Last updated: May 4, 2026
Comparison Calculator
Tropical vs Sidereal Calculator
Enter your birth details once. See your tropical and sidereal chart side by side, with a degree-by-degree delta and your choice of five ayanamsas (Lahiri default). An exact birth time gives the cleanest Ascendant comparison; approximate is fine for Sun, Moon, and planets.
What is the difference between tropical and sidereal?
The tropical zodiac anchors 0° Aries to the spring equinox. The sidereal zodiac anchors it to the actual constellations. Earth's axial precession has pulled the two systems about 24° apart since they last aligned roughly 2,000 years ago, which usually shifts a person's planetary placements one zodiac sign earlier in sidereal than in tropical.
Run your birth data through the calculator above and you get both. Same date, same time, same place. Two different zodiacs running over the top of one identical sky.
How the side by side calculator works
You enter your birth data once. We send it to the same chart engine that powers the rest of the site, get back the ecliptic longitudes of every body and angle, then frame those longitudes against two zodiacs at the same time.
The tropical column draws sign boundaries at the four cardinal points: 0° Aries at the March equinox, 0° Cancer at the June solstice, and so on around the year. The sidereal column subtracts the ayanamsa (the offset between the two systems) from each tropical longitude before it draws boundaries. Same ecliptic position. Different label.
The math is identical to what's running under the birth chart calculator and the sidereal chart calculator today. The new piece is that the two outputs are stitched together with a delta column, so you don't have to flip between tabs to see what shifted and by how much.
What changes when you switch from tropical to sidereal
Most pages that try to explain this drift into philosophy. Here is the part that's mechanical.
Sign placements
Almost everyone shifts one sign earlier in sidereal. A tropical Leo Sun usually reads as a sidereal Cancer Sun under Lahiri ayanamsa. The exception is anyone born in roughly the last 6° of a tropical sign, because the 24° subtraction keeps them inside the same sign rather than crossing the boundary back. If you have ever seen a tropical Sagittarius keep their Sagittarius Sun in a Vedic chart, that's why.
Degree positions
Every body shifts by the same amount: the ayanamsa value. Lahiri in 2026 sits at about 24°13′. So a tropical Sun at 12° Leo becomes a sidereal Sun at 17°47′ Cancer. Mercury at 3° Virgo becomes 8°47′ Leo. The shift is uniform across the chart, which is the part that catches people off guard the first time. There's no body that ignores precession.
House cusps
This depends on which house system you use. In whole sign houses (the Hellenistic and Vedic default), the rising sign shifts with everything else, and the houses ride along with it. If your tropical ascendant is 5° Libra, your tropical first house is the whole sign of Libra. Drop into sidereal Lahiri and your ascendant is now 10°47′ Virgo, so your sidereal first house is the whole sign of Virgo. Every house slides back a sign in lockstep.
In Placidus or any quadrant system, the cusp longitudes themselves don't move. What moves is the zodiac wallpaper they're sitting against. Your Placidus 10th cusp at 22° Cancer tropical becomes 27°47′ Gemini sidereal. The cusp is the same arc on the actual sky. It has just been re-labelled.
Aspects between planets
Identical. A trine in tropical is the same trine in sidereal, because aspect angles are differences in longitude, and subtracting the same ayanamsa from both ends of the difference cancels out. If you have a tropical Sun square Mars at 5° applying, you have a sidereal Sun square Mars at 5° applying. This is the single most reassuring thing for anyone afraid that going Vedic invalidates their Western chart reading.
Retrograde, speed, and dignity
Retrograde and speed describe how a planet is moving along the ecliptic, not where the ecliptic is divided. They don't change.
Essential dignities (rulership, exaltation, fall, detriment) are tied to signs, so they DO change in lockstep with the sign placements. Your tropical Mars in Aries (in domicile) becomes a sidereal Mars in Pisces (in detriment under modern rulerships, peregrine under traditional rulerships). For a lot of people, this catches them more than the sign change itself does.
What is ayanamsa, and which one should you pick?
Ayanamsa is a number. It's the angle, in degrees, between the start of tropical Aries and the start of sidereal Aries on a given date. Because the two systems drift by about 1° every 72 years, the ayanamsa keeps growing. You subtract it from a tropical longitude to get the corresponding sidereal longitude.
