Celestial Crossing

During the northern summer the Sun stays north of the equator (which is why it is summer), then it dips in the autumn crossing through the celestial equator again. When the Sun is on the celestial equator, night and day are equal in length, and each crossing point is often referred to as an equinox..... The crossing point of ecliptic and celestial equator which the Sun reaches in the northern spring is the vernal equinox (also known as The First Point of Aries) and denoted by the symbol U.