Equinoxes, Solstices, and Tropical Aries

Tropical Aries begins at the March equinox, the moment the Sun crosses the celestial equator heading north. Sidereal Mesha does not. Mesha is a 30-degree span tied to the stars. Precession has put about 24 degrees between those two starting lines, which is why "Aries season" in a Western magazine is not the month of Mesha.

Four solar events hold up the tropical zodiac. At the March equinox, on or about 20 March, day and night are nearly equal and the Sun's longitude is 0 degrees Aries. At the June solstice, on or about 21 June, the Sun reaches its furthest north and tropical Cancer begins. At the September equinox, on or about 23 September, the Sun crosses the equator heading south and tropical Libra begins. At the December solstice, on or about 21 December, the Sun reaches its furthest south and tropical Capricorn begins. The other eight signs are the 30-degree steps between these four corners. The system is a calendar of seasons. It would keep the same dates even if the constellations were renamed.

The stars have drifted against that calendar. The Earth's axis wobbles, so the equinox point slides west along the ecliptic by about one degree every 72 years. Hellenistic astronomers already knew the two zodiacs were slipping apart. They coincided roughly in the early centuries of the common era. The gap today is the ayanamsa. Lahiri, the value this site uses for Jyotish, places the star Chitra, Spica, at 180 degrees of the sidereal zodiac, and in the mid-2020s the resulting offset is a little over 24 degrees. The March equinox therefore falls in sidereal Pisces, not at the start of sidereal Aries. Tropical 0 Aries is about 6 degrees of sidereal Pisces. A person born a week later, near 8 degrees of tropical Aries, still has the Sun in sidereal Pisces, near 14 degrees of Meena.

What this does to a birthday

Western Aries season runs from the March equinox to about 20 April. For most of those days the Lahiri Sun is in Pisces, and only the last several degrees of tropical Aries reach sidereal Aries. Calling that whole month Mesha imports a season into a star chart. The birthday arithmetic is in how a sun sign is derived. The two reasons a rashi label then drifts further, zodiac and Moon versus Sun, are in why the sun sign and the Vedic rashi differ.

Questions people ask

Is the constellation Aries empty of the Sun in April?

The Sun still spends a month each year in the direction of the stars of Aries. That month is not the tropical Aries month. The constellation passage comes later, by about 24 degrees, which is roughly three weeks of solar motion.

Do solstices matter in Jyotish?

The seasons matter for weather and for festivals tied to a solar month. The sidereal signs are not defined by the solstice. A Makara Sankranti is a sidereal ingress, not the December solstice, and the two dates are weeks apart.

Will tropical Aries and Mesha ever line up again?

Precession continues. They will not snap back in a lifetime. Charts should use the ayanamsa of the year they claim to calculate, and they should say which zodiac they used.