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seasonal configuration of Earth and Sun
Earth's orbit around the Sun, with the positions of solstices and equinoxes.
vernal equinox
astronomy
Also known as: first point of Aries, spring equinox
vernal equinox, two moments in the year when the Sun is exactly above the Equator and day and night are of equal length; also, either of the two points in the sky where the ecliptic (the Sun’s annual pathway) and the celestial equator intersect. In the Northern Hemisphere the vernal equinox falls about March 20 or 21, as the Sun crosses the celestial equator going north.
What causes the seasons?In many parts of the world, weather cycles through the four seasons like clockwork: spring, summer, autumn, and winter.
See all videos for this articleIn the Southern Hemisphere the equinox occurs on September 22 or 23, when the Sun moves south across the celestial equator. According to the astronomical definition of the seasons, the vernal equinox also marks the beginning of spring, which lasts until the summer solstice (June 20 or 21 in the Northern Hemisphere, December 21 or 22 in the Southern Hemisphere).