function calculateSiderealSign() {
var dateInput = document.getElementById("birthDate").value;
var hourInput = parseInt(document.getElementById("birthHour").value);
var minuteInput = parseInt(document.getElementById("birthMinute").value);
if (!dateInput) {
alert("Please select a birth date.");
return;
}
var birthDate = new Date(dateInput);
var year = birthDate.getFullYear();
var month = birthDate.getMonth() + 1;
var day = birthDate.getDate();
// 1. Calculate Ayanamsa (Lahiri)
// Approximate Ayanamsa formula: 22.450848056 + 1.3974464 * T + 0.000176 * T^2
// where T is centuries from 1900
var t = (year – 1900) / 100;
var ayanamsa = 22.450848056 + (1.3974464 * t) + (0.000176 * t * t);
// 2. Calculate Approximate Tropical Sun Longitude
// Day of Year calculation
var start = new Date(year, 0, 0);
var diff = birthDate – start;
var oneDay = 1000 * 60 * 60 * 24;
var dayOfYear = Math.floor(diff / oneDay);
// March 21 (Vernal Equinox) is approx 0 degrees Tropical Aries
// Day 80 is roughly March 21st
var daySinceEquinox = dayOfYear – 80;
var tropicalLongitude = (daySinceEquinox * 360 / 365.25);
if (tropicalLongitude < 0) tropicalLongitude += 360;
// 3. Convert to Sidereal Longitude
var siderealLongitude = (tropicalLongitude – ayanamsa + 360) % 360;
// 4. Determine Sidereal Sign
var signs = ["Aries", "Taurus", "Gemini", "Cancer", "Leo", "Virgo", "Libra", "Scorpio", "Sagittarius", "Capricorn", "Aquarius", "Pisces"];
var signIndex = Math.floor(siderealLongitude / 30);
var currentSign = signs[signIndex];
// Display Results
document.getElementById("ayanamsaValue").innerHTML = "Applied Lahiri Ayanamsa: " + ayanamsa.toFixed(4) + "°";
document.getElementById("signOutput").innerHTML = "Your Sidereal Sun Sign: " + currentSign;
var descriptions = {
"Aries": "In the Sidereal system, Aries represents a bold, pioneering spirit. You are likely more grounded and pragmatic than a Western Aries.",
"Taurus": "Sidereal Taurus emphasizes stability and material connection. You possess a strong endurance and a love for sensory beauty.",
"Gemini": "Sidereal Gemini focuses on intellectual agility and communication. You likely have a highly versatile mind.",
"Cancer": "Sidereal Cancer is deeply intuitive and protective. Family and roots are your primary emotional foundation.",
"Leo": "Sidereal Leo represents leadership and creative self-expression. You have a natural warmth and a sense of duty.",
"Virgo": "Sidereal Virgo is analytical and service-oriented. You thrive when you can organize and improve your environment.",
"Libra": "Sidereal Libra seeks harmony and justice. You are naturally diplomatic and value partnership above all.",
"Scorpio": "Sidereal Scorpio is intense and transformative. You possess great psychological depth and resilience.",
"Sagittarius": "Sidereal Sagittarius is a seeker of truth. You are philosophical, honest, and value freedom of thought.",
"Capricorn": "Sidereal Capricorn is disciplined and ambitious. You value tradition and long-term achievement.",
"Aquarius": "Sidereal Aquarius is humanitarian and innovative. You often think outside the box for the benefit of the collective.",
"Pisces": "Sidereal Pisces is spiritual and compassionate. You have a strong connection to the subconscious and the arts."
};
document.getElementById("descriptionOutput").innerHTML = descriptions[currentSign];
document.getElementById("sidereal-result").style.display = "block";
}
Understanding the Sidereal Natal Chart
If you have ever looked at your Vedic astrology chart and noticed your Sun sign shifted back by about 24 degrees, you have experienced the difference between Tropical and Sidereal astrology. While Western astrology is based on the seasons (the Tropical zodiac), Sidereal astrology is based on the actual current positions of the stars and constellations.
What is Ayanamsa?
The core of any Sidereal natal chart calculator is the Ayanamsa. This is the longitudinal difference between the Tropical zodiac (fixed to the Equinox) and the Sidereal zodiac (fixed to the fixed stars). Because the Earth undergoes axial precession, the alignment of the stars relative to our seasons shifts by approximately 1 degree every 72 years.
The most common Ayanamsa used by astrologers today is the Lahiri Ayanamsa (also known as Chitra Paksha). This calculator utilizes the Lahiri method to adjust your birth data, providing a more accurate astronomical view of where the planets were positioned at your birth.
Sidereal vs. Tropical: Which is Correct?
Neither is "wrong," but they serve different purposes:
Tropical (Western): Focuses on the psychological archetypes and the internal journey, tied to the Earth's seasonal cycle.
Sidereal (Vedic/Eastern): Focuses on karma, destiny, and the actual astronomical placement of celestial bodies.
Example Calculation
Imagine someone born on April 15th. In Western (Tropical) astrology, they are an Aries. However, because the Ayanamsa correction is currently around 24 degrees, their Sidereal Sun placement would be approximately 21 degrees in Pisces.
Calculation Walkthrough:
Determine Tropical Sun Position: ~25° Aries.
Apply current Ayanamsa: ~24°.
Subtract: 25° (Aries) – 24° = 1° (Aries) or late degrees of the previous sign.
Result: The Sidereal Sun is likely in the very beginning of Aries or end of Pisces depending on the specific birth year.
Why Use a Sidereal Calculator?
Many people find that their Sidereal chart resonates more deeply with their external life circumstances and "earthly" personality traits. By calculating your Sidereal Sun sign, you gain a secondary perspective on your astrological identity, bridging the gap between ancient observational astronomy and modern self-discovery.