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Vernal Equinox, the Commencement of Iranian Calendar


20 March 2005



At 16:03:24 (Tehran time) on Sunday 30th of Esfand (20th of March) Iranian New Year will start. But how is this complicated calculation carried out to obtain such an accurate timing?

For the first step, we should be aware that in astronomy appearances and reality are far different and in order to have an easier look on this particular astronomical phenomenon we take the appearance. As the result we need to assume a single transparent sphere on which all celestial bodies including the sun and other planets of the solar system are situated. This “sky sphere” or “celestial sphere”, as we call it in astronomy, is crossed by the earth rotation axis, which is by a negligible difference the same as that of magnetic poles, at two points, one just above the North Pole called north celestial pole, the other above the South Pole. For the second step you just need more details on this celestial sphere so imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere. They are now called, respectively, declination and right ascension. This way, we have a celestial equator on this virtual sphere correspondent to that of earth. So if you stand on the Earth's equator, the celestial equator passes overhead and if you stand on either pole it passes below your horizon.

We see the sun every day crossing the sky east-west ward and thanks to Galileo Galilei, we all know this movement is due to earth’s axial rotation. But earth, a planet like others in solar system, is affected by a planetary revolution as well which means an orbital movement around the sun. Because of this revolution, had our sun been less radiant than what is today, we could have had this opportunity to be an eyewitness of this revolution effect. In fact this way, we could have observed a gradual west ward motion of sun on its background of celestial sphere day in day out and eventually as the result of this motion the sun reaches its original location as the first day after a period of 365.24 days. This period which is exactly equal to the period of one complete revolution is called one year.

By locating the sun’s movement on the background of 12 constellations of stars through out a year (called zodiacs) we realize that the sun appears to move in a great circle of the celestial sphere as viewed from the earth. This great circle which is inscribed on a terrestrial globe inclined at an approximate angel of 23° and 27′ to the earth’s equator is called ecliptic and is in fact the intersection plane of the earth’s orbit with the celestial sphere (the inclination angel mentioned above is due to the deviation of the earth’s rotation axis from the right angel from the plane of the earth’s orbit).

Ecliptic and celestial equator are not parallel and are intersected at two points called equinoxes. In other words, in its long journey round the ecliptic, the sun is sometimes below the celestial equator (during the summer of southern hemisphere and winter of northern hemisphere) and is sometimes above the celestial equator (during the winter of southern hemisphere and the summer of northern hemisphere). Meanwhile the intersection points, one vernal and the other autumnal equinox are situated opposite to each other and whenever the sun is located on either equinox, the length of the day and night are equal. This happens once on 21st of March (vernal equinox) and the other on 23rd of September (autumnal equinox).

The vernal equinox on which Iranian calendar is based, the beginning of spring and after that, days commence to lengthen day after day until summer revolution. The moment the sun reaches the vernal equinox, Iranian New Year starts and this is one of Iranians’ intangible masterpieces who opted the beginning of spring as the beginning of the New Year.



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