The Spring Equinox / First Day of Spring

A solar equinox occurs when the Sun crosses the Earth’s equator, or, more precisely, when the plane of Earth’s equator passes through the geometric centre of the Sun’s disk. This occurs when the Earth’ rotation axis is directly perpendicular to the Sun-Earth line.

When the Julian calendar was established back in 45 BC, Caesar set March 25th as the date of the Spring Equinox, which was already the starting day of both the Persian and Indian calendars. However, due to the length of the Julian calendar, it drifted, and by the 1580s the Spring Equinox had moved to March 11th!