What is the difference between tropical and sidereal astrology?

What does the tropical zodiac measure?

Position measured from the March equinox. The Sun’s apparent yearly path, the ecliptic, crosses the celestial equator at two points, and the tropical zodiac calls the March crossing 0° Aries. From there it divides the circle into twelve equal signs of 30°. Because the starting point is tied to the Earth’s seasons rather than to the stars, the Sun reaches 0° Aries at the March equinox every year, whatever stars lie behind it.

Calculated: at the 2026 March equinox the stars behind the Sun belonged to the constellation Pisces, yet by definition the Sun was at 0° Aries. The equinox fell at 14:45 UTC on 20 March.

On your OrbHeaven chart: every position is in the tropical zodiac, measured from the equinox of the birth date, and the line at the foot of the chart names it.

What does a sidereal zodiac measure?

Position measured from a starting point fixed against the background stars. A sidereal zodiac uses the same twelve equal 30° signs, but its 0° Aries is set by a chosen reference, such as a particular star, rather than by the equinox. The gap between the sidereal and tropical starting points on a given date is called the ayanamsa. Sidereal zodiacs are used mainly in Indian astrology, and by some Western astrologers.

Calculated: the Lahiri system takes its name from N. C. Lahiri, member secretary of India’s Calendar Reform Committee (1952–1955). It fixes its starting point 180° from the star Spica, and by that definition it coincided with the tropical starting point in 285 CE. At the 2026 March equinox, when the Sun was at tropical 0° Aries, it was at 5°46′ Pisces on the Lahiri zodiac.

What is a convention here: the stars give a fixed frame, but which point in that frame counts as 0° Aries is a decision each tradition makes. That choice is not something an observation can settle.

Why do the two zodiacs drift apart?

Because the equinox moves against the stars. The Earth’s axis slowly wobbles like a spinning top’s, a motion called precession, caused mainly by the pull of the Sun and Moon on the Earth’s equatorial bulge. One wobble takes about 25,800 years, and it slides the equinox backwards along the zodiac by about 50 arcseconds a year, roughly one degree every 72 years. A sidereal starting point stays with the stars, so the gap grows at that rate.

Calculated: on the Lahiri zodiac the gap was 23.17° in mid-1950, 23.73° in mid-1990 and 24.23° in mid-2026. Over the century to 2026 it grew by 1.40°, or 50.3 arcseconds a year: one degree every 71.6 years. So the size of the gap depends on the birth date as well as on the system.

The rate itself changes very slowly over thousands of years, which is why “roughly one degree every 72 years” is a good description and not an exact constant.

Do sidereal astrologers agree on the size of the gap?

No. Each sidereal system fixes its starting point by its own definition, so they disagree by a few degrees. On 1 July 2026, five sidereal systems put the gap between 22.78° and 25.11°. A figure such as “about 24 degrees” is therefore true of one system, Lahiri, at one time. A sidereal position means little unless it names the system that produced it.

Calculated: by the Swiss Ephemeris, on 1 July 2026, Raman 22.78°, Yukteshwar 22.85°, Krishnamurti 24.13°, Lahiri 24.23° and Fagan/Bradley 25.11°. Run backwards, each gap reaches zero in a different year: by its own definition, each system’s zodiac coincided with the tropical one in 221 CE (Fagan/Bradley), 285 (Lahiri), 292 (Krishnamurti), 385 (Yukteshwar) or 389 (Raman).

This is the same spread that makes two calculators disagree about a planet’s sign; see why astrology calculators give different results.

Are the signs the same as the constellations?

No, in either zodiac. The constellations are 88 regions of sky whose official boundaries the International Astronomical Union published in 1930, and the thirteen that the Sun’s path crosses are very unequal in width. Both zodiacs instead use twelve equal 30° signs, so a sidereal zodiac measures from the stars, not the constellations. The two often name the same thing, but not always.

Calculated: on 10 December 2026 the Sun was at 18°26′ Sagittarius in the tropical zodiac and 24°11′ Scorpio in the Lahiri zodiac, but the constellation behind it was Ophiuchus, which no zodiac counts as a sign. Along the Sun’s path the constellations range from 6.6° wide (Scorpius) to 44.0° (Virgo).

The constellations, Ophiuchus and the recurring “your star sign has changed” story are covered in why your star sign is not where the stars are.

Will your sign be different in the other zodiac?

Often, though nothing in the sky changes. A tropical position is converted by subtracting the chosen system’s gap for that date. A planet fewer degrees into its tropical sign than the gap falls in the previous sign; one further in keeps the same sign name. With Lahiri’s gap near 24°, most positions move back one sign. The Moon, the planets and the Ascendant all shift by the same angle.

Calculated: for a 2026 birth, any position in the first 24°13′ of a tropical sign falls in the previous Lahiri sign. Counting each day at noon UTC, the Sun’s Lahiri sign differs from its tropical sign on 282 days of 1950, 290 of 1990 and 295 of 2026. That is a count of days, for the Sun and one system; it is not a count of people, and a system with a larger gap moves more days.

On your OrbHeaven chart: every position is tropical, given to the degree and minute. There is no sidereal chart and no setting to switch zodiacs.

Which zodiac is right?

Neither has been measured to be correct. Each is a coordinate system with a different reference point: the tropical zodiac is anchored to the seasons, a sidereal one to the stars. The positions they describe are calculated and can be checked in either. What a sign is said to mean belongs to astrological tradition, and neither zodiac has been shown to predict events or to describe personality.

What is calculated, and what is not: the position of the Sun, Moon and planets, the date of the equinox and the size of each system’s gap are astronomy. The meanings given to the signs, in either zodiac, are interpretation.