Tropical zodiac vs. sidereal zodiac: the explanation
A zodiac is a band of sky centered on the ecliptic, the Sun’s apparent path through the year, divided into twelve sectors of 30 degrees.
What remains is choosing where to place the starting point. The tropical zodiac puts it at the spring equinox: Aries opens when day equals night, around March 20, and the four seasons open the four cardinal signs. The sidereal zodiac ties it to the distant stars, the way you pick out a milestone on a road.
But the equinox does not stay still among the stars: the Earth’s axis pivots very slowly, which shifts this point by 50.29 arcseconds each year. The two zodiacs therefore coincided long ago, in the first centuries of our era, then drifted apart. The site calculates the gap with the Lahiri ayanamsa, the official value in India: about 24 degrees in 2026. In practice, a person born on March 25 has the Sun in Aries in the tropical zodiac, but still in Pisces in the sidereal one.
This difference does not make either one wrong: they answer two different questions. The tropical zodiac describes the relationship of the Earth to the Sun and the seasons; the sidereal one describes the Sun’s position in front of the constellations. Versana uses the tropical zodiac, that of the Western tradition, for its sign, rising sign, and birth chart calculations, and says so each time the difference matters.
Family
The sky and the zodiac
The diagram
Two zodiacs, a growing gap
Read next
The precession of the equinoxes
History, debates, and sources
A little history
Ptolemy, in the 2nd century, starts the signs at the equinoxes and solstices in the Tetrabiblos: this is the source of the zodiac of the seasons, which remains that of Western astrology.
Yet he knew of the discovery made by Hipparchus, three centuries earlier, that the equinoxes slip relative to the stars. Indian astrology, for its part, kept a zodiac tied to the stars. In 1955, a committee tasked with reforming the Indian calendar adopted the Lahiri value as the official reference. Cyril Fagan, in 1951, had founded a Western sidereal school of his own.
Sources
4
Oldest
Claudius Ptolemy, 2nd century
Most recent
Cyril Fagan, 1951
What is debated
Supporters of the tropical zodiac hold that the signs mark the seasons and the Sun’s cycle, independent of the stars.
Supporters of the sidereal zodiac believe the zodiac should stay tied to the constellations, as it was originally. Even among sidereal astrologers, the starting point varies: Lahiri, Fagan-Bradley, Krishnamurti, or Raman differ by a few arcminutes up to more than a degree. No astronomical argument settles it, since both reference points are exact; the choice comes down to what each tradition is trying to describe.
Schools do not all agree: the site sets out the positions without deciding for you, and states the setting it uses by default.
Where this concept comes from
The authors and texts that establish or discuss this concept, with the passage when it is known.
2nd centuryClaudius PtolemyTetrabiblos, Book I
Starts the signs at the equinoxes and solstices: the zodiac of the seasons, known as tropical.
c. 130 BCEHipparchus(lost work, reported by Ptolemy, Almagest VII), see the Almagest, Book VII
Discovers the slow drift of the equinoxes relative to the stars, the origin of the gap between the two zodiacs.
1955Indian Calendar Reform Committee (chaired by Meghnad Saha)Report of the Calendar Reform Committee, whole report
Adopts the so-called Lahiri ayanamsa, the official value of the gap for Indian astrology and the Indian calendar.
The diagram’s figures are calculated by the site’s engine (Astronomy Engine) or taken from the recorded facts, never copied by hand.
Two zodiacs, a growing gap
Outside: tropical zodiac, Aries begins at the equinox. Inside: sidereal zodiac (Lahiri ayanamsa, about 24° 13′ in 2026).
Frequently asked questions: tropical zodiac vs. sidereal zodiac
What is the difference between the tropical and sidereal zodiacs?
The tropical zodiac measures the signs from the spring equinox, so according to the seasons. The sidereal zodiac measures them from a point fixed on the stars. At the start of our era the two reference points coincided; the precession of the equinoxes has since opened a gap of about 24 degrees in 2026, nearly one sign.
Does my sign change in sidereal astrology?
In about four cases out of five, yes: with a 24-degree offset, the Sun lands in the previous sign, except for the last six days of each tropical sign. The result also depends on the ayanamsa chosen. Your tropical sign remains the sign of Western astrology, the astrology this site presents.
Which zodiac does Versana use?
Versana uses the tropical zodiac, the most widespread in Western astrology and in the press. It calculates planetary positions with an astronomical engine, then sorts them into twelve equal sectors counted from the spring equinox. When a page mentions the sidereal zodiac, it says so explicitly.