See where your planets actually sit against the stars — not the seasons.
Free sidereal astrology calculator. Compare True Sidereal (with Ophiuchus) and tropical positions side by side from one birth time.
Free full chart with a written snapshot interpretation — no payment needed.Enter your birth date to see how your Sun shifts from tropical to sidereal. For your full chart across all five systems, use the calculator below.
Five systems. Your composition in 60 seconds.
Sidereal astrology measures planetary positions against the actual star constellations in the sky, not the seasons. This accounts for precession — the slow wobble of Earth's axis — which has opened a gap of roughly 31° between the tropical zodiac and the real constellation boundaries today.
When tropical astrology was created, 0° Aries aligned with the spring equinox and the constellation Aries. Today, the spring equinox occurs in Pisces. Tropical astrology stayed locked to the seasons; sidereal astrology stayed locked to the stars.
True sidereal vs. traditional Vedic: Traditional Vedic astrology uses Lahiri ayanamsa (equal 30° signs, shifted ~24° from tropical). True sidereal astrology maps to the actual constellation boundaries defined by the International Astronomical Union (IAU), which vary in size. This includes Ophiuchus, the 13th constellation the Sun passes through.
Your reading shows both tropical and true sidereal positions so you can compare. Most people find their Sun sign shifts backward by one sign in sidereal — and ~4% discover they're Ophiuchus.
Is this Vedic astrology? Not exactly. Sidereal astrology shares its sky-based zodiac with Vedic (Jyotish) astrology, but Synthesis uses true astronomical sidereal — the real IAU constellation boundaries, including Ophiuchus — not the Lahiri ayanamsa that most Vedic astrologers use. So the signs here can differ from a traditional Vedic chart.
| Aspect | Tropical (Western) | Sidereal (Star-Based) |
|---|---|---|
| Reference Point | Spring equinox (seasonal) | Actual star constellations |
| Zodiac Structure | 12 equal 30° signs | 13 constellations, unequal widths |
| 13th Sign | Not included | Ophiuchus (Nov 30 - Dec 17) |
| Current Offset | Aligned to seasons | ~31° behind tropical (varies by constellation) |
| What It Measures | Psychological/archetypal timing | Physical star energy/astronomy |
| Vedic Astrology | Not used | Vedic uses sidereal + Lahiri (equal signs, no Ophiuchus) |
Both systems are valid — they just measure different cosmic phenomena. Tropical tracks Earth's seasonal cycle; sidereal tracks actual stellar positions.
Once the calculator returns your chart you have two sets of positions for the same birth moment: tropical and true sidereal. The tempting question is which one is right. That is the wrong question. They measure different things, and the reading lives in the comparison. Take it in three passes.
Start with the big three and note which of them moved. Most placements land one constellation back from their tropical sign, but not all of them, and not by the same distance, because the constellations are not the same size. What matters is not the new label. It is whether the shift describes something you recognize. If your sidereal Sun reads truer than your tropical one, that is worth sitting with. If neither quite lands, the Moon and Rising usually carry more of the recognizable material anyway.
Go through each planet and mark whether the constellation changed. Planets that land in the same sign in both systems are the least ambiguous things in your chart: two different ways of dividing the sky agree about them. Planets that moved are where the two readings diverge, and the divergence is information rather than a contradiction. The tropical position describes the seasonal phase you were born into; the sidereal one describes the stars that were actually behind the planet. Where they disagree, most people recognize themselves in one considerably more than the other.
This is the pass no equal-sign calculator can offer you. True sidereal uses the real IAU constellation boundaries, and those constellations are wildly unequal: Virgo occupies 49.7° of the ecliptic while Ophiuchus gets 12.4°, a difference of four times. So sidereal signs are not equally common. Virgo covers 13.8% of the band the Sun travels through and Ophiuchus 3.4%, and the charts run here track those proportions closely. A tropical chart divides the sky into twelve identical 30° slices by definition, so in that system every sign is exactly as common as every other. Here they are not, and yours carries a rarity a tropical chart cannot express. It is also worth checking how far into your constellation a planet sits: near a boundary, a small difference in birth time moves it into the neighboring sign entirely.
Neither chart is the real one. Tropical measures where you sit in the seasonal year; sidereal measures where you sit against the stars. Both are computed from the same birth moment and both are internally consistent. Synthesis shows them together because agreement between independent systems is the strongest signal a chart offers, and disagreement is the most interesting.
A sidereal birth chart measures planetary positions against the actual star constellations, not the seasons. This accounts for precession — the drift accumulated since tropical astrology was created, now roughly 31° against the real constellation boundaries. Your sidereal chart shows where planets actually sit in the sky.
Traditional Vedic astrology uses Lahiri ayanamsa (equal 30° signs shifted ~24°). True sidereal astrology maps to the actual constellation boundaries defined by the IAU, including Ophiuchus as the 13th sign. Vedic is sidereal-based but uses different zodiac divisions.
Due to Earth's precession, the tropical zodiac (season-based) has drifted away from the actual star constellations — the gap is roughly 31° today. Most people find their Sun sign shifts backward by one sign in true sidereal — and some fall in Ophiuchus.
They measure different things. Tropical tracks seasons and psychological archetypes. Sidereal tracks actual star positions and energetic astronomy. Neither is 'wrong' — they're different lenses. Synthesis shows both in your reading so you can explore what resonates.
Most calculators that offer sidereal positions force you to leave the Tropical view and start over on a separate page. Synthesis computes Tropical, True Sidereal, and Draconic positions side by side from one input — so you can read the divergence between systems at a glance. The free chart is complete — no payment needed. It uses true astronomical sidereal (the real IAU constellation boundaries, with Ophiuchus), not the Lahiri ayanamsa that traditional Vedic astrology uses.
Yes. The full True Sidereal birth chart with all planetary positions, houses, and aspects is free at Synthesis. Optional paid readings ($28 one-time) provide a written multi-system analysis, but the chart math itself never sits behind a paywall.
Yes — “natal chart” and “birth chart” are interchangeable terms. This sidereal natal chart calculator computes the complete chart — planets, houses, and aspects — against the actual star positions, with the tropical version alongside for comparison.
True Sidereal mode uses the real IAU constellation boundaries, which include Ophiuchus as a 13th sign between Scorpius and Sagittarius. If your Sun or any planet falls in Ophiuchus, the chart shows it rather than rounding to a 12-sign wheel.
Your tropical Sun might be Capricorn while your sidereal Sun is Sagittarius. See both — across all five systems — and which one tracks closer to how you actually move through the world.
See your full free chart — tropical + sidereal →