Different astrological lineages pin the moment when the two systems were aligned to slightly different epochs, which is why there isn't a single agreed value. Five you'll see most:
Lahiri
Default in Vedic astrology and in India's official Rashtriya Panchang almanac. Aligned to Spica at 0° Libra (sidereal). Currently about 24°13′. This is the ayanamsa we use by default, and it's what most beginner Vedic books assume.
Fagan-Bradley
The Western sidereal value, formalized by Cyril Fagan and Donald Bradley in the 1950s. About 48 arcminutes larger than Lahiri, currently around 25°01′. If you have come to sidereal through Western sidereal authors (Fagan, Garth Allen) rather than Vedic teachers, this is the value those charts assume.
Krishnamurti (KP)
A small adjustment of Lahiri used in KP (Krishnamurti Paddhati) astrology, a 20th century South Indian system known for predictive technique using sub-lords and stellar division. Sits about 5′ behind Lahiri (around 24°08′ in 2026), so the placements look almost identical at sign level.
Raman
B.V. Raman's value, around 22°46′ in 2026. About 1°27′ smaller than Lahiri. Used by the Raman lineage of Vedic astrologers and the older issues of The Astrological Magazine. If you're reading a chart from before about 1980 in the Indian tradition, it may be on Raman.
Yukteswar
From Sri Yukteswar's The Holy Science(1894). Common in some yogic and Kriya traditions. Yukteswar's literature gives several ayanamsa values; this implementation follows the Swiss Ephemeris reading, which lands at roughly 23°44′ in 2026.
If you don't have a strong reason to pick a different one, start with Lahiri. The differences between the five are usually under 2°, which is enough to move a planet across a sign boundary in maybe one chart out of fifteen, but rarely changes the broad picture.
Why your sidereal sign is usually one sign earlier
Worked example. Born September 4 in any recent year, you have a tropical Sun somewhere in the first 12° of Virgo. Subtract Lahiri's ~24°13′ and you land at roughly 17°47′ Leo. The sign moved back one slot, which is what almost everyone sees.
The “almost” matters. Born September 21 with the Sun at 28° Virgo tropical, you subtract 24°13′ and end at 3°47′ Virgo sidereal. Same sign, different degree. About one person in fifteen ends up here, depending on the time of year and the exact ayanamsa.
The further into a tropical sign you were born, the better your odds of keeping that sign in sidereal. The first 6° of a tropical sign almost always shifts back. The last 6° almost always stays put. The middle is mixed.
Which system should you use?
Neither is wrong. They measure two different things using the same observed sky, and the right one is the one your tradition uses.
Use tropical if
You're working in modern Western astrology: psychological, evolutionary, traditional Hellenistic, or modern Western forecasting. Your aspect doctrine, your transits, your secondary progressions, your solar returns are all calibrated to tropical. Switching to sidereal scrambles the chart you've been reading without giving you a Vedic toolkit to compensate.
Use sidereal if
You're working in Vedic astrology: dashas, divisional charts, nakshatras, yogas, KP, Tajika annual returns. The whole predictive apparatus assumes sidereal positions and nakshatra placements. Feeding it tropical longitudes produces nonsense results.
Use both, side by side, if
You want to see whether your stated lived experience matches the symbolism better in one frame or the other. A lot of people find their sidereal Sun reads closer to who they are than their tropical Sun. A lot find the opposite. The side by side view is the one where you can test that without taking a side ahead of time.
This page is built for the third audience. If you already know which tradition you're working in, save a step and go straight to the tropical birth chart or the dedicated sidereal Lahiri chart. For the wider sidereal tradition, see the sidereal astrology guide and the sidereal sun sign date ranges.
The astronomy behind the offset
Earth's rotation axis isn't fixed. It traces a slow circle, taking roughly 25,800 years to complete one cycle, in the same way a top wobbles when it spins. That wobble is precession of the equinoxes.
Practical consequence: the point on the ecliptic where the Sun crosses the celestial equator going north (the March equinox, which tropical astrology uses to anchor 0° Aries) drifts backwards through the constellations at a rate of about 1° every 72 years. Two thousand years ago, that point sat at the start of the constellation Aries. Today it sits in the middle of the constellation Pisces. In about 600 years, it will move into Aquarius. (This is what people mean by the “Age of Aquarius.”)
