Sidereal vs tropical, in plain terms
Right now the two zodiacs are about 24° apart — nearly an entire sign. A tropical Sun at 10.6° Capricorn is a sidereal Sun at 16.8° Sagittarius today; in the year 300 the gap will be about 1° wider still, and in ~250 years your tropical Sun sign's sidereal equivalent will have slid one whole sign further back.
Neither is 'the real' zodiac: both are human conventions for dividing the same 360° ring. Tropical astrology ties the ring to the Sun's seasonal cycle (why signs match seasons in the northern hemisphere); sidereal, the system used in Vedic astrology, ties it to the backdrop of fixed stars. The 'you're actually a Sagittarius' meme is about nothing more — or less — than this offset.
How we compute it
We subtract the Lahiri (Chitrapaksha) ayanamsa, evaluated from the Julian date of your birth with the standard polynomial (23.795° + 1.397°/century, with the small quadratic correction), from every tropical longitude in your natal chart — planets, Ascendant and Midheaven alike. Lahiri anchors 0° Aries to the fixed star Spica at 180° from the traditional node point and matches the official Indian eclipse-table standard.
Because the method removes a single constant from the whole chart, it is a rigid rotation: every separation between two bodies stays exactly the same, so the technique preserves your aspect pattern to the arcsecond. Which is the deep answer to the tropical-versus-sidereal argument: the two charts tell the same relational story, just with different sign labels.
What you get
After entering one birth date, time and place, the report shows:
- The exact Lahiri ayanamsa at your birth moment, to three decimal places.
- All ten natal bodies plus Ascendant and Midheaven in both frames, with a 'sign changes?' column.
- How many of your bodies keep their sign versus shift back one — slowly moving planets near sign boundaries are the coin-flips.
- Element and modality tallies of the sidereal distribution, the traditional first-glance texture of a chart.
What the two answers mean for you
Siderealists argue their frame matches actual constellation positions; tropicalists answer that astrology was built on the seasonal cycle and that the tropical zodiac's signs are literally named after the seasons it tracks. You do not have to settle it to use this tool: run your chart in both and notice that everything structural — aspects, stellia, the node axis — is identical.
Where people do feel a difference is the label itself: if a 'Scorpio Moon' reads as Libra to you, that is a hint the sign-name layer is more conventional than causal. Our sidereal placements agree with standard Vedic software using Lahiri to within a small fraction of a degree; some Vedic apps use other ayanamsas (Raman, KP), which differ by up to a further degree.
A worked example: born 1 January 2000, 17:30 UTC, London
At this birth moment the Lahiri ayanamsa is 23.795°. The tropical chart has Sun 10.6° Capricorn, Moon 16.1° Scorpio, Mercury 2.2° Capricorn, Venus 1.8° Sagittarius, Mars 28.1° Aquarius, Jupiter 25.3° Aries, Saturn 10.4° Taurus, Ascendant Cancer 28.8°.
Subtract 23.795° from each and the sidereal chart reads: Sun 16.8° Sagittarius, Moon 22.3° Libra, Mercury 8.5° Sagittarius, Venus 8.0° Scorpio, Saturn 16.6° Aries — five bodies pushed back one whole sign — while Mars 4.3° Aquarius, Jupiter 1.5° Aries and the Ascendant at 5.0° Cancer stay in the same signs, just at very different dates within them. Note Jupiter and the Ascendant: both were close to their sign's start tropically, and both survive the shift — barely.
What this tool does not claim
The ayanamsa is computed from the standard Lahiri polynomial rather than a full precession theory; it matches the official Chitrapaksha value to within roughly 0.05° for modern dates — far under a minute of arc's effect on any reading. Some traditions will disagree about which ayanamsa is 'right'; that disagreement is about a constant, not about the sky.
There is no controlled evidence that either zodiac's readings work better than chance, and this page takes no side — it shows you the arithmetic both camps actually share. All computation runs locally; your birth data never leaves your browser.
Sidereal vs draconic vs heliocentric rebasing
This tool subtracts a time-dependent constant (the precession offset); the Draconic chart subtracts your North Node's degree instead, and the Heliocentric chart changes the observer rather than the zero point. Running the same birth data through all three is the fastest way to learn which parts of a chart are frame-dependent — most sign labels are — and which are not — all aspects and separations are.
Our method in one line
Tropical positions from sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine), rebased with the Lahiri ayanamsa polynomial evaluated at your birth's Julian date; both columns shown on the same page. Reviewed by Dr. Amara Osei, Astronomy Reviewer.
Step by step
- 1
Enter one birth record
Date, time and place — the same inputs as our natal chart; the moment is resolved to UTC to evaluate the ayanamsa.
- 2
Read the ayanamsa card
The exact offset at your birth, in degrees, is printed above the table — it is the whole difference between the systems.
- 3
Compare the columns
Every body, plus Ascendant and Midheaven, appears tropical and sidereal side by side with sign changes flagged.
- 4
Check the tallies
Element and modality counts update for the sidereal distribution — the fastest read of what the zodiac choice did to your chart.
Tip — Bodies sitting in the first ~24° of a sign tropically are the ones most likely to change; anything born within the last few degrees of a sign keeps its name and simply ages backwards in it.
Frequently asked questions
- What is an ayanamsa exactly?
- It is the name for the current angular gap between the tropical and sidereal zero points — the accumulated precession since the two frames last coincided (roughly the early centuries CE, depending on which sidereal canon you assume). Lahiri puts it at about 23.8° for births around the year 2000, growing by ~1.4° every century.
- Why does this page call it a 'Western' sidereal chart?
- Because the technique and presentation are the Western ones — ten planets, whole-sign-free degrees, Ascendant and MC — just rebased onto the star-anchored zodiac. A Vedic chart additionally uses lunar nakshatras, different house systems and different interpretive rules; this tool gives you the coordinate shift, not a full Vedic reading.
- Will my Ascendant change sign?
- Often, yes — the offset is nearly a whole sign, so most Ascendants move back one. In our worked example the Cancer Ascendant survives at 5.0° because it sat deep in the sign at 28.8°. The tool shows your real answer, and it is the same subtraction as for every planet.
- Do aspects change between the two zodiacs?
- No — this is the fact both camps should agree on. Subtracting one constant from every position leaves all separations identical, so it exactly preserves your aspects, stellia and wheel geometry. Only which named sign each body falls in changes.
- Is my birth data private?
- Calculations run entirely in your browser. Your birth data is never uploaded, never stored on a server, and never shared. The rebasing is a local calculation from the ephemeris; nothing is uploaded or stored.
Written by
Elena Marchetti
Founder & Staff Astrologer
Methodology reviewed by
Dr. Amara Osei
Astronomy Reviewer
Published 2026-10-01 · Updated 2026-10-01 · Calculated with sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine)