Zodiac, equinoX and the Wobble

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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.

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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.

Image Credits: EarthSky.org, MydarkSky.org

One Nation Many Calendars

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India has as many new years as states. But there is a method to the madness.

The calendars can be grouped in five main variations. Two are solar, three are lunar.

The solar new years correspond to a specific day of the Gregorian Calendar, which is also solar. The lunar new years start on a specific phase of the moon.

The first solar group starts on the Tropical Equinox, when the day and nights are equal, around 21 March. The Indian National Calendar and the Parsi Navroz are in this category.

The second solar group starts on the Sidereal Equinox, which adjusts for the wobble in the tilt of the earth. It starts around April 14. The calendars of Tamil Nadu, Kerala, Odisha, Assam (Bihu), West Bengal and Punjab (Baisakhi) are in this group. The equinox shifts away from the sidereal new year by about a day, every hundred years.

The first lunar group starts on the full moon following the equinox. This is Holi, celebrated as new year in Haryana, Bihar, Rajasthan, Madhya Pradesh, Uttarakhand, Jharkhand and Chhattisgarh.

The second lunar group starts on the new moon following the equinox. This is Ugadi in Karnataka, Andhra Pradesh, Telengana, Maharashtra and Konkan; Navreh in Kashmir and Vishu in Manipur.

And finally, Gujrat starts its calendar on Diwali, which falls on a new moon in autumn.