Sidereal or tropical?

There are two zodiacs, and both are honest. The tropical zodiac, the one Western astrology and Hali use, follows the seasons. The sidereal zodiac, the one Vedic astrology uses, follows the stars. Around the year 300 the two lay almost exactly on top of each other. Since then they have been drifting apart by one degree every 71 years, and today they sit 24.2 degrees apart. That is why 79% of people have a different Sun sign in each.

See your Sun, Moon and Rising in both, beside the stars that were really behind them.

What is the tropical zodiac?

The tropical zodiac begins where spring begins. Its first degree, 0° Aries, is the point where the Sun crosses the celestial equator heading north at the March equinox. From there the year is divided into twelve equal signs of thirty degrees, so every sign is a season within the seasons. Aries is the first rush of spring, Cancer the longest days, Libra the second equinox and Capricorn the deep dark of midwinter.

Western astrology has measured this way since Ptolemy. In the second century he wrote that the sign which begins with the spring equinox “is the starting-point of them all.” The equinox is a moment, not a place among the stars, and anyone can find it with a stick in the ground: on the equinox, the tip of its shadow draws a straight line across the day.

What is the sidereal zodiac?

The sidereal zodiac is pinned to the stars instead of the seasons. Its signs are the same twelve slices of thirty degrees, but they are anchored to a fixed star, so they stay put among the constellations while the seasons slowly slide past them.

It is the zodiac of Vedic astrology, where it also carries the twenty-seven lunar mansions called nakshatras. In the 1950s the Irish astrologer Cyril Fagan and the American Donald Bradley revived it in the West, arguing that the first zodiac, set out by Babylonian astronomers, had been measured against the stars. On that point the historians agree with them.

Why are the two zodiacs 24 degrees apart?

Because Earth wobbles. Our planet spins like a top, and like a top its axis slowly circles, tracing one full loop about every 26,000 years. Astronomers call it precession. It carries the north pole of the sky across the stars, which is why Polaris is our pole star today and Vega will be, around the year 14,000.

The same wobble moves the equinox. Every year the Sun crosses the equator a little earlier against the background stars, and every 71 years the seasons slip one more degree. The Greek astronomer Hipparchus noticed it around 130 BCE, when he compared the position of the star Spica with records made about 150 years before him.

By the Spica anchor, the two zodiacs agreed around the year 300. Today the gap is 24.2 degrees, close to a whole sign. That is why most Taurus Suns are Aries in the sidereal zodiac, and most Leo Suns are Cancer.

Why is my sign different in Vedic astrology?

Because a Vedic chart measures the same sky from a different starting point. Take about 24 degrees away from every planet in a Western chart and you have its sidereal twin. A planet in the first 24 degrees or so of its Western sign moves back one sign. Only placements in the last few degrees stay where they are, which is why just 21% of people keep the same Sun sign in both.

Your sign has not changed, and neither has the sky. You have simply been given a second map of it.

Which zodiac is more accurate?

That depends on what you ask of it.

As a map of the stars, the sidereal zodiac is far closer. By our count, the sidereal Sun sign matches the constellation the Sun is really standing in 80% of the time, and the tropical sign only 12%. It is still an approximation. The real constellations come in every size, the Sun crosses thirteen of them, and sidereal astrologers do not agree on where the stars begin. Anchor the zodiac to Aldebaran at 15° Taurus instead of Spica at 0° Libra, as some Western sidereal astrologers do, and everything shifts by almost a degree, enough to change the sidereal Sun sign for 3.2% of birthdays.

As a map of the seasons, the tropical zodiac is exact by definition. Its zero point is a moment, not a star, and every observatory on Earth agrees on when it happens.

As a way of predicting a life, neither has been shown to beat chance. In 1985 the physicist Shawn Carlson published a double-blind test of Western astrologers in the journal Nature. In 2009 the astrophysicist Jayant Narlikar and his colleagues tested Vedic astrologers in Pune. In both, the astrologers did no better than guessing. Hali takes those results seriously, which is why the calls in your daily reading are dated, specific and scored where you can see them.

Why does Hali use the tropical zodiac?

Because in Western astrology a sign has always been a season. Your Sun sign holds the light you were born into, the lengthening days of a spring birthday, the gold of late summer, the long dark of midwinter. That meaning belongs to the turning year, and the turning year is measured from the equinox.

Because its starting point is a moment anyone can check, not a choice between stars.

And because we love the stars too. Hali keeps your signs in the tropical zodiac and shows you the real sky beside them, the actual constellation behind every planet the moment you were born, Ophiuchus included. You can see yours on your real sky.

What about the southern hemisphere?

The seasons written into the signs are the northern ones. Born in Sydney or Cape Town, you arrived in Aries as the leaves were turning gold, not opening. Southern astrologers have debated this for a long time. Most still use the same signs, because the equinox itself, the moment the Sun crosses the equator, is the same moment for everyone on Earth.

Which sidereal zodiac does this page use?

The comparison above anchors the sidereal zodiac to the star Spica at exactly 0° Libra. That is the idea behind the Lahiri ayanamsa, adopted for India’s national calendar after the Calendar Reform Committee reported in 1955, and our values agree with published Lahiri figures to within a few arcminutes. An ayanamsa is simply the distance between the two zodiacs, today about 24.2 degrees.

• Sun, Moon and Rising positions come from the same astronomy engine behind every Hali birth chart.
• Spica’s place among the stars comes from the SIMBAD astronomical database, and the real stars use the official constellation borders of the International Astronomical Union.
• Without a birth time, Hali checks the whole day you were born, and a sign that changed that day shows both.
• Every figure on this page is computed by Hali for births from 1950 to 2029.