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From the Astronomical Dictionary


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the vernal equinox

The vernal equinox is the intersection of the equator of the sky and the ecliptic, through which the Sun passes during the March equinox, at the beginning of spring in the northern hemisphere, and autumn in the southern hemisphere. The vernal equinox is the zero point of longitude and latitude of the equatorial and ecliptic coordinate systems. The period between two successive passages of the Sun through the vernal equinox is the tropical year.