Nowruz and the spring equinox

 At the time of the equinox, the sun is observed to be directly over the equator, and the north and south poles of the Earth lie along the solar terminator; sunlight is evenly divided between the north and south hemispheres.

In ca. 11 century CE major reforms of Iranian calendars took place and whose principal purpose was to fix the beginning of the calendar year, i.e. Nowrūz, at the vernal equinox. Accordingly, the definition of Nowruz given by the Iranian scientist Ṭūsī was the following: "the first day of the official new year [Nowruz] was always the day on which the sun entered Aries before noon".