|
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.
Top
of Page
Source:
Relevant
News:
|
