The Western sidereal standard developed by Cyril Fagan and Donald Bradley in the mid 20th century, anchored to the star Aldebaran. About 0.85° behind Lahiri. If you have read Garth Allen, Rupert Gleadow, or any classical Western sidereal astrology, you have read Fagan-Bradley positions. This calculator does not output Fagan-Bradley; for charts in that frame, the difference will be roughly 51 arc minutes from what you see here.

True sidereal (or constellational). Not an ayanamsa at all. Instead of dividing the sky into 12 equal 30° signs, true sidereal uses constellation-based boundaries, often based on IAU spans or midpoint models. That produces unequal-sized signs and a 13th sign (Ophiuchus) in some implementations. If a calculator gave you Ophiuchus or told you your Sun was in a very short Scorpio segment, that was a true sidereal calculator, not a Lahiri one.

Short version: if you want a Lahiri sidereal chart for Vedic or Jyotish study, stay on this page. If you want a constellational sky map, true sidereal is what you want, and it is not what this calculator computes.

For most people the Sun moves back about one sign. A tropical Sun at 5° Taurus becomes a sidereal Sun near 11° Aries once the 24° ayanamsa is subtracted. The closer your tropical placement sits to the start of a sign, the more likely it is to cross into the previous sign. The closer it sits to the end, the more likely it is to stay in the same sign. As a rule of thumb: tropical positions in the first 24 degrees of a sign usually shift back in sidereal, while positions in the final six degrees tend to stay put.

The rising sign is the placement most likely to surprise you. It changes every two hours, so the same 24° shift can put the Ascendant in the previous sign. When the Ascendant sign changes, the sign-based chart ruler changes too, and the sidereal labels on the angles and house cusps can read differently. If you have only ever read your tropical chart, the sidereal Ascendant is where the biggest reframing happens.

What does not change: the angular distance between any two planets, the orb of any aspect, and whether an aspect is applying or separating. A Sun square Mars in tropical is the same configuration in sidereal, with identical orb. The signs the square connects may differ, but the underlying geometry is the same chart.

1. Start with the Sun, Moon, and rising sign. These are the placements most discussed in Vedic and sidereal traditions. If you are coming from tropical, write both side by side before drawing conclusions. The contrast often says more than either reading on its own.

2. Check the Ascendant and house cusps. If your Ascendant shifted, the chart ruler shifted with it. In Vedic astrology that ruler is called the Lagna Lord, and it is the most-referenced single placement after the Moon. A new ruler often changes which planet you should be reading first.

3. Read aspects from the same chart you read tropically. They have the same orb, the same separating or applying status, the same exactness. Sidereal does not change the geometry. It only changes the symbolic frame the geometry sits inside.

4. Note the Moon's sidereal degree.In Vedic astrology, the Moon's sidereal degree determines your nakshatra, the 27-fold lunar mansion system that drives the entire dasha (planetary period) timing tradition. This calculator does not compute nakshatras or dashas, but the Moon's sidereal longitude printed in your results is the only input a nakshatra calculator needs.

5. Compare to your tropical chart. The interpretive rule of thumb most practitioners who use both end up with: tropical signs describe seasonal psychology, sidereal signs describe a star-referenced zodiac corrected by ayanamsa. Treat them as different layers, not as competing answers to the same question. If you want both zodiacs side by side before you interpret the shift, use the tropical vs sidereal chart calculator.

There is no settled answer to which system is “correct.” The tradition you train in is usually the tradition you find accurate. That said, three cases tip in sidereal's favor.

You are working with Indian techniques. Dashas, varga charts, nakshatras, yogas, ashtakavarga, and most predictive Jyotish workflows assume sidereal positions. Many English-language resources default to Lahiri, but some lineages use other ayanamsas. Running those techniques on a tropical chart mixes frameworks.

You want a fixed-star reference instead of a seasonal reference. Lahiri does not trace the uneven borders of the visible constellations, but it does keep the zodiac tied to a star-based reference point instead of the March equinox. The 2,000 year drift from tropical is observable. For people who care about that non-seasonal reference, sidereal is often the more meaningful frame.

Your tropical chart never landed. Some people read their tropical chart for years and find it generic, then read their sidereal chart and recognize themselves. Others have the opposite experience. Both are real. If your tropical chart never quite described you, sidereal is worth a serious read before you abandon astrology.

On the other side: if you are reading most classical Western astrology (Hellenistic, medieval, Renaissance), tropical is usually the expected convention. Ptolemy gives a tropical rationale, and later Western astrology largely kept that frame even as sidereal approaches continued elsewhere.

For a deeper history of the split, see the sidereal astrology guide. For approximate sidereal Sun sign date ranges, see the sidereal Sun sign dates hub. The sidereal tradition descends from the Babylonian zodiac; you can compare your date with the Babylonian-style reference layer using the Babylonian zodiac calculator.