  We present a general description of the Iranian calendar and Nowruz, which
starts at the precise instant when the Sun coincides with the vernal equinox.
Although the paper aims at the basic astronomical elements of the calendar,
historical aspects are not ignored.
  Using the ephemeris provided by the French Institut de Mecanique Celeste et
de Calcul des Ephemerides (IMCCE) for the period A.D. -4000 to +2500, we
discuss the various definitions of the year and show that the Iranian calendar
is based upon the vernal-equinox year and not on the ``tropical year'' of
365.2422 days formulated by researchers in the field of celestial mechanics.
  Neglecting the short-term fluctuations, the mean length of the Iranian year
at the present epoch is 365.2424 solar days, corresponding to the mean interval
between two successive passages of the Sun through the vernal equinox.
  There is now a widely spread confusion between the "tropical" and
vernal-equinox years, leading to the erroneous value of 365.2422 days often
given for the mean interval between two successive passages of the Sun through
the vernal equinox.
  We also analyze the intercalation system of the Iranian calendar, which is
based on a 33-year cycle, and point out the astronomical foundation of this
cycle created by the famous Iranian mathematician, astronomer, and poet Omar
Khayyam.
