The Sun’s apparent position helps determine the exact moment of Nowruz.
Nowruz begins at the vernal equinox, the instant when the Sun appears to cross the celestial equator moving northward. At that point, day and night are approximately equal in length, and the Northern Hemisphere begins astronomical spring.
Because the equinox occurs at a precise moment rather than at midnight everywhere, the local calendar date can vary. In Iran and nearby regions, Nowruz generally falls on 20 or 21 March in the Gregorian calendar. The Solar Hijri year begins at that equinox-based transition.
The Moon does not set the date of Nowruz, which distinguishes it from lunar-calendar festivals. Nor is the festival determined simply by a fixed Gregorian date in every country; local time and calendar rules matter.