Sidereal astrology pins 0° Aries to a fixed point in the stars, with the choice of fixed point being what differs between the ayanamsas, so it stays aligned with the constellations as the equinox drifts away. Tropical astrology pins 0° Aries to the equinox itself, so it stays aligned with the seasons as the constellations drift away.
The numerical underpinning here, including the rate of precession and the planetary positions you're seeing in both columns, comes from NAIF's ANISE / JPL ephemeris data, the same source NASA uses for spacecraft navigation. Both columns of your chart agree on the underlying sky. They just frame it differently.
The sidereal tradition descends from the Babylonian zodiac; you can find your sign in the original 18 constellation system using the Babylonian zodiac calculator.
More Free Tools
Sidereal Astrology Calculator
Calculate your complete sidereal natal chart using the Lahiri ayanamsa. See Sun, Moon, rising sign, and all planetary placements in the sidereal zodiac.
Birth Chart Calculator
Calculate your complete natal chart with planet placements, houses, aspects, and chart summary.
Babylonian Zodiac Calculator
Discover your Babylonian zodiac sign, its original cuneiform name, and which of the 18 MUL.APIN constellations the ancient astronomers saw at your birth.
Frequently Asked Questions
What's my sidereal sign?
Run your birth data through the calculator above and read the sidereal column. If you only have your tropical Sun sign and no chart in front of you, the rule of thumb under Lahiri ayanamsa is one sign earlier, with the exception of births in the last 6° of a tropical sign, which often stay in place.
Why is my Vedic sign different from my Western sign?
Vedic astrology uses the sidereal zodiac, anchored to the constellations. Western astrology uses the tropical zodiac, anchored to the spring equinox. The two have drifted apart by about 24° since they last aligned 2,000 years ago, because of precession of the equinoxes. The drift is gradual, about 1° every 72 years, and it pushes most people's planets back one zodiac sign.
Is sidereal astrology more accurate than tropical?
This is the wrong question. Both systems are accurate at measuring what they were designed to measure: tropical tracks Earth's seasonal cycle, sidereal tracks the actual stars. The accuracy framing assumes there is a single right answer about what astrology refers to, and the two traditions disagree about that. Pick the system that matches the technique you're studying.
What is ayanamsa?
Ayanamsa is the angular offset between the start of tropical Aries and the start of sidereal Aries on a given date, currently about 24° for the most common (Lahiri) value. You subtract it from a tropical longitude to get the equivalent sidereal longitude. Different lineages use slightly different ayanamsa values, but they agree to within about 2°.
Do all the planets change when I switch to sidereal?
Yes. Every planet, the Moon, the lunar nodes, and all chart angles shift by the ayanamsa value. The shift is uniform: if your tropical Sun moves back 24°13′, every other body moves back 24°13′ too. What does not change between the two views is the angles between planets (your aspects), retrograde status, planetary speed, or the underlying ecliptic positions.
Does my rising sign change?
Almost always, yes. The ascendant shifts back by the same ayanamsa value as everything else, which usually puts it in the previous zodiac sign. The exception is anyone whose tropical ascendant sits in the last 6° of a sign; those charts keep the same rising sign in sidereal.
Does sidereal use the 13th sign (Ophiuchus)?
Mostly no. Lahiri, Fagan-Bradley, Krishnamurti, Raman, and Yukteswar all keep the 12-sign zodiac with each sign at exactly 30° of ecliptic. The 13-sign idea comes from true sidereal or IAU-constellation-boundary systems, which use the actual irregular widths of the modern astronomical constellations, including the strip of Ophiuchus that the Sun crosses in late November and early December. That is a much smaller subset of the sidereal world than the press coverage usually suggests.
Should beginners learn sidereal or tropical first?
Pick the lineage you want to study and use that lineage's zodiac. If you're working through modern Western forecasting or Hellenistic source texts, that's tropical. If you're working through any Vedic predictive text, that's sidereal. Trying to learn both zodiacs at the same time slows down both traditions. Better to anchor in one, get fluent, and run the other one as a comparison once you know what you're comparing.
Save your tropical and sidereal charts together
Create a free account to save both frames of your chart, see live transits in either zodiac, and watch each placement activate on the Astro Replay timeline.