Many years ago, the ancients devised the zodiac, based on, which constellation appears immediately behind the sun. Of course you can’t see the stars and the Sun at the same time. But watching the constellation that rises just before sunrise, and one that sets just after sunset, one can figure. With observations made throughout the year, a watch, and some mathematics, an accurate determination can be made. Aries became the first period of the zodiac, as they found the sun just entering the constellation Aries on the spring equinox day, every year.
today the equinox doesn’t occur on the day the sun enters Aries
However, the equinox is linked to the tilt in the earth’s axis, which, it turns out, has a wobble. The axis today points to Polaris, but it did not do so two thousands years ago. Result being, today the equinox doesn’t occur on the day the sun enters Aries. It has been slipping back by a day every seventy years. You can call it a precession, if you have a thing for such words.
In the tropical zodiac, Aries still starts on the equinox day (March 21), even though the sun really enters Aries a month later.
In the tropical zodiac, the western astronomers decided to keep the spring equinox as the reference, and by convention Aries still starts on the equinox day (March 21), even though the sun really enters Aries a month later. If someone is born on October 26th, like my wife, she’s a Scorpio based on this zodiac. However, the sun wasn’t actually in Scorpio when she was born. Take my well learned advice though, don’t take her any lightly for that.
In the sidereal zodiac, Aries starts on April 14, when the Sun actually enters Aries, even though the equinox is a month earlier.
In the sidereal zodiac, the eastern astronomers decided to use the sun’s actual position as the reference, and Aries starts on April 14, when the Sun actually enters Aries, even though the equinox is a month earlier. In this calendar, the new year on April 14 isn’t a real equinox. It is the day when the sun is in the same stellar position, where it was on the equinox day, on some reference year, 1500 years ago.
Which one is better? It is a matter of choice.
You could say that the sidereal zodiac is astronomically more accurate. The stars offer a sturdier frame of reference than the axis of earth. But the tropical zodiac corresponds to the seasons better. The seasons are after all, driven by the wobbly axis.
The Sidereal zodiac is the basis for the solar calendar used by Indian mathematicians. April 14th ( or 13th) is celebrated as New Year’s Day in the states of Tamil Nadu, Kerala, Odisha, Assam (Bihu), West Bengal and Punjab (Baisakhi). For more about the various New Year’s Days of India see the post on One Nation Many Calandars.