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Equinox
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Diagram showing Earth's elliptical orbit around the Sun with labeled points for aphelion, perihelion, solstices, and equinoxes, and distances from the Sun at aphelion and perihelion.
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A diagram of the celestial sphere with Earth at the center, showing the celestial equator, ecliptic, and labeled positions of the vernal and autumnal equinoxes, as well as the north celestial and ecliptic poles. sunlight strikes parts of Earth differently Diagram showing Earth's orbit around the Sun with labeled positions for the summer solstice, winter solstice, vernal equinox, and autumnal equinox, illustrating how Earth's tilt causes the seasons.
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Diagram showing Earth's elliptical orbit around the Sun with labeled points for aphelion, perihelion, solstices, and equinoxes, and distances from the Sun at aphelion and perihelion.
Earth's orbit On average, Earth is approximately 149.6 million km (about 92.96 million miles) away from the Sun. Earth's orbit around the Sun is nearly circular, with one full orbit marking the period of one year.

Equinox

astronomy
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Equinox, either of the two moments in the year when the Sun is exactly above the Equator; also, either of the two points in the sky where the ecliptic (the Sunā€™s annual pathway through the sky) and the celestial equator intersect. On March 20 or 21, the Sun moves north across the celestial equator, marking the vernal equinox, the beginning of spring in the Northern Hemisphere, and the autumnal equinox, the beginning of autumn in the Southern Hemisphere. On September 22 or 23, as the Sun crosses the celestial equator going south, marking the autumnal equinox, the beginning of autumn in the Northern Hemisphere, and the vernal equinox, the beginning of spring in the Southern Hemisphere.

A large tree with leaves turning orange in fall.
What Causes the Seasons?In many parts of the world, weather cycles through the four seasons like clockwork: spring, summer, autumn, and winter.
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Some astronomical coordinatesā€”e.g., right ascension and celestial longitudeā€”are measured from the vernal equinox. It is sometimes called the first point of Aries because it was at the beginning of that constellation some 2,000 years ago. The term is still used, though precession of the equinoxes has moved the vernal equinox into Pisces.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Erik Gregersen.