An equinox occurs twice yearly when the Sun crosses the celestial equator—Earth's equator projected into space—resulting in nearly equal daylight and darkness (approximately 12 hours each) across the globe. This astronomical event happens around March 20th (vernal equinox) and September 22nd (autumnal equinox), marking the official beginnings of spring and fall in the Northern Hemisphere.</p><p>During an equinox, the Sun appears directly overhead at noon along Earth's equator, and both poles experience equal illumination. The word "equinox" derives from Latin meaning "equal night," though due to atmospheric refraction, daylight actually lasts about 8-10 minutes longer than darkness.</p><p>Ancient civilizations recognized equinoxes thousands of years ago, building monuments like Stonehenge and Chichen Itza to track these solar events. The Great Pyramid of Giza aligns precisely with cardinal directions, demonstrating early astronomical knowledge. Modern farmers still use equinoxes for planting schedules, while cultures worldwide celebrate these transitions—from Nowruz (Persian New Year) during the spring equinox to harvest festivals in autumn.</p><p>Equinoxes occur because Earth's axis tilts 23.5 degrees relative to its orbital plane. As Earth orbits the Sun, this tilt creates our seasons, with equinoxes representing the balanced moments when neither hemisphere tilts toward or away from the Sun, creating equal solar exposure globally.