Birkat Hachama - The Vernal Equinox

The Vernal Equinox

As explained in the Talmud, there is a tradition that the Sun was created in its vernal equinox position at the beginning of the springtime Jewish lunar month of Nissan. The sages of the Talmud settled disputes over the halachic definition of the vernal equinox by establishing it on March 25 of the Julian calendar. Because both the Julian calendar and Jewish tradition define a solar year as exactly 365.25 days, the halachic vernal equinox historically fell out on March 25 every year. This halachic equinox now falls about 17 days after the true equinox, with the error increasing by about 3/4 of a day per century.

In summary, Birkat Hachama is recited when the halachic vernal equinox (the position at which the Sun was created) occurs at sundown on a Tuesday (the time at which the sun was created).