Both divide the Sun’s yearly path into twelve signs of 30°. They differ in where they start counting — and that starting point has drifted apart by about 24° over the last two thousand years.
The tropical zodiac: tied to the seasons
The tropical zodiac starts at the March equinox, the moment the Sun crosses the celestial equator going north. That point is 0° Aries; the June solstice is 0° Cancer, the September equinox 0° Libra and the December solstice 0° Capricorn.
Because it is defined by the equinoxes, the tropical zodiac stays in step with the seasons and with the calendar: the Sun enters Aries on 19, 20 or 21 March every year. This is the zodiac used by most Western astrologers, horoscope columns and MySkySoul.
The sidereal zodiac: tied to the stars
The sidereal zodiac fixes its signs to the background stars instead. Its 0° Aries is set by a reference star or point, and the difference between the two zodiacs is called the ayanamsa. Several ayanamsas are in use; the most common in Indian astrology (Lahiri) is currently a little over 24°.
Sidereal signs are still equal 30° segments. They are not the constellations themselves, which have very different sizes.
Why the two drifted apart: precession
The Earth’s axis slowly wobbles like a spinning top, completing one circle in about 25,800 years. This makes the equinox points slide backwards against the stars by about 1° every 72 years.
Around the 3rd or 4th century AD, depending on the ayanamsa chosen, the two zodiacs roughly coincided. Since then the gap has grown to about 24°, and it keeps growing by roughly 1° per human lifetime.
What it means for your signs
Subtract about 24° from any tropical position to get its sidereal position. If your tropical Sun is in the first 24° of a sign, your sidereal Sun is in the previous sign; only in the last 6° or so does it stay in the same sign. So for roughly four people in five, the sidereal Sun sign is the one before their tropical sign.
The same shift applies to the Moon, the Rising sign and every planet, so a whole chart can look different. Neither version is more “accurate”: they measure from different reference points, and each astrological tradition interprets its own zodiac.
Ophiuchus and the “13th sign”
Astronomers divide the whole sky into 88 constellations with official boundaries agreed by the International Astronomical Union in 1928. Along the Sun’s path those boundaries cross thirteen constellations, including Ophiuchus, and the Sun spends very different lengths of time in each — from about a week in Scorpius to over six weeks in Virgo. The Sun is in Ophiuchus from about 29 November to 17 December.
Astrological signs, tropical or sidereal, are twelve equal divisions, not these constellations, so Ophiuchus is not a sign in either system. Headlines claiming that NASA “added a 13th sign” describe the constellations, not a change to astrology.
Which one does MySkySoul use?
MySkySoul uses the tropical zodiac for every calculator. If you follow Vedic astrology, you can still use our positions: subtract your tradition’s ayanamsa from the degrees shown. Our “How we calculate” page lists every convention we use.
Sources
Astrology offers themes for reflection, not predictions or facts about who you must be.