Did you know? On Earth, twice each year (in March and September), daytime and nighttime are of approximately equal duration all over the planet.
Equinox is a unique phenomenon during which a planet’s subsolar point passes through its equator. On equinox days, the Earth's axis is perpendicular to the Sun's rays, implying that all regions of the planet receive approximately the same number of hours of sunlight. In other words, only during equinox is the Earth's 23.5° axis not tilting toward or away from the Sun. This is why it is called an “equinox”, derived from Latin words aequus, meaning equal or even, and nox, meaning night. However, this literal translation is not entirely accurate. In reality, equinox days do not have exactly 12 hours of daylight and 12 hours of darkness. Earth receives a few more minutes of light than darkness during an equinox. This is because sunrise is defined as the moment when the sun's tip crosses the horizon, and sunset is defined as the moment when the sun's other edge vanishes behind the skyline. Because the sun is a disc rather than a point source of light, at an equinox, Earth gets a few minutes of extra light (rather than darkness). Furthermore, the sun's light is refracted by the atmosphere and continues to travel to "nighttime" Earth for a short time after the sun has dipped below the horizon.
Every year, there are two equinoxes: one in March and the other in September. In March, the Sun crosses the equator from south to north. According to a definition, the March and September equinoxes mark the beginning of Earth's spring and autumn seasons. Sometimes, the equinoxes are named the “vernal equinox” (spring equinox) and the “autumnal equinox” (fall equinox). However, the Northern and Southern Hemispheres have different dates. The March equinox is known as the vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere. The September equinox is known as the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere.
The March equinox is when the Sun crosses the celestial equator—an imaginary line in the sky above Earth’s equator—from south to north which happens on March 19, 20, or 21 every year. The date of the equinoxes varies because a year in our calendar does not exactly match the length of the tropical year—the time it takes the Earth to complete an orbit around the Sun.
The second equinox, September equinox, takes place on or around September 22 every year. It is the Southern Hemisphere's spring equinox and the fall equinox in the Northern Hemisphere.
The subsolar point moves north or south before and after the equinox. It moves north as the Northern Hemisphere tilts toward the Sun after the March equinox. The subsolar point reaches the Tropic of Cancer (23.5°N) around June 21. The June solstice marks the start of the subsolar point's southward migration. This is the summer solstice when there is most sunlight of the year. The subsolar point continues to migrate south as the Southern Hemisphere tilts toward the Sun after the September equinox. Around December 21, the subsolar point reaches the Tropic of Capricorn (23.5°S). This is the winter solstice which observes the least sunlight of the year.
Therefore, an equinox happens twice a year (at the start of the spring and fall) when the sun is nearest to the equatorial plane resulting in equal lengths of day and night, and solstice is the time of the year (during the summer and the winter) when the sun is farthest from the equatorial plane resulting in long nights and days.