Definition of an Equinox
An equinox is an astronomical event that occurs twice a year when the Sun crosses the celestial equator. At this precise moment, the Earth's axis is neither tilted toward nor away from the Sun, resulting in nearly equal durations of daylight and darkness across most parts of the world.
When Equinoxes Occur
There are two equinoxes annually: the vernal (or spring) equinox, typically around March 20th, and the autumnal (or fall) equinox, usually around September 22nd. These events herald the beginning of spring and autumn, respectively, in the Northern Hemisphere, with opposite seasons in the Southern Hemisphere.
The Role of Earth's Axial Tilt
The phenomenon of an equinox is a direct consequence of the Earth's axial tilt (approximately 23.5 degrees) relative to its orbital plane around the Sun. During an equinox, the Sun's most direct rays shine perpendicularly on the Equator, illuminating both the Northern and Southern Hemispheres equally.
Global Daylight and Darkness
At the moment of an equinox, the line separating day and night (the terminator) passes through both the North and South Poles. This unique alignment means that every location on Earth experiences approximately 12 hours of daylight and 12 hours of night, leading to the term 'equal night'.