سال ِ هورگردی sâl-e hurgardi
*Fr.: année tropique*
The interval during which the Sun's mean longitude, referred to the mean equinox of
date, increases by 360 degrees. Its mean length for the epoch J2000.0 is
365.24217879 real solar days (approximately 365.2422 days). This concept of tropical
year, adopted by the International Astronomical Union at its General Assembly in Dublin,
September 1955, has often been confounded with the
→ *vernal-equinox year*.
In fact the mean period between two successive true vernal equinoxes is different from
the tropical year. This period, which is equal to 365.24236460 solar days
(about 365.2424 days), is the real mean length of the year in the Iranian calendar. The difference
between the two year lengths is due to the fact that the Earth's orbital velocity
around the Sun is not uniform, since the orbit is an ellipse.
At the perihelion of its orbit the Earth is closest to the Sun, and therefore
moves faster than average, while at aphelion, when it is farthest away from the Sun,
it moves slower. Therefore the interval between two successive vernal equinoxes is not the
same as the period between two successive summer solstices.
In fact the tropical year does not depend on a specific origin for the annual
apparent motion of the Sun. For detailed discussion see:
A concise review of the Iranian calendar. → *tropical*; → *year*. |