Tropical vs sidereal zodiac: two starting points for one sky

The tropical zodiac starts at the March equinox and is the one most Western astrology uses. The sidereal zodiac is tied to the stars and is used in Vedic astrology, the astrological tradition of India. The two are about 24 degrees apart, so most people's Sun is one sign earlier in the sidereal zodiac. Neither is wrong: they are two reference frames for the same sky.

What tropical and sidereal mean

Both zodiacs are a circle of 360 degrees, divided into twelve signs of 30 degrees. The signs have the same names and the same order. The only difference is where the circle begins, the point called 0° Aries.

• Tropical zodiac. 0° Aries is the place where the Sun stands at the March equinox, the moment around March 20 or 21 when day and night are about equally long. The exact day depends on the year and on your time zone. This zodiac is anchored to the seasons. The word has nothing to do with a tropical climate: it comes from the Greek word for turning and refers to the turning points of the Sun in the course of the year.
• Sidereal zodiac. 0° Aries is a fixed place among the stars. "Sidereal" means "of the stars". This zodiac is anchored to the starry background.

This guide uses the tropical zodiac throughout. The zodiac sign dates you know from Western horoscopes are tropical dates.

Why the two zodiacs drift apart: precession

Earth's axis is not perfectly steady. It wobbles very slowly, like the axis of a spinning top that traces a small circle while the top spins. One full wobble takes about 26,000 years. Astronomers call this precession.

This wobble has a consequence for the equinox: the place among the stars where the Sun stands at the March equinox slides slowly backward, by about one degree every 72 years. The tropical zodiac moves with the equinox, while the sidereal zodiac stays with the stars. So the two scales slide against each other.

In a human lifetime of 72 years that is one single degree. Over the centuries it adds up. In the 21st century the gap is about 24 degrees. At one degree every 72 years, that means the two zodiacs lined up roughly 1,700 years ago.

The exact size of the gap depends on which place among the stars is taken as the anchor. Sidereal schools differ slightly on this, so their figures differ slightly too.

What changes in your chart and what stays the same

Every position shifts by the same amount. The Sun, the Moon, each planet and the Rising point, the degree that was coming up in the east at your birth, all move back about 24 degrees. A sign is 30 degrees wide, and the Sun moves about one degree per day. That gives a simple rule:

Two examples. A Sun at 10° Leo in the tropical zodiac stands at 16° Cancer in the sidereal one. A Sun at 27° Leo stands at 3° Leo and keeps its sign. The same arithmetic applies to your Moon sign and your Rising sign, also called the Ascendant.

What stays the same:

• The sky itself. Both zodiacs describe the same Sun, Moon and planets at the same moment.
• The aspects. These are the angles between the planets. Because everything shifts by the same amount, no angle changes.
• The timing. A Full Moon or an eclipse happens at the same moment in both zodiacs. Only the sign named for it differs.

One thing does not carry over: the descriptions. What this guide says about Leo was written for the tropical Leo. Vedic astrology is a complete system of its own. It gives the Moon and the Rising sign more weight than the Sun sign and works with its own techniques. Taking a sidereal position and reading a Western description for it mixes two languages.

Is one of the two zodiacs wrong?

No. Think of Celsius and Fahrenheit. A room at 20 degrees Celsius is a room at 68 degrees Fahrenheit. Both numbers are correct, and the room is equally warm either way. The only mistake is to mix the scales.

The planets stand where they stand. The two zodiacs only disagree about where to start counting.

Each scale has its own logic. The tropical zodiac follows the year: the Sun enters Aries at the March equinox, Cancer at the June solstice, Libra at the September equinox and Capricorn at the December solstice. The sidereal zodiac follows the stars you can see at night.

A common objection goes: "The signs no longer match the constellations." For the tropical zodiac that is true, and it is by design. Its signs are named after constellations but measured from the equinox. The sidereal signs are not the constellations either. They are twelve equal sections of 30 degrees, while the real star patterns have very different sizes. That is also why the story of a 13th zodiac sign changes neither zodiac.

Which scale says something true about a person cannot be settled by looking through a telescope. Astrology is a symbolic language and a tool for self-reflection, not a measurement of forces. How astrology differs from astronomy

What to do next with your own chart

1. If you read Western astrology, stay with the tropical zodiac. Your sign is the one you have always known.
2. If Vedic astrology interests you, use the sidereal zodiac and read it in its own tradition.
3. Find the exact degree of your Sun. A calculated birth chart shows it, and with the rule above you can see at once whether your sign differs between the two.

A Birth Chart Reading is calculated from your exact birth data and written for your chart.