Skip to main content
If you have ever read a Western horoscope, you may be surprised to find that this app places your Sun — and often your Moon and rising sign — in a different sign than you expected. That is not an error, and it is not a quirk of this particular app. It is the single most fundamental difference between jyotiṣa and modern Western astrology: the two traditions measure the zodiac from different starting points, and after nearly two millennia of drift, those starting points are almost one full sign apart.

​
Two zodiacs, one sky

The zodiac is a 360° band around the sky divided into twelve 30° signs. The critical question is: where does 0° Aries begin? The tropical zodiac — the one used by most Western astrology — ties 0° Aries to the March equinox, the moment the Sun crosses the celestial equator heading north each spring. The signs are anchored to the seasons, so “Aries season” always means roughly late March to late April regardless of what the stars are doing. The sidereal zodiac — used by jyotiṣa and by this app — ties 0° Aries to the fixed stars, the actual star groups that make up the constellations. The signs stay locked to the stellar background, so “the sign Aries” always refers to the same patch of sky, year after year. These two starting points were nearly aligned around 1,700 years ago. Since then they have drifted steadily apart, because Earth’s rotational axis wobbles slowly like a spinning top — a motion astronomers call the precession of the equinoxes, which completes one full 360° cycle in about 25,800 years. Today the two zodiacs are offset by roughly 24°, a little less than one whole sign.
The practical effect on your chart: a planet that a tropical chart describes as sitting in “early Cancer” a sidereal chart will usually call “late Gemini.” Approximately four people in five find that their sidereal Sun sign is the sign immediately before their familiar Western one. This is expected and consistent — not a rounding error.

​
The ayanāṁśa — measuring the gap

The angular distance between the tropical and sidereal starting points at any given moment is called the ayanāṁśa (Sanskrit for “portion of the precessional motion”). To produce a sidereal chart, the calculation engine takes each planet’s tropical longitude and subtracts the ayanāṁśa for the exact date and time of birth. Because the gap has grown slowly over centuries and was estimated in slightly different ways by different scholars and institutions, several ayanāṁśa definitions remain in active use. This app supports three: The differences between these values are small — fractions of a degree — but they matter when a planet or the Lagna sits right on a sign boundary or a nakṣatra cusp, since a shift of even one arc-minute can move it into the adjacent sign or mansion. The app lets you switch among the three ayanāṁśas and recomputes the entire chart immediately, so you can see the effect directly rather than having to reason about it abstractly. For the full technical reference, see Ayanāṁśa options.

​
Why it matters for reading

Keeping the signs locked to the fixed stars is not merely a historical preference — it has specific consequences for how jyotiṣa is read, and it makes several of its most important tools coherent in a way the tropical frame cannot replicate. The 27 nakṣatras are real star groups. The lunar mansions (nakṣatras) are 13°20′ segments of the zodiac each anchored to a specific star or asterism along the Moon’s path through the sky. Because the sidereal frame stays fixed to those stars, the nakṣatra a planet occupies is genuinely the sky region where that planet appears. This is what makes the Vimśottarī daśā period system work: the Moon’s nakṣatra at birth determines which planetary period you are born into, and the period lengths are derived from the lunar mansions’ fixed proportions — a correspondence that only holds in the sidereal frame. The Moon’s sign is at least as important as the Sun’s. Because jyotiṣa anchors interpretation in the actual sky rather than in the seasonal calendar, the Moon’s placement carries enormous weight. The Moon sign (Rāśi) is often given as much emphasis as the rising sign in a reading, and the daśā system roots the entire life-timeline in the Moon’s nakṣatra rather than the Sun’s. Whole-sign houses stay tightly bound to the signs. In the whole-sign house system the app uses by default, each house is exactly one complete sign. Because the signs are fixed to the stellar background, a house is always the same patch of sky, making the system internally consistent with the rest of the sidereal framework. None of this means the sidereal approach is philosophically superior to the tropical in the abstract — the two systems answer different questions and rest on different premises. But if you want a chart in the tradition of Parāśara, Varāhamihira, and the classical Sanskrit texts, it must be sidereal, and that is precisely what this app computes.
To see how the engine constructs the sky for a given birth moment — ephemeris source, coordinate system, and house calculation — read The Sidereal Sky. When you are ready to apply all of this to an actual chart, head to The Individual Chart guide.